NAME
perlfunc - Perl builtin functions
DESCRIPTION
The functions in this section can serve as terms in an expression.
They fall into two major categories: list operators and named unary
operators. These differ in their precedence relationship with a
following comma. (See the precedence table in perlop.) List operators
take more than one argument, while unary operators can never take more
than one argument. Thus, a comma terminates the argument of a unary
operator, but merely separates the arguments of a list operator. A
unary operator generally provides a scalar context to its argument,
while a list operator may provide either scalar or list contexts for
its arguments. If it does both, the scalar arguments will be first,
and the list argument will follow. (Note that there can ever be only
one such list argument.) For instance, splice() has three scalar
arguments followed by a list, whereas gethostbyname() has four scalar
arguments.
In the syntax descriptions that follow, list operators that expect a
list (and provide list context for the elements of the list) are shown
with LIST as an argument. Such a list may consist of any combination
of scalar arguments or list values; the list values will be included in
the list as if each individual element were interpolated at that point
in the list, forming a longer single-dimensional list value. Commas
should separate elements of the LIST.
Any function in the list below may be used either with or without
parentheses around its arguments. (The syntax descriptions omit the
parentheses.) If you use the parentheses, the simple (but occasionally
surprising) rule is this: It looks like a function, therefore it is a
function, and precedence doesn’t matter. Otherwise it’s a list
operator or unary operator, and precedence does matter. And whitespace
between the function and left parenthesis doesn’t count--so you need to
be careful sometimes:
print 1+2+4; # Prints 7.
print(1+2) + 4; # Prints 3.
print (1+2)+4; # Also prints 3!
print +(1+2)+4; # Prints 7.
print ((1+2)+4); # Prints 7.
If you run Perl with the -w switch it can warn you about this. For
example, the third line above produces:
print (...) interpreted as function at - line 1.
Useless use of integer addition in void context at - line 1.
A few functions take no arguments at all, and therefore work as neither
unary nor list operators. These include such functions as "time" and
"endpwent". For example, "time+86_400" always means "time() + 86_400".
For functions that can be used in either a scalar or list context,
nonabortive failure is generally indicated in a scalar context by
returning the undefined value, and in a list context by returning the
null list.
Remember the following important rule: There is no rule that relates
the behavior of an expression in list context to its behavior in scalar
context, or vice versa. It might do two totally different things.
Each operator and function decides which sort of value it would be most
appropriate to return in scalar context. Some operators return the
length of the list that would have been returned in list context. Some
operators return the first value in the list. Some operators return
the last value in the list. Some operators return a count of
successful operations. In general, they do what you want, unless you
want consistency.
A named array in scalar context is quite different from what would at
first glance appear to be a list in scalar context. You can’t get a
list like "(1,2,3)" into being in scalar context, because the compiler
knows the context at compile time. It would generate the scalar comma
operator there, not the list construction version of the comma. That
means it was never a list to start with.
In general, functions in Perl that serve as wrappers for system calls
of the same name (like chown(2), fork(2), closedir(2), etc.) all return
true when they succeed and "undef" otherwise, as is usually mentioned
in the descriptions below. This is different from the C interfaces,
which return "-1" on failure. Exceptions to this rule are "wait",
"waitpid", and "syscall". System calls also set the special $!
variable on failure. Other functions do not, except accidentally.
Perl Functions by Category
Here are Perl’s functions (including things that look like functions,
like some keywords and named operators) arranged by category. Some
functions appear in more than one place.
Functions for SCALARs or strings
"chomp", "chop", "chr", "crypt", "hex", "index", "lc", "lcfirst",
"length", "oct", "ord", "pack", "q//", "qq//", "reverse", "rindex",
"sprintf", "substr", "tr///", "uc", "ucfirst", "y///"
Regular expressions and pattern matching
"m//", "pos", "quotemeta", "s///", "split", "study", "qr//"
Numeric functions
"abs", "atan2", "cos", "exp", "hex", "int", "log", "oct", "rand",
"sin", "sqrt", "srand"
Functions for real @ARRAYs
"pop", "push", "shift", "splice", "unshift"
Functions for list data
"grep", "join", "map", "qw//", "reverse", "sort", "unpack"
Functions for real %HASHes
"delete", "each", "exists", "keys", "values"
Input and output functions
"binmode", "close", "closedir", "dbmclose", "dbmopen", "die",
"eof", "fileno", "flock", "format", "getc", "print", "printf",
"read", "readdir", "rewinddir", "say", "seek", "seekdir", "select",
"syscall", "sysread", "sysseek", "syswrite", "tell", "telldir",
"truncate", "warn", "write"
Functions for fixed length data or records
"pack", "read", "syscall", "sysread", "syswrite", "unpack", "vec"
Functions for filehandles, files, or directories
"-X", "chdir", "chmod", "chown", "chroot", "fcntl", "glob",
"ioctl", "link", "lstat", "mkdir", "open", "opendir", "readlink",
"rename", "rmdir", "stat", "symlink", "sysopen", "umask", "unlink",
"utime"
Keywords related to the control flow of your Perl program
"caller", "continue", "die", "do", "dump", "eval", "exit", "goto",
"last", "next", "redo", "return", "sub", "wantarray"
Keywords related to switch
"break", "continue", "given", "when", "default"
(These are only available if you enable the "switch" feature. See
feature and "Switch statements" in perlsyn.)
Keywords related to scoping
"caller", "import", "local", "my", "our", "state", "package", "use"
("state" is only available if the "state" feature is enabled. See
feature.)
Miscellaneous functions
"defined", "dump", "eval", "formline", "local", "my", "our",
"reset", "scalar", "state", "undef", "wantarray"
Functions for processes and process groups
"alarm", "exec", "fork", "getpgrp", "getppid", "getpriority",
"kill", "pipe", "qx//", "setpgrp", "setpriority", "sleep",
"system", "times", "wait", "waitpid"
Keywords related to perl modules
"do", "import", "no", "package", "require", "use"
Keywords related to classes and object-orientation
"bless", "dbmclose", "dbmopen", "package", "ref", "tie", "tied",
"untie", "use"
Low-level socket functions
"accept", "bind", "connect", "getpeername", "getsockname",
"getsockopt", "listen", "recv", "send", "setsockopt", "shutdown",
"socket", "socketpair"
System V interprocess communication functions
"msgctl", "msgget", "msgrcv", "msgsnd", "semctl", "semget",
"semop", "shmctl", "shmget", "shmread", "shmwrite"
Fetching user and group info
"endgrent", "endhostent", "endnetent", "endpwent", "getgrent",
"getgrgid", "getgrnam", "getlogin", "getpwent", "getpwnam",
"getpwuid", "setgrent", "setpwent"
Fetching network info
"endprotoent", "endservent", "gethostbyaddr", "gethostbyname",
"gethostent", "getnetbyaddr", "getnetbyname", "getnetent",
"getprotobyname", "getprotobynumber", "getprotoent",
"getservbyname", "getservbyport", "getservent", "sethostent",
"setnetent", "setprotoent", "setservent"
Time-related functions
"gmtime", "localtime", "time", "times"
Functions new in perl5
"abs", "bless", "break", "chomp", "chr", "continue", "default",
"exists", "formline", "given", "glob", "import", "lc", "lcfirst",
"lock", "map", "my", "no", "our", "prototype", "qr//", "qw//",
"qx//", "readline", "readpipe", "ref", "sub"*, "sysopen", "tie",
"tied", "uc", "ucfirst", "untie", "use", "when"
* - "sub" was a keyword in perl4, but in perl5 it is an operator,
which can be used in expressions.
Functions obsoleted in perl5
"dbmclose", "dbmopen"
Portability
Perl was born in Unix and can therefore access all common Unix system
calls. In non-Unix environments, the functionality of some Unix system
calls may not be available, or details of the available functionality
may differ slightly. The Perl functions affected by this are:
"-X", "binmode", "chmod", "chown", "chroot", "crypt", "dbmclose",
"dbmopen", "dump", "endgrent", "endhostent", "endnetent",
"endprotoent", "endpwent", "endservent", "exec", "fcntl", "flock",
"fork", "getgrent", "getgrgid", "gethostbyname", "gethostent",
"getlogin", "getnetbyaddr", "getnetbyname", "getnetent", "getppid",
"getpgrp", "getpriority", "getprotobynumber", "getprotoent",
"getpwent", "getpwnam", "getpwuid", "getservbyport", "getservent",
"getsockopt", "glob", "ioctl", "kill", "link", "lstat", "msgctl",
"msgget", "msgrcv", "msgsnd", "open", "pipe", "readlink", "rename",
"select", "semctl", "semget", "semop", "setgrent", "sethostent",
"setnetent", "setpgrp", "setpriority", "setprotoent", "setpwent",
"setservent", "setsockopt", "shmctl", "shmget", "shmread", "shmwrite",
"socket", "socketpair", "stat", "symlink", "syscall", "sysopen",
"system", "times", "truncate", "umask", "unlink", "utime", "wait",
"waitpid"
For more information about the portability of these functions, see
perlport and other available platform-specific documentation.
Alphabetical Listing of Perl Functions
-X FILEHANDLE
-X EXPR
-X DIRHANDLE
-X A file test, where X is one of the letters listed below. This
unary operator takes one argument, either a filename, a
filehandle, or a dirhandle, and tests the associated file to
see if something is true about it. If the argument is omitted,
tests $_, except for "-t", which tests STDIN. Unless otherwise
documented, it returns 1 for true and '' for false, or the
undefined value if the file doesn’t exist. Despite the funny
names, precedence is the same as any other named unary
operator. The operator may be any of:
-r File is readable by effective uid/gid.
-w File is writable by effective uid/gid.
-x File is executable by effective uid/gid.
-o File is owned by effective uid.
-R File is readable by real uid/gid.
-W File is writable by real uid/gid.
-X File is executable by real uid/gid.
-O File is owned by real uid.
-e File exists.
-z File has zero size (is empty).
-s File has nonzero size (returns size in bytes).
-f File is a plain file.
-d File is a directory.
-l File is a symbolic link.
-p File is a named pipe (FIFO), or Filehandle is a pipe.
-S File is a socket.
-b File is a block special file.
-c File is a character special file.
-t Filehandle is opened to a tty.
-u File has setuid bit set.
-g File has setgid bit set.
-k File has sticky bit set.
-T File is an ASCII text file (heuristic guess).
-B File is a "binary" file (opposite of -T).
-M Script start time minus file modification time, in days.
-A Same for access time.
-C Same for inode change time (Unix, may differ for other platforms)
Example:
while (<>) {
chomp;
next unless -f $_; # ignore specials
#...
}
The interpretation of the file permission operators "-r", "-R",
"-w", "-W", "-x", and "-X" is by default based solely on the
mode of the file and the uids and gids of the user. There may
be other reasons you can’t actually read, write, or execute the
file: for example network filesystem access controls, ACLs
(access control lists), read-only filesystems, and unrecognized
executable formats. Note that the use of these six specific
operators to verify if some operation is possible is usually a
mistake, because it may be open to race conditions.
Also note that, for the superuser on the local filesystems, the
"-r", "-R", "-w", and "-W" tests always return 1, and "-x" and
"-X" return 1 if any execute bit is set in the mode. Scripts
run by the superuser may thus need to do a stat() to determine
the actual mode of the file, or temporarily set their effective
uid to something else.
If you are using ACLs, there is a pragma called "filetest" that
may produce more accurate results than the bare stat() mode
bits. When under the "use filetest 'access'" the above-
mentioned filetests will test whether the permission can (not)
be granted using the access() family of system calls. Also
note that the "-x" and "-X" may under this pragma return true
even if there are no execute permission bits set (nor any extra
execute permission ACLs). This strangeness is due to the
underlying system calls’ definitions. Note also that, due to
the implementation of "use filetest 'access'", the "_" special
filehandle won’t cache the results of the file tests when this
pragma is in effect. Read the documentation for the "filetest"
pragma for more information.
Note that "-s/a/b/" does not do a negated substitution. Saying
"-exp($foo)" still works as expected, however--only single
letters following a minus are interpreted as file tests.
The "-T" and "-B" switches work as follows. The first block or
so of the file is examined for odd characters such as strange
control codes or characters with the high bit set. If too many
strange characters (>30%) are found, it’s a "-B" file;
otherwise it’s a "-T" file. Also, any file containing null in
the first block is considered a binary file. If "-T" or "-B"
is used on a filehandle, the current IO buffer is examined
rather than the first block. Both "-T" and "-B" return true on
a null file, or a file at EOF when testing a filehandle.
Because you have to read a file to do the "-T" test, on most
occasions you want to use a "-f" against the file first, as in
"next unless -f $file && -T $file".
If any of the file tests (or either the "stat" or "lstat"
operators) are given the special filehandle consisting of a
solitary underline, then the stat structure of the previous
file test (or stat operator) is used, saving a system call.
(This doesn’t work with "-t", and you need to remember that
lstat() and "-l" will leave values in the stat structure for
the symbolic link, not the real file.) (Also, if the stat
buffer was filled by an "lstat" call, "-T" and "-B" will reset
it with the results of "stat _"). Example:
print "Can do.\n" if -r $a || -w _ || -x _;
stat($filename);
print "Readable\n" if -r _;
print "Writable\n" if -w _;
print "Executable\n" if -x _;
print "Setuid\n" if -u _;
print "Setgid\n" if -g _;
print "Sticky\n" if -k _;
print "Text\n" if -T _;
print "Binary\n" if -B _;
As of Perl 5.9.1, as a form of purely syntactic sugar, you can
stack file test operators, in a way that "-f -w -x $file" is
equivalent to "-x $file && -w _ && -f _". (This is only syntax
fancy: if you use the return value of "-f $file" as an argument
to another filetest operator, no special magic will happen.)
abs VALUE
abs Returns the absolute value of its argument. If VALUE is
omitted, uses $_.
accept NEWSOCKET,GENERICSOCKET
Accepts an incoming socket connect, just as the accept(2)
system call does. Returns the packed address if it succeeded,
false otherwise. See the example in "Sockets: Client/Server
Communication" in perlipc.
On systems that support a close-on-exec flag on files, the flag
will be set for the newly opened file descriptor, as determined
by the value of $^F. See "$^F" in perlvar.
alarm SECONDS
alarm Arranges to have a SIGALRM delivered to this process after the
specified number of wallclock seconds has elapsed. If SECONDS
is not specified, the value stored in $_ is used. (On some
machines, unfortunately, the elapsed time may be up to one
second less or more than you specified because of how seconds
are counted, and process scheduling may delay the delivery of
the signal even further.)
Only one timer may be counting at once. Each call disables the
previous timer, and an argument of 0 may be supplied to cancel
the previous timer without starting a new one. The returned
value is the amount of time remaining on the previous timer.
For delays of finer granularity than one second, the
Time::HiRes module (from CPAN, and starting from Perl 5.8 part
of the standard distribution) provides ualarm(). You may also
use Perl’s four-argument version of select() leaving the first
three arguments undefined, or you might be able to use the
"syscall" interface to access setitimer(2) if your system
supports it. See perlfaq8 for details.
It is usually a mistake to intermix "alarm" and "sleep" calls.
("sleep" may be internally implemented in your system with
"alarm")
If you want to use "alarm" to time out a system call you need
to use an "eval"/"die" pair. You can’t rely on the alarm
causing the system call to fail with $! set to "EINTR" because
Perl sets up signal handlers to restart system calls on some
systems. Using "eval"/"die" always works, modulo the caveats
given in "Signals" in perlipc.
eval {
local $SIG{ALRM} = sub { die "alarm\n" }; # NB: \n required
alarm $timeout;
$nread = sysread SOCKET, $buffer, $size;
alarm 0;
};
if ($@) {
die unless $@ eq "alarm\n"; # propagate unexpected errors
# timed out
}
else {
# didn't
}
For more information see perlipc.
atan2 Y,X
Returns the arctangent of Y/X in the range -PI to PI.
For the tangent operation, you may use the "Math::Trig::tan"
function, or use the familiar relation:
sub tan { sin($_[0]) / cos($_[0]) }
The return value for "atan2(0,0)" is implementation-defined;
consult your atan2(3) manpage for more information.
bind SOCKET,NAME
Binds a network address to a socket, just as the bind system
call does. Returns true if it succeeded, false otherwise.
NAME should be a packed address of the appropriate type for the
socket. See the examples in "Sockets: Client/Server
Communication" in perlipc.
binmode FILEHANDLE, LAYER
binmode FILEHANDLE
Arranges for FILEHANDLE to be read or written in "binary" or
"text" mode on systems where the run-time libraries distinguish
between binary and text files. If FILEHANDLE is an expression,
the value is taken as the name of the filehandle. Returns true
on success, otherwise it returns "undef" and sets $! (errno).
On some systems (in general, DOS and Windows-based systems)
binmode() is necessary when you’re not working with a text
file. For the sake of portability it is a good idea to always
use it when appropriate, and to never use it when it isn’t
appropriate. Also, people can set their I/O to be by default
UTF-8 encoded Unicode, not bytes.
In other words: regardless of platform, use binmode() on binary
data, like for example images.
If LAYER is present it is a single string, but may contain
multiple directives. The directives alter the behaviour of the
file handle. When LAYER is present using binmode on a text
file makes sense.
If LAYER is omitted or specified as ":raw" the filehandle is
made suitable for passing binary data. This includes turning
off possible CRLF translation and marking it as bytes (as
opposed to Unicode characters). Note that, despite what may be
implied in "Programming Perl" (the Camel) or elsewhere, ":raw"
is not simply the inverse of ":crlf" -- other layers which
would affect the binary nature of the stream are also disabled.
See PerlIO, perlrun and the discussion about the PERLIO
environment variable.
The ":bytes", ":crlf", and ":utf8", and any other directives of
the form ":...", are called I/O layers. The "open" pragma can
be used to establish default I/O layers. See open.
The LAYER parameter of the binmode() function is described as
"DISCIPLINE" in "Programming Perl, 3rd Edition". However,
since the publishing of this book, by many known as "Camel
III", the consensus of the naming of this functionality has
moved from "discipline" to "layer". All documentation of this
version of Perl therefore refers to "layers" rather than to
"disciplines". Now back to the regularly scheduled
documentation...
To mark FILEHANDLE as UTF-8, use ":utf8" or ":encoding(utf8)".
":utf8" just marks the data as UTF-8 without further checking,
while ":encoding(utf8)" checks the data for actually being
valid UTF-8. More details can be found in PerlIO::encoding.
In general, binmode() should be called after open() but before
any I/O is done on the filehandle. Calling binmode() will
normally flush any pending buffered output data (and perhaps
pending input data) on the handle. An exception to this is the
":encoding" layer that changes the default character encoding
of the handle, see open. The ":encoding" layer sometimes needs
to be called in mid-stream, and it doesn’t flush the stream.
The ":encoding" also implicitly pushes on top of itself the
":utf8" layer because internally Perl will operate on UTF-8
encoded Unicode characters.
The operating system, device drivers, C libraries, and Perl
run-time system all work together to let the programmer treat a
single character ("\n") as the line terminator, irrespective of
the external representation. On many operating systems, the
native text file representation matches the internal
representation, but on some platforms the external
representation of "\n" is made up of more than one character.
Mac OS, all variants of Unix, and Stream_LF files on VMS use a
single character to end each line in the external
representation of text (even though that single character is
CARRIAGE RETURN on Mac OS and LINE FEED on Unix and most VMS
files). In other systems like OS/2, DOS and the various flavors
of MS-Windows your program sees a "\n" as a simple "\cJ", but
what’s stored in text files are the two characters "\cM\cJ".
That means that, if you don’t use binmode() on these systems,
"\cM\cJ" sequences on disk will be converted to "\n" on input,
and any "\n" in your program will be converted back to "\cM\cJ"
on output. This is what you want for text files, but it can be
disastrous for binary files.
Another consequence of using binmode() (on some systems) is
that special end-of-file markers will be seen as part of the
data stream. For systems from the Microsoft family this means
that if your binary data contains "\cZ", the I/O subsystem will
regard it as the end of the file, unless you use binmode().
binmode() is not only important for readline() and print()
operations, but also when using read(), seek(), sysread(),
syswrite() and tell() (see perlport for more details). See the
$/ and "$\" variables in perlvar for how to manually set your
input and output line-termination sequences.
bless REF,CLASSNAME
bless REF
This function tells the thingy referenced by REF that it is now
an object in the CLASSNAME package. If CLASSNAME is omitted,
the current package is used. Because a "bless" is often the
last thing in a constructor, it returns the reference for
convenience. Always use the two-argument version if a derived
class might inherit the function doing the blessing. See
perltoot and perlobj for more about the blessing (and
blessings) of objects.
Consider always blessing objects in CLASSNAMEs that are mixed
case. Namespaces with all lowercase names are considered
reserved for Perl pragmata. Builtin types have all uppercase
names. To prevent confusion, you may wish to avoid such package
names as well. Make sure that CLASSNAME is a true value.
See "Perl Modules" in perlmod.
break Break out of a "given()" block.
This keyword is enabled by the "switch" feature: see feature
for more information.
caller EXPR
caller Returns the context of the current subroutine call. In scalar
context, returns the caller’s package name if there is a
caller, that is, if we’re in a subroutine or "eval" or
"require", and the undefined value otherwise. In list context,
returns
# 0 1 2
($package, $filename, $line) = caller;
With EXPR, it returns some extra information that the debugger
uses to print a stack trace. The value of EXPR indicates how
many call frames to go back before the current one.
# 0 1 2 3 4
($package, $filename, $line, $subroutine, $hasargs,
# 5 6 7 8 9 10
$wantarray, $evaltext, $is_require, $hints, $bitmask, $hinthash)
= caller($i);
Here $subroutine may be "(eval)" if the frame is not a
subroutine call, but an "eval". In such a case additional
elements $evaltext and $is_require are set: $is_require is true
if the frame is created by a "require" or "use" statement,
$evaltext contains the text of the "eval EXPR" statement. In
particular, for an "eval BLOCK" statement, $subroutine is
"(eval)", but $evaltext is undefined. (Note also that each
"use" statement creates a "require" frame inside an "eval EXPR"
frame.) $subroutine may also be "(unknown)" if this particular
subroutine happens to have been deleted from the symbol table.
$hasargs is true if a new instance of @_ was set up for the
frame. $hints and $bitmask contain pragmatic hints that the
caller was compiled with. The $hints and $bitmask values are
subject to change between versions of Perl, and are not meant
for external use.
$hinthash is a reference to a hash containing the value of
"%^H" when the caller was compiled, or "undef" if "%^H" was
empty. Do not modify the values of this hash, as they are the
actual values stored in the optree.
Furthermore, when called from within the DB package, caller
returns more detailed information: it sets the list variable
@DB::args to be the arguments with which the subroutine was
invoked.
Be aware that the optimizer might have optimized call frames
away before "caller" had a chance to get the information. That
means that caller(N) might not return information about the
call frame you expect it do, for "N > 1". In particular,
@DB::args might have information from the previous time
"caller" was called.
chdir EXPR
chdir FILEHANDLE
chdir DIRHANDLE
chdir Changes the working directory to EXPR, if possible. If EXPR is
omitted, changes to the directory specified by $ENV{HOME}, if
set; if not, changes to the directory specified by
$ENV{LOGDIR}. (Under VMS, the variable $ENV{SYS$LOGIN} is also
checked, and used if it is set.) If neither is set, "chdir"
does nothing. It returns true upon success, false otherwise.
See the example under "die".
On systems that support fchdir, you might pass a file handle or
directory handle as argument. On systems that don’t support
fchdir, passing handles produces a fatal error at run time.
chmod LIST
Changes the permissions of a list of files. The first element
of the list must be the numerical mode, which should probably
be an octal number, and which definitely should not be a string
of octal digits: 0644 is okay, '0644' is not. Returns the
number of files successfully changed. See also "oct", if all
you have is a string.
$cnt = chmod 0755, 'foo', 'bar';
chmod 0755, @executables;
$mode = '0644'; chmod $mode, 'foo'; # !!! sets mode to
# --w----r-T
$mode = '0644'; chmod oct($mode), 'foo'; # this is better
$mode = 0644; chmod $mode, 'foo'; # this is best
On systems that support fchmod, you might pass file handles
among the files. On systems that don’t support fchmod, passing
file handles produces a fatal error at run time. The file
handles must be passed as globs or references to be recognized.
Barewords are considered file names.
open(my $fh, "<", "foo");
my $perm = (stat $fh)[2] & 07777;
chmod($perm | 0600, $fh);
You can also import the symbolic "S_I*" constants from the
Fcntl module:
use Fcntl ':mode';
chmod S_IRWXU|S_IRGRP|S_IXGRP|S_IROTH|S_IXOTH, @executables;
# This is identical to the chmod 0755 of the above example.
chomp VARIABLE
chomp( LIST )
chomp This safer version of "chop" removes any trailing string that
corresponds to the current value of $/ (also known as
$INPUT_RECORD_SEPARATOR in the "English" module). It returns
the total number of characters removed from all its arguments.
It’s often used to remove the newline from the end of an input
record when you’re worried that the final record may be missing
its newline. When in paragraph mode ("$/ = """), it removes
all trailing newlines from the string. When in slurp mode ("$/
= undef") or fixed-length record mode ($/ is a reference to an
integer or the like, see perlvar) chomp() won’t remove
anything. If VARIABLE is omitted, it chomps $_. Example:
while (<>) {
chomp; # avoid \n on last field
@array = split(/:/);
# ...
}
If VARIABLE is a hash, it chomps the hash’s values, but not its
keys.
You can actually chomp anything that’s an lvalue, including an
assignment:
chomp($cwd = `pwd`);
chomp($answer = <STDIN>);
If you chomp a list, each element is chomped, and the total
number of characters removed is returned.
Note that parentheses are necessary when you’re chomping
anything that is not a simple variable. This is because "chomp
$cwd = `pwd`;" is interpreted as "(chomp $cwd) = `pwd`;",
rather than as "chomp( $cwd = `pwd` )" which you might expect.
Similarly, "chomp $a, $b" is interpreted as "chomp($a), $b"
rather than as "chomp($a, $b)".
chop VARIABLE
chop( LIST )
chop Chops off the last character of a string and returns the
character chopped. It is much more efficient than "s/.$//s"
because it neither scans nor copies the string. If VARIABLE is
omitted, chops $_. If VARIABLE is a hash, it chops the hash’s
values, but not its keys.
You can actually chop anything that’s an lvalue, including an
assignment.
If you chop a list, each element is chopped. Only the value of
the last "chop" is returned.
Note that "chop" returns the last character. To return all but
the last character, use "substr($string, 0, -1)".
See also "chomp".
chown LIST
Changes the owner (and group) of a list of files. The first
two elements of the list must be the numeric uid and gid, in
that order. A value of -1 in either position is interpreted by
most systems to leave that value unchanged. Returns the number
of files successfully changed.
$cnt = chown $uid, $gid, 'foo', 'bar';
chown $uid, $gid, @filenames;
On systems that support fchown, you might pass file handles
among the files. On systems that don’t support fchown, passing
file handles produces a fatal error at run time. The file
handles must be passed as globs or references to be recognized.
Barewords are considered file names.
Here’s an example that looks up nonnumeric uids in the passwd
file:
print "User: ";
chomp($user = <STDIN>);
print "Files: ";
chomp($pattern = <STDIN>);
($login,$pass,$uid,$gid) = getpwnam($user)
or die "$user not in passwd file";
@ary = glob($pattern); # expand filenames
chown $uid, $gid, @ary;
On most systems, you are not allowed to change the ownership of
the file unless you’re the superuser, although you should be
able to change the group to any of your secondary groups. On
insecure systems, these restrictions may be relaxed, but this
is not a portable assumption. On POSIX systems, you can detect
this condition this way:
use POSIX qw(sysconf _PC_CHOWN_RESTRICTED);
$can_chown_giveaway = not sysconf(_PC_CHOWN_RESTRICTED);
chr NUMBER
chr Returns the character represented by that NUMBER in the
character set. For example, "chr(65)" is "A" in either ASCII
or Unicode, and chr(0x263a) is a Unicode smiley face.
Negative values give the Unicode replacement character
(chr(0xfffd)), except under the bytes pragma, where low eight
bits of the value (truncated to an integer) are used.
If NUMBER is omitted, uses $_.
For the reverse, use "ord".
Note that characters from 128 to 255 (inclusive) are by default
internally not encoded as UTF-8 for backward compatibility
reasons.
See perlunicode for more about Unicode.
chroot FILENAME
chroot This function works like the system call by the same name: it
makes the named directory the new root directory for all
further pathnames that begin with a "/" by your process and all
its children. (It doesn’t change your current working
directory, which is unaffected.) For security reasons, this
call is restricted to the superuser. If FILENAME is omitted,
does a "chroot" to $_.
close FILEHANDLE
close Closes the file or pipe associated with the file handle,
flushes the IO buffers, and closes the system file descriptor.
Returns true if those operations have succeeded and if no error
was reported by any PerlIO layer. Closes the currently
selected filehandle if the argument is omitted.
You don’t have to close FILEHANDLE if you are immediately going
to do another "open" on it, because "open" will close it for
you. (See "open".) However, an explicit "close" on an input
file resets the line counter ($.), while the implicit close
done by "open" does not.
If the file handle came from a piped open, "close" will
additionally return false if one of the other system calls
involved fails, or if the program exits with non-zero status.
(If the only problem was that the program exited non-zero, $!
will be set to 0.) Closing a pipe also waits for the process
executing on the pipe to complete, in case you want to look at
the output of the pipe afterwards, and implicitly puts the exit
status value of that command into $? and
"${^CHILD_ERROR_NATIVE}".
Prematurely closing the read end of a pipe (i.e. before the
process writing to it at the other end has closed it) will
result in a SIGPIPE being delivered to the writer. If the
other end can’t handle that, be sure to read all the data
before closing the pipe.
Example:
open(OUTPUT, '|sort >foo') # pipe to sort
or die "Can't start sort: $!";
#... # print stuff to output
close OUTPUT # wait for sort to finish
or warn $! ? "Error closing sort pipe: $!"
: "Exit status $? from sort";
open(INPUT, 'foo') # get sort's results
or die "Can't open 'foo' for input: $!";
FILEHANDLE may be an expression whose value can be used as an
indirect filehandle, usually the real filehandle name.
closedir DIRHANDLE
Closes a directory opened by "opendir" and returns the success
of that system call.
connect SOCKET,NAME
Attempts to connect to a remote socket, just as the connect
system call does. Returns true if it succeeded, false
otherwise. NAME should be a packed address of the appropriate
type for the socket. See the examples in "Sockets:
Client/Server Communication" in perlipc.
continue BLOCK
continue
"continue" is actually a flow control statement rather than a
function. If there is a "continue" BLOCK attached to a BLOCK
(typically in a "while" or "foreach"), it is always executed
just before the conditional is about to be evaluated again,
just like the third part of a "for" loop in C. Thus it can be
used to increment a loop variable, even when the loop has been
continued via the "next" statement (which is similar to the C
"continue" statement).
"last", "next", or "redo" may appear within a "continue" block.
"last" and "redo" will behave as if they had been executed
within the main block. So will "next", but since it will
execute a "continue" block, it may be more entertaining.
while (EXPR) {
### redo always comes here
do_something;
} continue {
### next always comes here
do_something_else;
# then back the top to re-check EXPR
}
### last always comes here
Omitting the "continue" section is semantically equivalent to
using an empty one, logically enough. In that case, "next"
goes directly back to check the condition at the top of the
loop.
If the "switch" feature is enabled, "continue" is also a
function that will break out of the current "when" or "default"
block, and fall through to the next case. See feature and
"Switch statements" in perlsyn for more information.
cos EXPR
cos Returns the cosine of EXPR (expressed in radians). If EXPR is
omitted, takes cosine of $_.
For the inverse cosine operation, you may use the
"Math::Trig::acos()" function, or use this relation:
sub acos { atan2( sqrt(1 - $_[0] * $_[0]), $_[0] ) }
crypt PLAINTEXT,SALT
Creates a digest string exactly like the crypt(3) function in
the C library (assuming that you actually have a version there
that has not been extirpated as a potential munition).
crypt() is a one-way hash function. The PLAINTEXT and SALT is
turned into a short string, called a digest, which is returned.
The same PLAINTEXT and SALT will always return the same string,
but there is no (known) way to get the original PLAINTEXT from
the hash. Small changes in the PLAINTEXT or SALT will result
in large changes in the digest.
There is no decrypt function. This function isn’t all that
useful for cryptography (for that, look for Crypt modules on
your nearby CPAN mirror) and the name "crypt" is a bit of a
misnomer. Instead it is primarily used to check if two pieces
of text are the same without having to transmit or store the
text itself. An example is checking if a correct password is
given. The digest of the password is stored, not the password
itself. The user types in a password that is crypt()’d with
the same salt as the stored digest. If the two digests match
the password is correct.
When verifying an existing digest string you should use the
digest as the salt (like "crypt($plain, $digest) eq $digest").
The SALT used to create the digest is visible as part of the
digest. This ensures crypt() will hash the new string with the
same salt as the digest. This allows your code to work with
the standard crypt and with more exotic implementations. In
other words, do not assume anything about the returned string
itself, or how many bytes in the digest matter.
Traditionally the result is a string of 13 bytes: two first
bytes of the salt, followed by 11 bytes from the set
"[./0-9A-Za-z]", and only the first eight bytes of PLAINTEXT
mattered. But alternative hashing schemes (like MD5), higher
level security schemes (like C2), and implementations on non-
UNIX platforms may produce different strings.
When choosing a new salt create a random two character string
whose characters come from the set "[./0-9A-Za-z]" (like "join
'', ('.', '/', 0..9, 'A'..'Z', 'a'..'z')[rand 64, rand 64]").
This set of characters is just a recommendation; the characters
allowed in the salt depend solely on your system’s crypt
library, and Perl can’t restrict what salts "crypt()" accepts.
Here’s an example that makes sure that whoever runs this
program knows their password:
$pwd = (getpwuid($<))[1];
system "stty -echo";
print "Password: ";
chomp($word = <STDIN>);
print "\n";
system "stty echo";
if (crypt($word, $pwd) ne $pwd) {
die "Sorry...\n";
} else {
print "ok\n";
}
Of course, typing in your own password to whoever asks you for
it is unwise.
The crypt function is unsuitable for hashing large quantities
of data, not least of all because you can’t get the information
back. Look at the Digest module for more robust algorithms.
If using crypt() on a Unicode string (which potentially has
characters with codepoints above 255), Perl tries to make sense
of the situation by trying to downgrade (a copy of the string)
the string back to an eight-bit byte string before calling
crypt() (on that copy). If that works, good. If not, crypt()
dies with "Wide character in crypt".
dbmclose HASH
[This function has been largely superseded by the "untie"
function.]
Breaks the binding between a DBM file and a hash.
dbmopen HASH,DBNAME,MASK
[This function has been largely superseded by the "tie"
function.]
This binds a dbm(3), ndbm(3), sdbm(3), gdbm(3), or Berkeley DB
file to a hash. HASH is the name of the hash. (Unlike normal
"open", the first argument is not a filehandle, even though it
looks like one). DBNAME is the name of the database (without
the .dir or .pag extension if any). If the database does not
exist, it is created with protection specified by MASK (as
modified by the "umask"). If your system supports only the
older DBM functions, you may perform only one "dbmopen" in your
program. In older versions of Perl, if your system had neither
DBM nor ndbm, calling "dbmopen" produced a fatal error; it now
falls back to sdbm(3).
If you don’t have write access to the DBM file, you can only
read hash variables, not set them. If you want to test whether
you can write, either use file tests or try setting a dummy
hash entry inside an "eval", which will trap the error.
Note that functions such as "keys" and "values" may return huge
lists when used on large DBM files. You may prefer to use the
"each" function to iterate over large DBM files. Example:
# print out history file offsets
dbmopen(%HIST,'/usr/lib/news/history',0666);
while (($key,$val) = each %HIST) {
print $key, ' = ', unpack('L',$val), "\n";
}
dbmclose(%HIST);
See also AnyDBM_File for a more general description of the pros
and cons of the various dbm approaches, as well as DB_File for
a particularly rich implementation.
You can control which DBM library you use by loading that
library before you call dbmopen():
use DB_File;
dbmopen(%NS_Hist, "$ENV{HOME}/.netscape/history.db")
or die "Can't open netscape history file: $!";
defined EXPR
defined Returns a Boolean value telling whether EXPR has a value other
than the undefined value "undef". If EXPR is not present, $_
will be checked.
Many operations return "undef" to indicate failure, end of
file, system error, uninitialized variable, and other
exceptional conditions. This function allows you to
distinguish "undef" from other values. (A simple Boolean test
will not distinguish among "undef", zero, the empty string, and
"0", which are all equally false.) Note that since "undef" is
a valid scalar, its presence doesn’t necessarily indicate an
exceptional condition: "pop" returns "undef" when its argument
is an empty array, or when the element to return happens to be
"undef".
You may also use "defined(&func)" to check whether subroutine
&func has ever been defined. The return value is unaffected by
any forward declarations of &func. Note that a subroutine
which is not defined may still be callable: its package may
have an "AUTOLOAD" method that makes it spring into existence
the first time that it is called -- see perlsub.
Use of "defined" on aggregates (hashes and arrays) is
deprecated. It used to report whether memory for that
aggregate has ever been allocated. This behavior may disappear
in future versions of Perl. You should instead use a simple
test for size:
if (@an_array) { print "has array elements\n" }
if (%a_hash) { print "has hash members\n" }
When used on a hash element, it tells you whether the value is
defined, not whether the key exists in the hash. Use "exists"
for the latter purpose.
Examples:
print if defined $switch{'D'};
print "$val\n" while defined($val = pop(@ary));
die "Can't readlink $sym: $!"
unless defined($value = readlink $sym);
sub foo { defined &$bar ? &$bar(@_) : die "No bar"; }
$debugging = 0 unless defined $debugging;
Note: Many folks tend to overuse "defined", and then are
surprised to discover that the number 0 and "" (the zero-length
string) are, in fact, defined values. For example, if you say
"ab" =~ /a(.*)b/;
The pattern match succeeds, and $1 is defined, despite the fact
that it matched "nothing". It didn’t really fail to match
anything. Rather, it matched something that happened to be
zero characters long. This is all very above-board and honest.
When a function returns an undefined value, it’s an admission
that it couldn’t give you an honest answer. So you should use
"defined" only when you’re questioning the integrity of what
you’re trying to do. At other times, a simple comparison to 0
or "" is what you want.
See also "undef", "exists", "ref".
delete EXPR
Given an expression that specifies a hash element, array
element, hash slice, or array slice, deletes the specified
element(s) from the hash or array. In the case of an array, if
the array elements happen to be at the end, the size of the
array will shrink to the highest element that tests true for
exists() (or 0 if no such element exists).
Returns a list with the same number of elements as the number
of elements for which deletion was attempted. Each element of
that list consists of either the value of the element deleted,
or the undefined value. In scalar context, this means that you
get the value of the last element deleted (or the undefined
value if that element did not exist).
%hash = (foo => 11, bar => 22, baz => 33);
$scalar = delete $hash{foo}; # $scalar is 11
$scalar = delete @hash{qw(foo bar)}; # $scalar is 22
@array = delete @hash{qw(foo bar baz)}; # @array is (undef,undef,33)
Deleting from %ENV modifies the environment. Deleting from a
hash tied to a DBM file deletes the entry from the DBM file.
Deleting from a "tie"d hash or array may not necessarily return
anything.
Deleting an array element effectively returns that position of
the array to its initial, uninitialized state. Subsequently
testing for the same element with exists() will return false.
Also, deleting array elements in the middle of an array will
not shift the index of the elements after them down. Use
splice() for that. See "exists".
The following (inefficiently) deletes all the values of %HASH
and @ARRAY:
foreach $key (keys %HASH) {
delete $HASH{$key};
}
foreach $index (0 .. $#ARRAY) {
delete $ARRAY[$index];
}
And so do these:
delete @HASH{keys %HASH};
delete @ARRAY[0 .. $#ARRAY];
But both of these are slower than just assigning the empty list
or undefining %HASH or @ARRAY:
%HASH = (); # completely empty %HASH
undef %HASH; # forget %HASH ever existed
@ARRAY = (); # completely empty @ARRAY
undef @ARRAY; # forget @ARRAY ever existed
Note that the EXPR can be arbitrarily complicated as long as
the final operation is a hash element, array element, hash
slice, or array slice lookup:
delete $ref->[$x][$y]{$key};
delete @{$ref->[$x][$y]}{$key1, $key2, @morekeys};
delete $ref->[$x][$y][$index];
delete @{$ref->[$x][$y]}[$index1, $index2, @moreindices];
die LIST
Outside an "eval", prints the value of LIST to "STDERR" and
exits with the current value of $! (errno). If $! is 0, exits
with the value of "($? >> 8)" (backtick ‘command‘ status). If
"($? >> 8)" is 0, exits with 255. Inside an "eval()," the
error message is stuffed into $@ and the "eval" is terminated
with the undefined value. This makes "die" the way to raise an
exception.
Equivalent examples:
die "Can't cd to spool: $!\n" unless chdir '/usr/spool/news';
chdir '/usr/spool/news' or die "Can't cd to spool: $!\n"
If the last element of LIST does not end in a newline, the
current script line number and input line number (if any) are
also printed, and a newline is supplied. Note that the "input
line number" (also known as "chunk") is subject to whatever
notion of "line" happens to be currently in effect, and is also
available as the special variable $.. See "$/" in perlvar and
"$." in perlvar.
Hint: sometimes appending ", stopped" to your message will
cause it to make better sense when the string "at foo line 123"
is appended. Suppose you are running script "canasta".
die "/etc/games is no good";
die "/etc/games is no good, stopped";
produce, respectively
/etc/games is no good at canasta line 123.
/etc/games is no good, stopped at canasta line 123.
See also exit(), warn(), and the Carp module.
If LIST is empty and $@ already contains a value (typically
from a previous eval) that value is reused after appending
"\t...propagated". This is useful for propagating exceptions:
eval { ... };
die unless $@ =~ /Expected exception/;
If LIST is empty and $@ contains an object reference that has a
"PROPAGATE" method, that method will be called with additional
file and line number parameters. The return value replaces the
value in $@. i.e. as if "$@ = eval { $@->PROPAGATE(__FILE__,
__LINE__) };" were called.
If $@ is empty then the string "Died" is used.
die() can also be called with a reference argument. If this
happens to be trapped within an eval(), $@ contains the
reference. This behavior permits a more elaborate exception
handling implementation using objects that maintain arbitrary
state about the nature of the exception. Such a scheme is
sometimes preferable to matching particular string values of $@
using regular expressions. Because $@ is a global variable,
and eval() may be used within object implementations, care must
be taken that analyzing the error object doesn’t replace the
reference in the global variable. The easiest solution is to
make a local copy of the reference before doing other
manipulations. Here’s an example:
use Scalar::Util 'blessed';
eval { ... ; die Some::Module::Exception->new( FOO => "bar" ) };
if (my $ev_err = $@) {
if (blessed($ev_err) && $ev_err->isa("Some::Module::Exception")) {
# handle Some::Module::Exception
}
else {
# handle all other possible exceptions
}
}
Because perl will stringify uncaught exception messages before
displaying them, you may want to overload stringification
operations on such custom exception objects. See overload for
details about that.
You can arrange for a callback to be run just before the "die"
does its deed, by setting the $SIG{__DIE__} hook. The
associated handler will be called with the error text and can
change the error message, if it sees fit, by calling "die"
again. See "$SIG{expr}" in perlvar for details on setting %SIG
entries, and "eval BLOCK" for some examples. Although this
feature was to be run only right before your program was to
exit, this is not currently the case--the $SIG{__DIE__} hook is
currently called even inside eval()ed blocks/strings! If one
wants the hook to do nothing in such situations, put
die @_ if $^S;
as the first line of the handler (see "$^S" in perlvar).
Because this promotes strange action at a distance, this
counterintuitive behavior may be fixed in a future release.
do BLOCK
Not really a function. Returns the value of the last command
in the sequence of commands indicated by BLOCK. When modified
by the "while" or "until" loop modifier, executes the BLOCK
once before testing the loop condition. (On other statements
the loop modifiers test the conditional first.)
"do BLOCK" does not count as a loop, so the loop control
statements "next", "last", or "redo" cannot be used to leave or
restart the block. See perlsyn for alternative strategies.
do SUBROUTINE(LIST)
This form of subroutine call is deprecated. See perlsub.
do EXPR Uses the value of EXPR as a filename and executes the contents
of the file as a Perl script.
do 'stat.pl';
is just like
eval `cat stat.pl`;
except that it’s more efficient and concise, keeps track of the
current filename for error messages, searches the @INC
directories, and updates %INC if the file is found. See
"Predefined Names" in perlvar for these variables. It also
differs in that code evaluated with "do FILENAME" cannot see
lexicals in the enclosing scope; "eval STRING" does. It’s the
same, however, in that it does reparse the file every time you
call it, so you probably don’t want to do this inside a loop.
If "do" cannot read the file, it returns undef and sets $! to
the error. If "do" can read the file but cannot compile it, it
returns undef and sets an error message in $@. If the file is
successfully compiled, "do" returns the value of the last
expression evaluated.
Note that inclusion of library modules is better done with the
"use" and "require" operators, which also do automatic error
checking and raise an exception if there’s a problem.
You might like to use "do" to read in a program configuration
file. Manual error checking can be done this way:
# read in config files: system first, then user
for $file ("/share/prog/defaults.rc",
"$ENV{HOME}/.someprogrc")
{
unless ($return = do $file) {
warn "couldn't parse $file: $@" if $@;
warn "couldn't do $file: $!" unless defined $return;
warn "couldn't run $file" unless $return;
}
}
dump LABEL
dump This function causes an immediate core dump. See also the -u
command-line switch in perlrun, which does the same thing.
Primarily this is so that you can use the undump program (not
supplied) to turn your core dump into an executable binary
after having initialized all your variables at the beginning of
the program. When the new binary is executed it will begin by
executing a "goto LABEL" (with all the restrictions that "goto"
suffers). Think of it as a goto with an intervening core dump
and reincarnation. If "LABEL" is omitted, restarts the program
from the top.
WARNING: Any files opened at the time of the dump will not be
open any more when the program is reincarnated, with possible
resulting confusion on the part of Perl.
This function is now largely obsolete, mostly because it’s very
hard to convert a core file into an executable. That’s why you
should now invoke it as "CORE::dump()", if you don’t want to be
warned against a possible typo.
each HASH
When called in list context, returns a 2-element list
consisting of the key and value for the next element of a hash,
so that you can iterate over it. When called in scalar
context, returns only the key for the next element in the hash.
Entries are returned in an apparently random order. The actual
random order is subject to change in future versions of perl,
but it is guaranteed to be in the same order as either the
"keys" or "values" function would produce on the same
(unmodified) hash. Since Perl 5.8.2 the ordering can be
different even between different runs of Perl for security
reasons (see "Algorithmic Complexity Attacks" in perlsec).
When the hash is entirely read, a null array is returned in
list context (which when assigned produces a false (0) value),
and "undef" in scalar context. The next call to "each" after
that will start iterating again. There is a single iterator
for each hash, shared by all "each", "keys", and "values"
function calls in the program; it can be reset by reading all
the elements from the hash, or by evaluating "keys HASH" or
"values HASH". If you add or delete elements of a hash while
you’re iterating over it, you may get entries skipped or
duplicated, so don’t. Exception: It is always safe to delete
the item most recently returned by "each()", which means that
the following code will work:
while (($key, $value) = each %hash) {
print $key, "\n";
delete $hash{$key}; # This is safe
}
The following prints out your environment like the printenv(1)
program, only in a different order:
while (($key,$value) = each %ENV) {
print "$key=$value\n";
}
See also "keys", "values" and "sort".
eof FILEHANDLE
eof ()
eof Returns 1 if the next read on FILEHANDLE will return end of
file, or if FILEHANDLE is not open. FILEHANDLE may be an
expression whose value gives the real filehandle. (Note that
this function actually reads a character and then "ungetc"s it,
so isn’t very useful in an interactive context.) Do not read
from a terminal file (or call "eof(FILEHANDLE)" on it) after
end-of-file is reached. File types such as terminals may lose
the end-of-file condition if you do.
An "eof" without an argument uses the last file read. Using
"eof()" with empty parentheses is very different. It refers to
the pseudo file formed from the files listed on the command
line and accessed via the "<>" operator. Since "<>" isn’t
explicitly opened, as a normal filehandle is, an "eof()" before
"<>" has been used will cause @ARGV to be examined to determine
if input is available. Similarly, an "eof()" after "<>" has
returned end-of-file will assume you are processing another
@ARGV list, and if you haven’t set @ARGV, will read input from
"STDIN"; see "I/O Operators" in perlop.
In a "while (<>)" loop, "eof" or "eof(ARGV)" can be used to
detect the end of each file, "eof()" will only detect the end
of the last file. Examples:
# reset line numbering on each input file
while (<>) {
next if /^\s*#/; # skip comments
print "$.\t$_";
} continue {
close ARGV if eof; # Not eof()!
}
# insert dashes just before last line of last file
while (<>) {
if (eof()) { # check for end of last file
print "--------------\n";
}
print;
last if eof(); # needed if we're reading from a terminal
}
Practical hint: you almost never need to use "eof" in Perl,
because the input operators typically return "undef" when they
run out of data, or if there was an error.
eval EXPR
eval BLOCK
eval In the first form, the return value of EXPR is parsed and
executed as if it were a little Perl program. The value of the
expression (which is itself determined within scalar context)
is first parsed, and if there weren’t any errors, executed in
the lexical context of the current Perl program, so that any
variable settings or subroutine and format definitions remain
afterwards. Note that the value is parsed every time the
"eval" executes. If EXPR is omitted, evaluates $_. This form
is typically used to delay parsing and subsequent execution of
the text of EXPR until run time.
In the second form, the code within the BLOCK is parsed only
once--at the same time the code surrounding the "eval" itself
was parsed--and executed within the context of the current Perl
program. This form is typically used to trap exceptions more
efficiently than the first (see below), while also providing
the benefit of checking the code within BLOCK at compile time.
The final semicolon, if any, may be omitted from the value of
EXPR or within the BLOCK.
In both forms, the value returned is the value of the last
expression evaluated inside the mini-program; a return
statement may be also used, just as with subroutines. The
expression providing the return value is evaluated in void,
scalar, or list context, depending on the context of the "eval"
itself. See "wantarray" for more on how the evaluation context
can be determined.
If there is a syntax error or runtime error, or a "die"
statement is executed, "eval" returns an undefined value in
scalar context or an empty list in list context, and $@ is set
to the error message. If there was no error, $@ is guaranteed
to be a null string. Beware that using "eval" neither silences
perl from printing warnings to STDERR, nor does it stuff the
text of warning messages into $@. To do either of those, you
have to use the $SIG{__WARN__} facility, or turn off warnings
inside the BLOCK or EXPR using "no warnings 'all'". See
"warn", perlvar, warnings and perllexwarn.
Note that, because "eval" traps otherwise-fatal errors, it is
useful for determining whether a particular feature (such as
"socket" or "symlink") is implemented. It is also Perl’s
exception trapping mechanism, where the die operator is used to
raise exceptions.
If you want to trap errors when loading an XS module, some
problems with the binary interface (such as Perl version skew)
may be fatal even with "eval" unless $ENV{PERL_DL_NONLAZY} is
set. See perlrun.
If the code to be executed doesn’t vary, you may use the eval-
BLOCK form to trap run-time errors without incurring the
penalty of recompiling each time. The error, if any, is still
returned in $@. Examples:
# make divide-by-zero nonfatal
eval { $answer = $a / $b; }; warn $@ if $@;
# same thing, but less efficient
eval '$answer = $a / $b'; warn $@ if $@;
# a compile-time error
eval { $answer = }; # WRONG
# a run-time error
eval '$answer ='; # sets $@
Using the "eval{}" form as an exception trap in libraries does
have some issues. Due to the current arguably broken state of
"__DIE__" hooks, you may wish not to trigger any "__DIE__"
hooks that user code may have installed. You can use the
"local $SIG{__DIE__}" construct for this purpose, as shown in
this example:
# a very private exception trap for divide-by-zero
eval { local $SIG{'__DIE__'}; $answer = $a / $b; };
warn $@ if $@;
This is especially significant, given that "__DIE__" hooks can
call "die" again, which has the effect of changing their error
messages:
# __DIE__ hooks may modify error messages
{
local $SIG{'__DIE__'} =
sub { (my $x = $_[0]) =~ s/foo/bar/g; die $x };
eval { die "foo lives here" };
print $@ if $@; # prints "bar lives here"
}
Because this promotes action at a distance, this
counterintuitive behavior may be fixed in a future release.
With an "eval", you should be especially careful to remember
what’s being looked at when:
eval $x; # CASE 1
eval "$x"; # CASE 2
eval '$x'; # CASE 3
eval { $x }; # CASE 4
eval "\$$x++"; # CASE 5
$$x++; # CASE 6
Cases 1 and 2 above behave identically: they run the code
contained in the variable $x. (Although case 2 has misleading
double quotes making the reader wonder what else might be
happening (nothing is).) Cases 3 and 4 likewise behave in the
same way: they run the code '$x', which does nothing but return
the value of $x. (Case 4 is preferred for purely visual
reasons, but it also has the advantage of compiling at compile-
time instead of at run-time.) Case 5 is a place where normally
you would like to use double quotes, except that in this
particular situation, you can just use symbolic references
instead, as in case 6.
The assignment to $@ occurs before restoration of localised
variables, which means a temporary is required if you want to
mask some but not all errors:
# alter $@ on nefarious repugnancy only
{
my $e;
{
local $@; # protect existing $@
eval { test_repugnancy() };
# $@ =~ /nefarious/ and die $@; # DOES NOT WORK
$@ =~ /nefarious/ and $e = $@;
}
die $e if defined $e
}
"eval BLOCK" does not count as a loop, so the loop control
statements "next", "last", or "redo" cannot be used to leave or
restart the block.
Note that as a very special case, an "eval ''" executed within
the "DB" package doesn’t see the usual surrounding lexical
scope, but rather the scope of the first non-DB piece of code
that called it. You don’t normally need to worry about this
unless you are writing a Perl debugger.
exec LIST
exec PROGRAM LIST
The "exec" function executes a system command and never
returns-- use "system" instead of "exec" if you want it to
return. It fails and returns false only if the command does
not exist and it is executed directly instead of via your
system’s command shell (see below).
Since it’s a common mistake to use "exec" instead of "system",
Perl warns you if there is a following statement which isn’t
"die", "warn", or "exit" (if "-w" is set - but you always do
that). If you really want to follow an "exec" with some other
statement, you can use one of these styles to avoid the
warning:
exec ('foo') or print STDERR "couldn't exec foo: $!";
{ exec ('foo') }; print STDERR "couldn't exec foo: $!";
If there is more than one argument in LIST, or if LIST is an
array with more than one value, calls execvp(3) with the
arguments in LIST. If there is only one scalar argument or an
array with one element in it, the argument is checked for shell
metacharacters, and if there are any, the entire argument is
passed to the system’s command shell for parsing (this is
"/bin/sh -c" on Unix platforms, but varies on other platforms).
If there are no shell metacharacters in the argument, it is
split into words and passed directly to "execvp", which is more
efficient. Examples:
exec '/bin/echo', 'Your arguments are: ', @ARGV;
exec "sort $outfile | uniq";
If you don’t really want to execute the first argument, but
want to lie to the program you are executing about its own
name, you can specify the program you actually want to run as
an "indirect object" (without a comma) in front of the LIST.
(This always forces interpretation of the LIST as a multivalued
list, even if there is only a single scalar in the list.)
Example:
$shell = '/bin/csh';
exec $shell '-sh'; # pretend it's a login shell
or, more directly,
exec {'/bin/csh'} '-sh'; # pretend it's a login shell
When the arguments get executed via the system shell, results
will be subject to its quirks and capabilities. See "‘STRING‘"
in perlop for details.
Using an indirect object with "exec" or "system" is also more
secure. This usage (which also works fine with system())
forces interpretation of the arguments as a multivalued list,
even if the list had just one argument. That way you’re safe
from the shell expanding wildcards or splitting up words with
whitespace in them.
@args = ( "echo surprise" );
exec @args; # subject to shell escapes
# if @args == 1
exec { $args[0] } @args; # safe even with one-arg list
The first version, the one without the indirect object, ran the
echo program, passing it "surprise" an argument. The second
version didn’t--it tried to run a program literally called
"echo surprise", didn’t find it, and set $? to a non-zero value
indicating failure.
Beginning with v5.6.0, Perl will attempt to flush all files
opened for output before the exec, but this may not be
supported on some platforms (see perlport). To be safe, you
may need to set $| ($AUTOFLUSH in English) or call the
"autoflush()" method of "IO::Handle" on any open handles in
order to avoid lost output.
Note that "exec" will not call your "END" blocks, nor will it
call any "DESTROY" methods in your objects.
exists EXPR
Given an expression that specifies a hash element or array
element, returns true if the specified element in the hash or
array has ever been initialized, even if the corresponding
value is undefined.
print "Exists\n" if exists $hash{$key};
print "Defined\n" if defined $hash{$key};
print "True\n" if $hash{$key};
print "Exists\n" if exists $array[$index];
print "Defined\n" if defined $array[$index];
print "True\n" if $array[$index];
A hash or array element can be true only if it’s defined, and
defined if it exists, but the reverse doesn’t necessarily hold
true.
Given an expression that specifies the name of a subroutine,
returns true if the specified subroutine has ever been
declared, even if it is undefined. Mentioning a subroutine
name for exists or defined does not count as declaring it.
Note that a subroutine which does not exist may still be
callable: its package may have an "AUTOLOAD" method that makes
it spring into existence the first time that it is called --
see perlsub.
print "Exists\n" if exists &subroutine;
print "Defined\n" if defined &subroutine;
Note that the EXPR can be arbitrarily complicated as long as
the final operation is a hash or array key lookup or subroutine
name:
if (exists $ref->{A}->{B}->{$key}) { }
if (exists $hash{A}{B}{$key}) { }
if (exists $ref->{A}->{B}->[$ix]) { }
if (exists $hash{A}{B}[$ix]) { }
if (exists &{$ref->{A}{B}{$key}}) { }
Although the deepest nested array or hash will not spring into
existence just because its existence was tested, any
intervening ones will. Thus "$ref->{"A"}" and
"$ref->{"A"}->{"B"}" will spring into existence due to the
existence test for the $key element above. This happens
anywhere the arrow operator is used, including even:
undef $ref;
if (exists $ref->{"Some key"}) { }
print $ref; # prints HASH(0x80d3d5c)
This surprising autovivification in what does not at first--or
even second--glance appear to be an lvalue context may be fixed
in a future release.
Use of a subroutine call, rather than a subroutine name, as an
argument to exists() is an error.
exists ⊂ # OK
exists &sub(); # Error
exit EXPR
exit Evaluates EXPR and exits immediately with that value.
Example:
$ans = <STDIN>;
exit 0 if $ans =~ /^[Xx]/;
See also "die". If EXPR is omitted, exits with 0 status. The
only universally recognized values for EXPR are 0 for success
and 1 for error; other values are subject to interpretation
depending on the environment in which the Perl program is
running. For example, exiting 69 (EX_UNAVAILABLE) from a
sendmail incoming-mail filter will cause the mailer to return
the item undelivered, but that’s not true everywhere.
Don’t use "exit" to abort a subroutine if there’s any chance
that someone might want to trap whatever error happened. Use
"die" instead, which can be trapped by an "eval".
The exit() function does not always exit immediately. It calls
any defined "END" routines first, but these "END" routines may
not themselves abort the exit. Likewise any object destructors
that need to be called are called before the real exit. If
this is a problem, you can call "POSIX:_exit($status)" to avoid
END and destructor processing. See perlmod for details.
exp EXPR
exp Returns e (the natural logarithm base) to the power of EXPR.
If EXPR is omitted, gives "exp($_)".
fcntl FILEHANDLE,FUNCTION,SCALAR
Implements the fcntl(2) function. You’ll probably have to say
use Fcntl;
first to get the correct constant definitions. Argument
processing and value return works just like "ioctl" below. For
example:
use Fcntl;
fcntl($filehandle, F_GETFL, $packed_return_buffer)
or die "can't fcntl F_GETFL: $!";
You don’t have to check for "defined" on the return from
"fcntl". Like "ioctl", it maps a 0 return from the system call
into "0 but true" in Perl. This string is true in boolean
context and 0 in numeric context. It is also exempt from the
normal -w warnings on improper numeric conversions.
Note that "fcntl" will produce a fatal error if used on a
machine that doesn’t implement fcntl(2). See the Fcntl module
or your fcntl(2) manpage to learn what functions are available
on your system.
Here’s an example of setting a filehandle named "REMOTE" to be
non-blocking at the system level. You’ll have to negotiate $|
on your own, though.
use Fcntl qw(F_GETFL F_SETFL O_NONBLOCK);
$flags = fcntl(REMOTE, F_GETFL, 0)
or die "Can't get flags for the socket: $!\n";
$flags = fcntl(REMOTE, F_SETFL, $flags | O_NONBLOCK)
or die "Can't set flags for the socket: $!\n";
fileno FILEHANDLE
Returns the file descriptor for a filehandle, or undefined if
the filehandle is not open. This is mainly useful for
constructing bitmaps for "select" and low-level POSIX tty-
handling operations. If FILEHANDLE is an expression, the value
is taken as an indirect filehandle, generally its name.
You can use this to find out whether two handles refer to the
same underlying descriptor:
if (fileno(THIS) == fileno(THAT)) {
print "THIS and THAT are dups\n";
}
(Filehandles connected to memory objects via new features of
"open" may return undefined even though they are open.)
flock FILEHANDLE,OPERATION
Calls flock(2), or an emulation of it, on FILEHANDLE. Returns
true for success, false on failure. Produces a fatal error if
used on a machine that doesn’t implement flock(2), fcntl(2)
locking, or lockf(3). "flock" is Perl’s portable file locking
interface, although it locks only entire files, not records.
Two potentially non-obvious but traditional "flock" semantics
are that it waits indefinitely until the lock is granted, and
that its locks merely advisory. Such discretionary locks are
more flexible, but offer fewer guarantees. This means that
programs that do not also use "flock" may modify files locked
with "flock". See perlport, your port’s specific
documentation, or your system-specific local manpages for
details. It’s best to assume traditional behavior if you’re
writing portable programs. (But if you’re not, you should as
always feel perfectly free to write for your own system’s
idiosyncrasies (sometimes called "features"). Slavish
adherence to portability concerns shouldn’t get in the way of
your getting your job done.)
OPERATION is one of LOCK_SH, LOCK_EX, or LOCK_UN, possibly
combined with LOCK_NB. These constants are traditionally
valued 1, 2, 8 and 4, but you can use the symbolic names if you
import them from the Fcntl module, either individually, or as a
group using the ’:flock’ tag. LOCK_SH requests a shared lock,
LOCK_EX requests an exclusive lock, and LOCK_UN releases a
previously requested lock. If LOCK_NB is bitwise-or’ed with
LOCK_SH or LOCK_EX then "flock" will return immediately rather
than blocking waiting for the lock (check the return status to
see if you got it).
To avoid the possibility of miscoordination, Perl now flushes
FILEHANDLE before locking or unlocking it.
Note that the emulation built with lockf(3) doesn’t provide
shared locks, and it requires that FILEHANDLE be open with
write intent. These are the semantics that lockf(3)
implements. Most if not all systems implement lockf(3) in
terms of fcntl(2) locking, though, so the differing semantics
shouldn’t bite too many people.
Note that the fcntl(2) emulation of flock(3) requires that
FILEHANDLE be open with read intent to use LOCK_SH and requires
that it be open with write intent to use LOCK_EX.
Note also that some versions of "flock" cannot lock things over
the network; you would need to use the more system-specific
"fcntl" for that. If you like you can force Perl to ignore
your system’s flock(2) function, and so provide its own
fcntl(2)-based emulation, by passing the switch "-Ud_flock" to
the Configure program when you configure perl.
Here’s a mailbox appender for BSD systems.
use Fcntl qw(:flock SEEK_END); # import LOCK_* and SEEK_END constants
sub lock {
my ($fh) = @_;
flock($fh, LOCK_EX) or die "Cannot lock mailbox - $!\n";
# and, in case someone appended while we were waiting...
seek($fh, 0, SEEK_END) or die "Cannot seek - $!\n";
}
sub unlock {
my ($fh) = @_;
flock($fh, LOCK_UN) or die "Cannot unlock mailbox - $!\n";
}
open(my $mbox, ">>", "/usr/spool/mail/$ENV{'USER'}")
or die "Can't open mailbox: $!";
lock($mbox);
print $mbox $msg,"\n\n";
unlock($mbox);
On systems that support a real flock(), locks are inherited
across fork() calls, whereas those that must resort to the more
capricious fcntl() function lose the locks, making it harder to
write servers.
See also DB_File for other flock() examples.
fork Does a fork(2) system call to create a new process running the
same program at the same point. It returns the child pid to
the parent process, 0 to the child process, or "undef" if the
fork is unsuccessful. File descriptors (and sometimes locks on
those descriptors) are shared, while everything else is copied.
On most systems supporting fork(), great care has gone into
making it extremely efficient (for example, using copy-on-write
technology on data pages), making it the dominant paradigm for
multitasking over the last few decades.
Beginning with v5.6.0, Perl will attempt to flush all files
opened for output before forking the child process, but this
may not be supported on some platforms (see perlport). To be
safe, you may need to set $| ($AUTOFLUSH in English) or call
the "autoflush()" method of "IO::Handle" on any open handles in
order to avoid duplicate output.
If you "fork" without ever waiting on your children, you will
accumulate zombies. On some systems, you can avoid this by
setting $SIG{CHLD} to "IGNORE". See also perlipc for more
examples of forking and reaping moribund children.
Note that if your forked child inherits system file descriptors
like STDIN and STDOUT that are actually connected by a pipe or
socket, even if you exit, then the remote server (such as, say,
a CGI script or a backgrounded job launched from a remote
shell) won’t think you’re done. You should reopen those to
/dev/null if it’s any issue.
format Declare a picture format for use by the "write" function. For
example:
format Something =
Test: @<<<<<<<< @||||| @>>>>>
$str, $%, '$' . int($num)
.
$str = "widget";
$num = $cost/$quantity;
$~ = 'Something';
write;
See perlform for many details and examples.
formline PICTURE,LIST
This is an internal function used by "format"s, though you may
call it, too. It formats (see perlform) a list of values
according to the contents of PICTURE, placing the output into
the format output accumulator, $^A (or $ACCUMULATOR in
English). Eventually, when a "write" is done, the contents of
$^A are written to some filehandle. You could also read $^A
and then set $^A back to "". Note that a format typically does
one "formline" per line of form, but the "formline" function
itself doesn’t care how many newlines are embedded in the
PICTURE. This means that the "~" and "~~" tokens will treat
the entire PICTURE as a single line. You may therefore need to
use multiple formlines to implement a single record format,
just like the format compiler.
Be careful if you put double quotes around the picture, because
an "@" character may be taken to mean the beginning of an array
name. "formline" always returns true. See perlform for other
examples.
getc FILEHANDLE
getc Returns the next character from the input file attached to
FILEHANDLE, or the undefined value at end of file, or if there
was an error (in the latter case $! is set). If FILEHANDLE is
omitted, reads from STDIN. This is not particularly efficient.
However, it cannot be used by itself to fetch single characters
without waiting for the user to hit enter. For that, try
something more like:
if ($BSD_STYLE) {
system "stty cbreak </dev/tty >/dev/tty 2>&1";
}
else {
system "stty", '-icanon', 'eol', "\001";
}
$key = getc(STDIN);
if ($BSD_STYLE) {
system "stty -cbreak </dev/tty >/dev/tty 2>&1";
}
else {
system "stty", 'icanon', 'eol', '^@'; # ASCII null
}
print "\n";
Determination of whether $BSD_STYLE should be set is left as an
exercise to the reader.
The "POSIX::getattr" function can do this more portably on
systems purporting POSIX compliance. See also the
"Term::ReadKey" module from your nearest CPAN site; details on
CPAN can be found on "CPAN" in perlmodlib.
getlogin
This implements the C library function of the same name, which
on most systems returns the current login from /etc/utmp, if
any. If null, use "getpwuid".
$login = getlogin || getpwuid($<) || "Kilroy";
Do not consider "getlogin" for authentication: it is not as
secure as "getpwuid".
getpeername SOCKET
Returns the packed sockaddr address of other end of the SOCKET
connection.
use Socket;
$hersockaddr = getpeername(SOCK);
($port, $iaddr) = sockaddr_in($hersockaddr);
$herhostname = gethostbyaddr($iaddr, AF_INET);
$herstraddr = inet_ntoa($iaddr);
getpgrp PID
Returns the current process group for the specified PID. Use a
PID of 0 to get the current process group for the current
process. Will raise an exception if used on a machine that
doesn’t implement getpgrp(2). If PID is omitted, returns
process group of current process. Note that the POSIX version
of "getpgrp" does not accept a PID argument, so only "PID==0"
is truly portable.
getppid Returns the process id of the parent process.
Note for Linux users: on Linux, the C functions "getpid()" and
"getppid()" return different values from different threads. In
order to be portable, this behavior is not reflected by the
perl-level function "getppid()", that returns a consistent
value across threads. If you want to call the underlying
"getppid()", you may use the CPAN module "Linux::Pid".
getpriority WHICH,WHO
Returns the current priority for a process, a process group, or
a user. (See getpriority(2).) Will raise a fatal exception if
used on a machine that doesn’t implement getpriority(2).
getpwnam NAME
getgrnam NAME
gethostbyname NAME
getnetbyname NAME
getprotobyname NAME
getpwuid UID
getgrgid GID
getservbyname NAME,PROTO
gethostbyaddr ADDR,ADDRTYPE
getnetbyaddr ADDR,ADDRTYPE
getprotobynumber NUMBER
getservbyport PORT,PROTO
getpwent
getgrent
gethostent
getnetent
getprotoent
getservent
setpwent
setgrent
sethostent STAYOPEN
setnetent STAYOPEN
setprotoent STAYOPEN
setservent STAYOPEN
endpwent
endgrent
endhostent
endnetent
endprotoent
endservent
These routines perform the same functions as their counterparts
in the system library. In list context, the return values from
the various get routines are as follows:
($name,$passwd,$uid,$gid,
$quota,$comment,$gcos,$dir,$shell,$expire) = getpw*
($name,$passwd,$gid,$members) = getgr*
($name,$aliases,$addrtype,$length,@addrs) = gethost*
($name,$aliases,$addrtype,$net) = getnet*
($name,$aliases,$proto) = getproto*
($name,$aliases,$port,$proto) = getserv*
(If the entry doesn’t exist you get a null list.)
The exact meaning of the $gcos field varies but it usually
contains the real name of the user (as opposed to the login
name) and other information pertaining to the user. Beware,
however, that in many system users are able to change this
information and therefore it cannot be trusted and therefore
the $gcos is tainted (see perlsec). The $passwd and $shell,
user’s encrypted password and login shell, are also tainted,
because of the same reason.
In scalar context, you get the name, unless the function was a
lookup by name, in which case you get the other thing, whatever
it is. (If the entry doesn’t exist you get the undefined
value.) For example:
$uid = getpwnam($name);
$name = getpwuid($num);
$name = getpwent();
$gid = getgrnam($name);
$name = getgrgid($num);
$name = getgrent();
#etc.
In getpw*() the fields $quota, $comment, and $expire are
special cases in the sense that in many systems they are
unsupported. If the $quota is unsupported, it is an empty
scalar. If it is supported, it usually encodes the disk quota.
If the $comment field is unsupported, it is an empty scalar.
If it is supported it usually encodes some administrative
comment about the user. In some systems the $quota field may
be $change or $age, fields that have to do with password aging.
In some systems the $comment field may be $class. The $expire
field, if present, encodes the expiration period of the account
or the password. For the availability and the exact meaning of
these fields in your system, please consult your getpwnam(3)
documentation and your pwd.h file. You can also find out from
within Perl what your $quota and $comment fields mean and
whether you have the $expire field by using the "Config" module
and the values "d_pwquota", "d_pwage", "d_pwchange",
"d_pwcomment", and "d_pwexpire". Shadow password files are
only supported if your vendor has implemented them in the
intuitive fashion that calling the regular C library routines
gets the shadow versions if you’re running under privilege or
if there exists the shadow(3) functions as found in System V
(this includes Solaris and Linux.) Those systems that
implement a proprietary shadow password facility are unlikely
to be supported.
The $members value returned by getgr*() is a space separated
list of the login names of the members of the group.
For the gethost*() functions, if the "h_errno" variable is
supported in C, it will be returned to you via $? if the
function call fails. The @addrs value returned by a successful
call is a list of the raw addresses returned by the
corresponding system library call. In the Internet domain,
each address is four bytes long and you can unpack it by saying
something like:
($a,$b,$c,$d) = unpack('W4',$addr[0]);
The Socket library makes this slightly easier:
use Socket;
$iaddr = inet_aton("127.1"); # or whatever address
$name = gethostbyaddr($iaddr, AF_INET);
# or going the other way
$straddr = inet_ntoa($iaddr);
In the opposite way, to resolve a hostname to the IP address
you can write this:
use Socket;
$packed_ip = gethostbyname("www.perl.org");
if (defined $packed_ip) {
$ip_address = inet_ntoa($packed_ip);
}
Make sure <gethostbyname()> is called in SCALAR context and
that its return value is checked for definedness.
If you get tired of remembering which element of the return
list contains which return value, by-name interfaces are
provided in standard modules: "File::stat", "Net::hostent",
"Net::netent", "Net::protoent", "Net::servent", "Time::gmtime",
"Time::localtime", and "User::grent". These override the
normal built-ins, supplying versions that return objects with
the appropriate names for each field. For example:
use File::stat;
use User::pwent;
$is_his = (stat($filename)->uid == pwent($whoever)->uid);
Even though it looks like they’re the same method calls (uid),
they aren’t, because a "File::stat" object is different from a
"User::pwent" object.
getsockname SOCKET
Returns the packed sockaddr address of this end of the SOCKET
connection, in case you don’t know the address because you have
several different IPs that the connection might have come in
on.
use Socket;
$mysockaddr = getsockname(SOCK);
($port, $myaddr) = sockaddr_in($mysockaddr);
printf "Connect to %s [%s]\n",
scalar gethostbyaddr($myaddr, AF_INET),
inet_ntoa($myaddr);
getsockopt SOCKET,LEVEL,OPTNAME
Queries the option named OPTNAME associated with SOCKET at a
given LEVEL. Options may exist at multiple protocol levels
depending on the socket type, but at least the uppermost socket
level SOL_SOCKET (defined in the "Socket" module) will exist.
To query options at another level the protocol number of the
appropriate protocol controlling the option should be supplied.
For example, to indicate that an option is to be interpreted by
the TCP protocol, LEVEL should be set to the protocol number of
TCP, which you can get using getprotobyname.
The call returns a packed string representing the requested
socket option, or "undef" if there is an error (the error
reason will be in $!). What exactly is in the packed string
depends in the LEVEL and OPTNAME, consult your system
documentation for details. A very common case however is that
the option is an integer, in which case the result will be a
packed integer which you can decode using unpack with the "i"
(or "I") format.
An example testing if Nagle’s algorithm is turned on on a
socket:
use Socket qw(:all);
defined(my $tcp = getprotobyname("tcp"))
or die "Could not determine the protocol number for tcp";
# my $tcp = IPPROTO_TCP; # Alternative
my $packed = getsockopt($socket, $tcp, TCP_NODELAY)
or die "Could not query TCP_NODELAY socket option: $!";
my $nodelay = unpack("I", $packed);
print "Nagle's algorithm is turned ", $nodelay ? "off\n" : "on\n";
glob EXPR
glob In list context, returns a (possibly empty) list of filename
expansions on the value of EXPR such as the standard Unix shell
/bin/csh would do. In scalar context, glob iterates through
such filename expansions, returning undef when the list is
exhausted. This is the internal function implementing the
"<*.c>" operator, but you can use it directly. If EXPR is
omitted, $_ is used. The "<*.c>" operator is discussed in more
detail in "I/O Operators" in perlop.
Note that "glob" will split its arguments on whitespace,
treating each segment as separate pattern. As such, "glob('*.c
*.h')" would match all files with a .c or .h extension. The
expression "glob('.* *')" would match all files in the current
working directory.
Beginning with v5.6.0, this operator is implemented using the
standard "File::Glob" extension. See File::Glob for details,
including "bsd_glob" which does not treat whitespace as a
pattern separator.
gmtime EXPR
gmtime Works just like localtime but the returned values are localized
for the standard Greenwich time zone.
Note: when called in list context, $isdst, the last value
returned by gmtime is always 0. There is no Daylight Saving
Time in GMT.
See "gmtime" in perlport for portability concerns.
goto LABEL
goto EXPR
goto &NAME
The "goto-LABEL" form finds the statement labeled with LABEL
and resumes execution there. It may not be used to go into any
construct that requires initialization, such as a subroutine or
a "foreach" loop. It also can’t be used to go into a construct
that is optimized away, or to get out of a block or subroutine
given to "sort". It can be used to go almost anywhere else
within the dynamic scope, including out of subroutines, but
it’s usually better to use some other construct such as "last"
or "die". The author of Perl has never felt the need to use
this form of "goto" (in Perl, that is--C is another matter).
(The difference being that C does not offer named loops
combined with loop control. Perl does, and this replaces most
structured uses of "goto" in other languages.)
The "goto-EXPR" form expects a label name, whose scope will be
resolved dynamically. This allows for computed "goto"s per
FORTRAN, but isn’t necessarily recommended if you’re optimizing
for maintainability:
goto ("FOO", "BAR", "GLARCH")[$i];
The "goto-&NAME" form is quite different from the other forms
of "goto". In fact, it isn’t a goto in the normal sense at
all, and doesn’t have the stigma associated with other gotos.
Instead, it exits the current subroutine (losing any changes
set by local()) and immediately calls in its place the named
subroutine using the current value of @_. This is used by
"AUTOLOAD" subroutines that wish to load another subroutine and
then pretend that the other subroutine had been called in the
first place (except that any modifications to @_ in the current
subroutine are propagated to the other subroutine.) After the
"goto", not even "caller" will be able to tell that this
routine was called first.
NAME needn’t be the name of a subroutine; it can be a scalar
variable containing a code reference, or a block that evaluates
to a code reference.
grep BLOCK LIST
grep EXPR,LIST
This is similar in spirit to, but not the same as, grep(1) and
its relatives. In particular, it is not limited to using
regular expressions.
Evaluates the BLOCK or EXPR for each element of LIST (locally
setting $_ to each element) and returns the list value
consisting of those elements for which the expression evaluated
to true. In scalar context, returns the number of times the
expression was true.
@foo = grep(!/^#/, @bar); # weed out comments
or equivalently,
@foo = grep {!/^#/} @bar; # weed out comments
Note that $_ is an alias to the list value, so it can be used
to modify the elements of the LIST. While this is useful and
supported, it can cause bizarre results if the elements of LIST
are not variables. Similarly, grep returns aliases into the
original list, much as a for loop’s index variable aliases the
list elements. That is, modifying an element of a list
returned by grep (for example, in a "foreach", "map" or another
"grep") actually modifies the element in the original list.
This is usually something to be avoided when writing clear
code.
If $_ is lexical in the scope where the "grep" appears (because
it has been declared with "my $_") then, in addition to being
locally aliased to the list elements, $_ keeps being lexical
inside the block; i.e. it can’t be seen from the outside,
avoiding any potential side-effects.
See also "map" for a list composed of the results of the BLOCK
or EXPR.
hex EXPR
hex Interprets EXPR as a hex string and returns the corresponding
value. (To convert strings that might start with either 0,
"0x", or "0b", see "oct".) If EXPR is omitted, uses $_.
print hex '0xAf'; # prints '175'
print hex 'aF'; # same
Hex strings may only represent integers. Strings that would
cause integer overflow trigger a warning. Leading whitespace
is not stripped, unlike oct(). To present something as hex,
look into "printf", "sprintf", or "unpack".
import LIST
There is no builtin "import" function. It is just an ordinary
method (subroutine) defined (or inherited) by modules that wish
to export names to another module. The "use" function calls
the "import" method for the package used. See also "use",
perlmod, and Exporter.
index STR,SUBSTR,POSITION
index STR,SUBSTR
The index function searches for one string within another, but
without the wildcard-like behavior of a full regular-expression
pattern match. It returns the position of the first occurrence
of SUBSTR in STR at or after POSITION. If POSITION is omitted,
starts searching from the beginning of the string. POSITION
before the beginning of the string or after its end is treated
as if it were the beginning or the end, respectively. POSITION
and the return value are based at 0 (or whatever you’ve set the
$[ variable to--but don’t do that). If the substring is not
found, "index" returns one less than the base, ordinarily "-1".
int EXPR
int Returns the integer portion of EXPR. If EXPR is omitted, uses
$_. You should not use this function for rounding: one because
it truncates towards 0, and two because machine representations
of floating point numbers can sometimes produce
counterintuitive results. For example, "int(-6.725/0.025)"
produces -268 rather than the correct -269; that’s because it’s
really more like -268.99999999999994315658 instead. Usually,
the "sprintf", "printf", or the "POSIX::floor" and
"POSIX::ceil" functions will serve you better than will int().
ioctl FILEHANDLE,FUNCTION,SCALAR
Implements the ioctl(2) function. You’ll probably first have
to say
require "sys/ioctl.ph"; # probably in $Config{archlib}/sys/ioctl.ph
to get the correct function definitions. If sys/ioctl.ph
doesn’t exist or doesn’t have the correct definitions you’ll
have to roll your own, based on your C header files such as
<sys/ioctl.h>. (There is a Perl script called h2ph that comes
with the Perl kit that may help you in this, but it’s
nontrivial.) SCALAR will be read and/or written depending on
the FUNCTION--a pointer to the string value of SCALAR will be
passed as the third argument of the actual "ioctl" call. (If
SCALAR has no string value but does have a numeric value, that
value will be passed rather than a pointer to the string value.
To guarantee this to be true, add a 0 to the scalar before
using it.) The "pack" and "unpack" functions may be needed to
manipulate the values of structures used by "ioctl".
The return value of "ioctl" (and "fcntl") is as follows:
if OS returns: then Perl returns:
-1 undefined value
0 string "0 but true"
anything else that number
Thus Perl returns true on success and false on failure, yet you
can still easily determine the actual value returned by the
operating system:
$retval = ioctl(...) || -1;
printf "System returned %d\n", $retval;
The special string "0 but true" is exempt from -w complaints
about improper numeric conversions.
join EXPR,LIST
Joins the separate strings of LIST into a single string with
fields separated by the value of EXPR, and returns that new
string. Example:
$rec = join(':', $login,$passwd,$uid,$gid,$gcos,$home,$shell);
Beware that unlike "split", "join" doesn’t take a pattern as
its first argument. Compare "split".
keys HASH
Returns a list consisting of all the keys of the named hash.
(In scalar context, returns the number of keys.)
The keys are returned in an apparently random order. The
actual random order is subject to change in future versions of
perl, but it is guaranteed to be the same order as either the
"values" or "each" function produces (given that the hash has
not been modified). Since Perl 5.8.1 the ordering is different
even between different runs of Perl for security reasons (see
"Algorithmic Complexity Attacks" in perlsec).
As a side effect, calling keys() resets the HASH’s internal
iterator (see "each"). In particular, calling keys() in void
context resets the iterator with no other overhead.
Here is yet another way to print your environment:
@keys = keys %ENV;
@values = values %ENV;
while (@keys) {
print pop(@keys), '=', pop(@values), "\n";
}
or how about sorted by key:
foreach $key (sort(keys %ENV)) {
print $key, '=', $ENV{$key}, "\n";
}
The returned values are copies of the original keys in the
hash, so modifying them will not affect the original hash.
Compare "values".
To sort a hash by value, you’ll need to use a "sort" function.
Here’s a descending numeric sort of a hash by its values:
foreach $key (sort { $hash{$b} <=> $hash{$a} } keys %hash) {
printf "%4d %s\n", $hash{$key}, $key;
}
As an lvalue "keys" allows you to increase the number of hash
buckets allocated for the given hash. This can gain you a
measure of efficiency if you know the hash is going to get big.
(This is similar to pre-extending an array by assigning a
larger number to $#array.) If you say
keys %hash = 200;
then %hash will have at least 200 buckets allocated for it--256
of them, in fact, since it rounds up to the next power of two.
These buckets will be retained even if you do "%hash = ()", use
"undef %hash" if you want to free the storage while %hash is
still in scope. You can’t shrink the number of buckets
allocated for the hash using "keys" in this way (but you
needn’t worry about doing this by accident, as trying has no
effect).
See also "each", "values" and "sort".
kill SIGNAL, LIST
Sends a signal to a list of processes. Returns the number of
processes successfully signaled (which is not necessarily the
same as the number actually killed).
$cnt = kill 1, $child1, $child2;
kill 9, @goners;
If SIGNAL is zero, no signal is sent to the process, but the
kill(2) system call will check whether it’s possible to send a
signal to it (that means, to be brief, that the process is
owned by the same user, or we are the super-user). This is a
useful way to check that a child process is alive (even if only
as a zombie) and hasn’t changed its UID. See perlport for
notes on the portability of this construct.
Unlike in the shell, if SIGNAL is negative, it kills process
groups instead of processes. (On System V, a negative PROCESS
number will also kill process groups, but that’s not portable.)
That means you usually want to use positive not negative
signals. You may also use a signal name in quotes.
See "Signals" in perlipc for more details.
last LABEL
last The "last" command is like the "break" statement in C (as used
in loops); it immediately exits the loop in question. If the
LABEL is omitted, the command refers to the innermost enclosing
loop. The "continue" block, if any, is not executed:
LINE: while (<STDIN>) {
last LINE if /^$/; # exit when done with header
#...
}
"last" cannot be used to exit a block which returns a value
such as "eval {}", "sub {}" or "do {}", and should not be used
to exit a grep() or map() operation.
Note that a block by itself is semantically identical to a loop
that executes once. Thus "last" can be used to effect an early
exit out of such a block.
See also "continue" for an illustration of how "last", "next",
and "redo" work.
lc EXPR
lc Returns a lowercased version of EXPR. This is the internal
function implementing the "\L" escape in double-quoted strings.
Respects current LC_CTYPE locale if "use locale" in force. See
perllocale and perlunicode for more details about locale and
Unicode support.
If EXPR is omitted, uses $_.
lcfirst EXPR
lcfirst Returns the value of EXPR with the first character lowercased.
This is the internal function implementing the "\l" escape in
double-quoted strings. Respects current LC_CTYPE locale if
"use locale" in force. See perllocale and perlunicode for more
details about locale and Unicode support.
If EXPR is omitted, uses $_.
length EXPR
length Returns the length in characters of the value of EXPR. If EXPR
is omitted, returns length of $_. Note that this cannot be
used on an entire array or hash to find out how many elements
these have. For that, use "scalar @array" and "scalar keys
%hash" respectively.
Note the characters: if the EXPR is in Unicode, you will get
the number of characters, not the number of bytes. To get the
length of the internal string in bytes, use
"bytes::length(EXPR)", see bytes. Note that the internal
encoding is variable, and the number of bytes usually
meaningless. To get the number of bytes that the string would
have when encoded as UTF-8, use
"length(Encoding::encode_utf8(EXPR))".
link OLDFILE,NEWFILE
Creates a new filename linked to the old filename. Returns
true for success, false otherwise.
listen SOCKET,QUEUESIZE
Does the same thing that the listen system call does. Returns
true if it succeeded, false otherwise. See the example in
"Sockets: Client/Server Communication" in perlipc.
local EXPR
You really probably want to be using "my" instead, because
"local" isn’t what most people think of as "local". See
"Private Variables via my()" in perlsub for details.
A local modifies the listed variables to be local to the
enclosing block, file, or eval. If more than one value is
listed, the list must be placed in parentheses. See "Temporary
Values via local()" in perlsub for details, including issues
with tied arrays and hashes.
localtime EXPR
localtime
Converts a time as returned by the time function to a 9-element
list with the time analyzed for the local time zone. Typically
used as follows:
# 0 1 2 3 4 5 6 7 8
($sec,$min,$hour,$mday,$mon,$year,$wday,$yday,$isdst) =
localtime(time);
All list elements are numeric, and come straight out of the C
‘struct tm’. $sec, $min, and $hour are the seconds, minutes,
and hours of the specified time.
$mday is the day of the month, and $mon is the month itself, in
the range 0..11 with 0 indicating January and 11 indicating
December. This makes it easy to get a month name from a list:
my @abbr = qw( Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec );
print "$abbr[$mon] $mday";
# $mon=9, $mday=18 gives "Oct 18"
$year is the number of years since 1900, not just the last two
digits of the year. That is, $year is 123 in year 2023. The
proper way to get a complete 4-digit year is simply:
$year += 1900;
Otherwise you create non-Y2K-compliant programs--and you
wouldn’t want to do that, would you?
To get the last two digits of the year (e.g., ’01’ in 2001) do:
$year = sprintf("%02d", $year % 100);
$wday is the day of the week, with 0 indicating Sunday and 3
indicating Wednesday. $yday is the day of the year, in the
range 0..364 (or 0..365 in leap years.)
$isdst is true if the specified time occurs during Daylight
Saving Time, false otherwise.
If EXPR is omitted, "localtime()" uses the current time (as
returned by time(3)).
In scalar context, "localtime()" returns the ctime(3) value:
$now_string = localtime; # e.g., "Thu Oct 13 04:54:34 1994"
This scalar value is not locale dependent but is a Perl
builtin. For GMT instead of local time use the "gmtime"
builtin. See also the "Time::Local" module (to convert the
second, minutes, hours, ... back to the integer value returned
by time()), and the POSIX module’s strftime(3) and mktime(3)
functions.
To get somewhat similar but locale dependent date strings, set
up your locale environment variables appropriately (please see
perllocale) and try for example:
use POSIX qw(strftime);
$now_string = strftime "%a %b %e %H:%M:%S %Y", localtime;
# or for GMT formatted appropriately for your locale:
$now_string = strftime "%a %b %e %H:%M:%S %Y", gmtime;
Note that the %a and %b, the short forms of the day of the week
and the month of the year, may not necessarily be three
characters wide.
See "localtime" in perlport for portability concerns.
The Time::gmtime and Time::localtime modules provides a
convenient, by-name access mechanism to the gmtime() and
localtime() functions, respectively.
For a comprehensive date and time representation look at the
DateTime module on CPAN.
lock THING
This function places an advisory lock on a shared variable, or
referenced object contained in THING until the lock goes out of
scope.
lock() is a "weak keyword" : this means that if you’ve defined
a function by this name (before any calls to it), that function
will be called instead. (However, if you’ve said "use threads",
lock() is always a keyword.) See threads.
log EXPR
log Returns the natural logarithm (base e) of EXPR. If EXPR is
omitted, returns log of $_. To get the log of another base,
use basic algebra: The base-N log of a number is equal to the
natural log of that number divided by the natural log of N.
For example:
sub log10 {
my $n = shift;
return log($n)/log(10);
}
See also "exp" for the inverse operation.
lstat EXPR
lstat Does the same thing as the "stat" function (including setting
the special "_" filehandle) but stats a symbolic link instead
of the file the symbolic link points to. If symbolic links are
unimplemented on your system, a normal "stat" is done. For
much more detailed information, please see the documentation
for "stat".
If EXPR is omitted, stats $_.
m// The match operator. See "Regexp Quote-Like Operators" in
perlop.
map BLOCK LIST
map EXPR,LIST
Evaluates the BLOCK or EXPR for each element of LIST (locally
setting $_ to each element) and returns the list value composed
of the results of each such evaluation. In scalar context,
returns the total number of elements so generated. Evaluates
BLOCK or EXPR in list context, so each element of LIST may
produce zero, one, or more elements in the returned value.
@chars = map(chr, @nums);
translates a list of numbers to the corresponding characters.
And
%hash = map { get_a_key_for($_) => $_ } @array;
is just a funny way to write
%hash = ();
foreach (@array) {
$hash{get_a_key_for($_)} = $_;
}
Note that $_ is an alias to the list value, so it can be used
to modify the elements of the LIST. While this is useful and
supported, it can cause bizarre results if the elements of LIST
are not variables. Using a regular "foreach" loop for this
purpose would be clearer in most cases. See also "grep" for an
array composed of those items of the original list for which
the BLOCK or EXPR evaluates to true.
If $_ is lexical in the scope where the "map" appears (because
it has been declared with "my $_"), then, in addition to being
locally aliased to the list elements, $_ keeps being lexical
inside the block; that is, it can’t be seen from the outside,
avoiding any potential side-effects.
"{" starts both hash references and blocks, so "map { ..."
could be either the start of map BLOCK LIST or map EXPR, LIST.
Because perl doesn’t look ahead for the closing "}" it has to
take a guess at which its dealing with based what it finds just
after the "{". Usually it gets it right, but if it doesn’t it
won’t realize something is wrong until it gets to the "}" and
encounters the missing (or unexpected) comma. The syntax error
will be reported close to the "}" but you’ll need to change
something near the "{" such as using a unary "+" to give perl
some help:
%hash = map { "\L$_", 1 } @array # perl guesses EXPR. wrong
%hash = map { +"\L$_", 1 } @array # perl guesses BLOCK. right
%hash = map { ("\L$_", 1) } @array # this also works
%hash = map { lc($_), 1 } @array # as does this.
%hash = map +( lc($_), 1 ), @array # this is EXPR and works!
%hash = map ( lc($_), 1 ), @array # evaluates to (1, @array)
or to force an anon hash constructor use "+{":
@hashes = map +{ lc($_), 1 }, @array # EXPR, so needs , at end
and you get list of anonymous hashes each with only 1 entry.
mkdir FILENAME,MASK
mkdir FILENAME
mkdir Creates the directory specified by FILENAME, with permissions
specified by MASK (as modified by "umask"). If it succeeds it
returns true, otherwise it returns false and sets $! (errno).
If omitted, MASK defaults to 0777. If omitted, FILENAME
defaults to $_.
In general, it is better to create directories with permissive
MASK, and let the user modify that with their "umask", than it
is to supply a restrictive MASK and give the user no way to be
more permissive. The exceptions to this rule are when the file
or directory should be kept private (mail files, for instance).
The perlfunc(1) entry on "umask" discusses the choice of MASK
in more detail.
Note that according to the POSIX 1003.1-1996 the FILENAME may
have any number of trailing slashes. Some operating and
filesystems do not get this right, so Perl automatically
removes all trailing slashes to keep everyone happy.
In order to recursively create a directory structure look at
the "mkpath" function of the File::Path module.
msgctl ID,CMD,ARG
Calls the System V IPC function msgctl(2). You’ll probably
have to say
use IPC::SysV;
first to get the correct constant definitions. If CMD is
"IPC_STAT", then ARG must be a variable that will hold the
returned "msqid_ds" structure. Returns like "ioctl": the
undefined value for error, "0 but true" for zero, or the actual
return value otherwise. See also "SysV IPC" in perlipc,
"IPC::SysV", and "IPC::Semaphore" documentation.
msgget KEY,FLAGS
Calls the System V IPC function msgget(2). Returns the message
queue id, or the undefined value if there is an error. See
also "SysV IPC" in perlipc and "IPC::SysV" and "IPC::Msg"
documentation.
msgrcv ID,VAR,SIZE,TYPE,FLAGS
Calls the System V IPC function msgrcv to receive a message
from message queue ID into variable VAR with a maximum message
size of SIZE. Note that when a message is received, the
message type as a native long integer will be the first thing
in VAR, followed by the actual message. This packing may be
opened with "unpack("l! a*")". Taints the variable. Returns
true if successful, or false if there is an error. See also
"SysV IPC" in perlipc, "IPC::SysV", and "IPC::SysV::Msg"
documentation.
msgsnd ID,MSG,FLAGS
Calls the System V IPC function msgsnd to send the message MSG
to the message queue ID. MSG must begin with the native long
integer message type, and be followed by the length of the
actual message, and finally the message itself. This kind of
packing can be achieved with "pack("l! a*", $type, $message)".
Returns true if successful, or false if there is an error. See
also "IPC::SysV" and "IPC::SysV::Msg" documentation.
my EXPR
my TYPE EXPR
my EXPR : ATTRS
my TYPE EXPR : ATTRS
A "my" declares the listed variables to be local (lexically) to
the enclosing block, file, or "eval". If more than one value
is listed, the list must be placed in parentheses.
The exact semantics and interface of TYPE and ATTRS are still
evolving. TYPE is currently bound to the use of "fields"
pragma, and attributes are handled using the "attributes"
pragma, or starting from Perl 5.8.0 also via the
"Attribute::Handlers" module. See "Private Variables via my()"
in perlsub for details, and fields, attributes, and
Attribute::Handlers.
next LABEL
next The "next" command is like the "continue" statement in C; it
starts the next iteration of the loop:
LINE: while (<STDIN>) {
next LINE if /^#/; # discard comments
#...
}
Note that if there were a "continue" block on the above, it
would get executed even on discarded lines. If the LABEL is
omitted, the command refers to the innermost enclosing loop.
"next" cannot be used to exit a block which returns a value
such as "eval {}", "sub {}" or "do {}", and should not be used
to exit a grep() or map() operation.
Note that a block by itself is semantically identical to a loop
that executes once. Thus "next" will exit such a block early.
See also "continue" for an illustration of how "last", "next",
and "redo" work.
no Module VERSION LIST
no Module VERSION
no Module LIST
no Module
no VERSION
See the "use" function, of which "no" is the opposite.
oct EXPR
oct Interprets EXPR as an octal string and returns the
corresponding value. (If EXPR happens to start off with "0x",
interprets it as a hex string. If EXPR starts off with "0b",
it is interpreted as a binary string. Leading whitespace is
ignored in all three cases.) The following will handle
decimal, binary, octal, and hex in the standard Perl or C
notation:
$val = oct($val) if $val =~ /^0/;
If EXPR is omitted, uses $_. To go the other way (produce a
number in octal), use sprintf() or printf():
$perms = (stat("filename"))[2] & 07777;
$oct_perms = sprintf "%lo", $perms;
The oct() function is commonly used when a string such as 644
needs to be converted into a file mode, for example. (Although
perl will automatically convert strings into numbers as needed,
this automatic conversion assumes base 10.)
open FILEHANDLE,EXPR
open FILEHANDLE,MODE,EXPR
open FILEHANDLE,MODE,EXPR,LIST
open FILEHANDLE,MODE,REFERENCE
open FILEHANDLE
Opens the file whose filename is given by EXPR, and associates
it with FILEHANDLE.
Simple examples to open a file for reading:
open(my $fh, '<', "input.txt") or die $!;
and for writing:
open(my $fh, '>', "output.txt") or die $!;
(The following is a comprehensive reference to open(): for a
gentler introduction you may consider perlopentut.)
If FILEHANDLE is an undefined scalar variable (or array or hash
element) the variable is assigned a reference to a new
anonymous filehandle, otherwise if FILEHANDLE is an expression,
its value is used as the name of the real filehandle wanted.
(This is considered a symbolic reference, so "use strict
'refs'" should not be in effect.)
If EXPR is omitted, the scalar variable of the same name as the
FILEHANDLE contains the filename. (Note that lexical
variables--those declared with "my"--will not work for this
purpose; so if you’re using "my", specify EXPR in your call to
open.)
If three or more arguments are specified then the mode of
opening and the file name are separate. If MODE is '<' or
nothing, the file is opened for input. If MODE is '>', the
file is truncated and opened for output, being created if
necessary. If MODE is '>>', the file is opened for appending,
again being created if necessary.
You can put a '+' in front of the '>' or '<' to indicate that
you want both read and write access to the file; thus '+<' is
almost always preferred for read/write updates--the '+>' mode
would clobber the file first. You can’t usually use either
read-write mode for updating textfiles, since they have
variable length records. See the -i switch in perlrun for a
better approach. The file is created with permissions of 0666
modified by the process’ "umask" value.
These various prefixes correspond to the fopen(3) modes of 'r',
'r+', 'w', 'w+', 'a', and 'a+'.
In the 2-arguments (and 1-argument) form of the call the mode
and filename should be concatenated (in this order), possibly
separated by spaces. It is possible to omit the mode in these
forms if the mode is '<'.
If the filename begins with '|', the filename is interpreted as
a command to which output is to be piped, and if the filename
ends with a '|', the filename is interpreted as a command which
pipes output to us. See "Using open() for IPC" in perlipc for
more examples of this. (You are not allowed to "open" to a
command that pipes both in and out, but see IPC::Open2,
IPC::Open3, and "Bidirectional Communication with Another
Process" in perlipc for alternatives.)
For three or more arguments if MODE is '|-', the filename is
interpreted as a command to which output is to be piped, and if
MODE is '-|', the filename is interpreted as a command which
pipes output to us. In the 2-arguments (and 1-argument) form
one should replace dash ('-') with the command. See "Using
open() for IPC" in perlipc for more examples of this. (You are
not allowed to "open" to a command that pipes both in and out,
but see IPC::Open2, IPC::Open3, and "Bidirectional
Communication" in perlipc for alternatives.)
In the three-or-more argument form of pipe opens, if LIST is
specified (extra arguments after the command name) then LIST
becomes arguments to the command invoked if the platform
supports it. The meaning of "open" with more than three
arguments for non-pipe modes is not yet specified. Experimental
"layers" may give extra LIST arguments meaning.
In the 2-arguments (and 1-argument) form opening '-' opens
STDIN and opening '>-' opens STDOUT.
You may use the three-argument form of open to specify IO
"layers" (sometimes also referred to as "disciplines") to be
applied to the handle that affect how the input and output are
processed (see open and PerlIO for more details). For example
open(my $fh, "<:encoding(UTF-8)", "file")
will open the UTF-8 encoded file containing Unicode characters,
see perluniintro. Note that if layers are specified in the
three-arg form then default layers stored in ${^OPEN} (see
perlvar; usually set by the open pragma or the switch -CioD)
are ignored.
Open returns nonzero upon success, the undefined value
otherwise. If the "open" involved a pipe, the return value
happens to be the pid of the subprocess.
If you’re running Perl on a system that distinguishes between
text files and binary files, then you should check out
"binmode" for tips for dealing with this. The key distinction
between systems that need "binmode" and those that don’t is
their text file formats. Systems like Unix, Mac OS, and Plan
9, which delimit lines with a single character, and which
encode that character in C as "\n", do not need "binmode". The
rest need it.
When opening a file, it’s usually a bad idea to continue normal
execution if the request failed, so "open" is frequently used
in connection with "die". Even if "die" won’t do what you want
(say, in a CGI script, where you want to make a nicely
formatted error message (but there are modules that can help
with that problem)) you should always check the return value
from opening a file. The infrequent exception is when working
with an unopened filehandle is actually what you want to do.
As a special case the 3-arg form with a read/write mode and the
third argument being "undef":
open(my $tmp, "+>", undef) or die ...
opens a filehandle to an anonymous temporary file. Also using
"+<" works for symmetry, but you really should consider writing
something to the temporary file first. You will need to seek()
to do the reading.
Since v5.8.0, perl has built using PerlIO by default. Unless
you’ve changed this (i.e. Configure -Uuseperlio), you can open
file handles to "in memory" files held in Perl scalars via:
open($fh, '>', \$variable) || ..
Though if you try to re-open "STDOUT" or "STDERR" as an "in
memory" file, you have to close it first:
close STDOUT;
open STDOUT, '>', \$variable or die "Can't open STDOUT: $!";
Examples:
$ARTICLE = 100;
open ARTICLE or die "Can't find article $ARTICLE: $!\n";
while (<ARTICLE>) {...
open(LOG, '>>/usr/spool/news/twitlog'); # (log is reserved)
# if the open fails, output is discarded
open(my $dbase, '+<', 'dbase.mine') # open for update
or die "Can't open 'dbase.mine' for update: $!";
open(my $dbase, '+<dbase.mine') # ditto
or die "Can't open 'dbase.mine' for update: $!";
open(ARTICLE, '-|', "caesar <$article") # decrypt article
or die "Can't start caesar: $!";
open(ARTICLE, "caesar <$article |") # ditto
or die "Can't start caesar: $!";
open(EXTRACT, "|sort >Tmp$$") # $$ is our process id
or die "Can't start sort: $!";
# in memory files
open(MEMORY,'>', \$var)
or die "Can't open memory file: $!";
print MEMORY "foo!\n"; # output will end up in $var
# process argument list of files along with any includes
foreach $file (@ARGV) {
process($file, 'fh00');
}
sub process {
my($filename, $input) = @_;
$input++; # this is a string increment
unless (open($input, $filename)) {
print STDERR "Can't open $filename: $!\n";
return;
}
local $_;
while (<$input>) { # note use of indirection
if (/^#include "(.*)"/) {
process($1, $input);
next;
}
#... # whatever
}
}
See perliol for detailed info on PerlIO.
You may also, in the Bourne shell tradition, specify an EXPR
beginning with '>&', in which case the rest of the string is
interpreted as the name of a filehandle (or file descriptor, if
numeric) to be duped (as dup(2)) and opened. You may use "&"
after ">", ">>", "<", "+>", "+>>", and "+<". The mode you
specify should match the mode of the original filehandle.
(Duping a filehandle does not take into account any existing
contents of IO buffers.) If you use the 3-arg form then you can
pass either a number, the name of a filehandle or the normal
"reference to a glob".
Here is a script that saves, redirects, and restores "STDOUT"
and "STDERR" using various methods:
#!/usr/bin/perl
open my $oldout, ">&STDOUT" or die "Can't dup STDOUT: $!";
open OLDERR, ">&", \*STDERR or die "Can't dup STDERR: $!";
open STDOUT, '>', "foo.out" or die "Can't redirect STDOUT: $!";
open STDERR, ">&STDOUT" or die "Can't dup STDOUT: $!";
select STDERR; $| = 1; # make unbuffered
select STDOUT; $| = 1; # make unbuffered
print STDOUT "stdout 1\n"; # this works for
print STDERR "stderr 1\n"; # subprocesses too
open STDOUT, ">&", $oldout or die "Can't dup \$oldout: $!";
open STDERR, ">&OLDERR" or die "Can't dup OLDERR: $!";
print STDOUT "stdout 2\n";
print STDERR "stderr 2\n";
If you specify '<&=X', where "X" is a file descriptor number or
a filehandle, then Perl will do an equivalent of C’s "fdopen"
of that file descriptor (and not call dup(2)); this is more
parsimonious of file descriptors. For example:
# open for input, reusing the fileno of $fd
open(FILEHANDLE, "<&=$fd")
or
open(FILEHANDLE, "<&=", $fd)
or
# open for append, using the fileno of OLDFH
open(FH, ">>&=", OLDFH)
or
open(FH, ">>&=OLDFH")
Being parsimonious on filehandles is also useful (besides being
parsimonious) for example when something is dependent on file
descriptors, like for example locking using flock(). If you do
just "open(A, '>>&B')", the filehandle A will not have the same
file descriptor as B, and therefore flock(A) will not flock(B),
and vice versa. But with "open(A, '>>&=B')" the filehandles
will share the same file descriptor.
Note that if you are using Perls older than 5.8.0, Perl will be
using the standard C libraries’ fdopen() to implement the "="
functionality. On many UNIX systems fdopen() fails when file
descriptors exceed a certain value, typically 255. For Perls
5.8.0 and later, PerlIO is most often the default.
You can see whether Perl has been compiled with PerlIO or not
by running "perl -V" and looking for "useperlio=" line. If
"useperlio" is "define", you have PerlIO, otherwise you don’t.
If you open a pipe on the command '-', i.e., either '|-' or
'-|' with 2-arguments (or 1-argument) form of open(), then
there is an implicit fork done, and the return value of open is
the pid of the child within the parent process, and 0 within
the child process. (Use "defined($pid)" to determine whether
the open was successful.) The filehandle behaves normally for
the parent, but i/o to that filehandle is piped from/to the
STDOUT/STDIN of the child process. In the child process the
filehandle isn’t opened--i/o happens from/to the new STDOUT or
STDIN. Typically this is used like the normal piped open when
you want to exercise more control over just how the pipe
command gets executed, such as when you are running setuid, and
don’t want to have to scan shell commands for metacharacters.
The following triples are more or less equivalent:
open(FOO, "|tr '[a-z]' '[A-Z]'");
open(FOO, '|-', "tr '[a-z]' '[A-Z]'");
open(FOO, '|-') || exec 'tr', '[a-z]', '[A-Z]';
open(FOO, '|-', "tr", '[a-z]', '[A-Z]');
open(FOO, "cat -n '$file'|");
open(FOO, '-|', "cat -n '$file'");
open(FOO, '-|') || exec 'cat', '-n', $file;
open(FOO, '-|', "cat", '-n', $file);
The last example in each block shows the pipe as "list form",
which is not yet supported on all platforms. A good rule of
thumb is that if your platform has true "fork()" (in other
words, if your platform is UNIX) you can use the list form.
See "Safe Pipe Opens" in perlipc for more examples of this.
Beginning with v5.6.0, Perl will attempt to flush all files
opened for output before any operation that may do a fork, but
this may not be supported on some platforms (see perlport). To
be safe, you may need to set $| ($AUTOFLUSH in English) or call
the "autoflush()" method of "IO::Handle" on any open handles.
On systems that support a close-on-exec flag on files, the flag
will be set for the newly opened file descriptor as determined
by the value of $^F. See "$^F" in perlvar.
Closing any piped filehandle causes the parent process to wait
for the child to finish, and returns the status value in $? and
"${^CHILD_ERROR_NATIVE}".
The filename passed to 2-argument (or 1-argument) form of
open() will have leading and trailing whitespace deleted, and
the normal redirection characters honored. This property,
known as "magic open", can often be used to good effect. A
user could specify a filename of "rsh cat file |", or you could
change certain filenames as needed:
$filename =~ s/(.*\.gz)\s*$/gzip -dc < $1|/;
open(FH, $filename) or die "Can't open $filename: $!";
Use 3-argument form to open a file with arbitrary weird
characters in it,
open(FOO, '<', $file);
otherwise it’s necessary to protect any leading and trailing
whitespace:
$file =~ s#^(\s)#./$1#;
open(FOO, "< $file\0");
(this may not work on some bizarre filesystems). One should
conscientiously choose between the magic and 3-arguments form
of open():
open IN, $ARGV[0];
will allow the user to specify an argument of the form "rsh cat
file |", but will not work on a filename which happens to have
a trailing space, while
open IN, '<', $ARGV[0];
will have exactly the opposite restrictions.
If you want a "real" C "open" (see open(2) on your system),
then you should use the "sysopen" function, which involves no
such magic (but may use subtly different filemodes than Perl
open(), which is mapped to C fopen()). This is another way to
protect your filenames from interpretation. For example:
use IO::Handle;
sysopen(HANDLE, $path, O_RDWR|O_CREAT|O_EXCL)
or die "sysopen $path: $!";
$oldfh = select(HANDLE); $| = 1; select($oldfh);
print HANDLE "stuff $$\n";
seek(HANDLE, 0, 0);
print "File contains: ", <HANDLE>;
Using the constructor from the "IO::Handle" package (or one of
its subclasses, such as "IO::File" or "IO::Socket"), you can
generate anonymous filehandles that have the scope of whatever
variables hold references to them, and automatically close
whenever and however you leave that scope:
use IO::File;
#...
sub read_myfile_munged {
my $ALL = shift;
my $handle = IO::File->new;
open($handle, "myfile") or die "myfile: $!";
$first = <$handle>
or return (); # Automatically closed here.
mung $first or die "mung failed"; # Or here.
return $first, <$handle> if $ALL; # Or here.
$first; # Or here.
}
See "seek" for some details about mixing reading and writing.
opendir DIRHANDLE,EXPR
Opens a directory named EXPR for processing by "readdir",
"telldir", "seekdir", "rewinddir", and "closedir". Returns
true if successful. DIRHANDLE may be an expression whose value
can be used as an indirect dirhandle, usually the real
dirhandle name. If DIRHANDLE is an undefined scalar variable
(or array or hash element), the variable is assigned a
reference to a new anonymous dirhandle. DIRHANDLEs have their
own namespace separate from FILEHANDLEs.
See example at "readdir".
ord EXPR
ord Returns the numeric (the native 8-bit encoding, like ASCII or
EBCDIC, or Unicode) value of the first character of EXPR. If
EXPR is omitted, uses $_.
For the reverse, see "chr". See perlunicode for more about
Unicode.
our EXPR
our TYPE EXPR
our EXPR : ATTRS
our TYPE EXPR : ATTRS
"our" associates a simple name with a package variable in the
current package for use within the current scope. When "use
strict 'vars'" is in effect, "our" lets you use declared global
variables without qualifying them with package names, within
the lexical scope of the "our" declaration. In this way "our"
differs from "use vars", which is package scoped.
Unlike "my", which both allocates storage for a variable and
associates a simple name with that storage for use within the
current scope, "our" associates a simple name with a package
variable in the current package, for use within the current
scope. In other words, "our" has the same scoping rules as
"my", but does not necessarily create a variable.
If more than one value is listed, the list must be placed in
parentheses.
our $foo;
our($bar, $baz);
An "our" declaration declares a global variable that will be
visible across its entire lexical scope, even across package
boundaries. The package in which the variable is entered is
determined at the point of the declaration, not at the point of
use. This means the following behavior holds:
package Foo;
our $bar; # declares $Foo::bar for rest of lexical scope
$bar = 20;
package Bar;
print $bar; # prints 20, as it refers to $Foo::bar
Multiple "our" declarations with the same name in the same
lexical scope are allowed if they are in different packages.
If they happen to be in the same package, Perl will emit
warnings if you have asked for them, just like multiple "my"
declarations. Unlike a second "my" declaration, which will
bind the name to a fresh variable, a second "our" declaration
in the same package, in the same scope, is merely redundant.
use warnings;
package Foo;
our $bar; # declares $Foo::bar for rest of lexical scope
$bar = 20;
package Bar;
our $bar = 30; # declares $Bar::bar for rest of lexical scope
print $bar; # prints 30
our $bar; # emits warning but has no other effect
print $bar; # still prints 30
An "our" declaration may also have a list of attributes
associated with it.
The exact semantics and interface of TYPE and ATTRS are still
evolving. TYPE is currently bound to the use of "fields"
pragma, and attributes are handled using the "attributes"
pragma, or starting from Perl 5.8.0 also via the
"Attribute::Handlers" module. See "Private Variables via my()"
in perlsub for details, and fields, attributes, and
Attribute::Handlers.
pack TEMPLATE,LIST
Takes a LIST of values and converts it into a string using the
rules given by the TEMPLATE. The resulting string is the
concatenation of the converted values. Typically, each
converted value looks like its machine-level representation.
For example, on 32-bit machines an integer may be represented
by a sequence of 4 bytes that will be converted to a sequence
of 4 characters.
The TEMPLATE is a sequence of characters that give the order
and type of values, as follows:
a A string with arbitrary binary data, will be null padded.
A A text (ASCII) string, will be space padded.
Z A null terminated (ASCIZ) string, will be null padded.
b A bit string (ascending bit order inside each byte, like vec()).
B A bit string (descending bit order inside each byte).
h A hex string (low nybble first).
H A hex string (high nybble first).
c A signed char (8-bit) value.
C An unsigned char (octet) value.
W An unsigned char value (can be greater than 255).
s A signed short (16-bit) value.
S An unsigned short value.
l A signed long (32-bit) value.
L An unsigned long value.
q A signed quad (64-bit) value.
Q An unsigned quad value.
(Quads are available only if your system supports 64-bit
integer values _and_ if Perl has been compiled to support those.
Causes a fatal error otherwise.)
i A signed integer value.
I A unsigned integer value.
(This 'integer' is _at_least_ 32 bits wide. Its exact
size depends on what a local C compiler calls 'int'.)
n An unsigned short (16-bit) in "network" (big-endian) order.
N An unsigned long (32-bit) in "network" (big-endian) order.
v An unsigned short (16-bit) in "VAX" (little-endian) order.
V An unsigned long (32-bit) in "VAX" (little-endian) order.
j A Perl internal signed integer value (IV).
J A Perl internal unsigned integer value (UV).
f A single-precision float in the native format.
d A double-precision float in the native format.
F A Perl internal floating point value (NV) in the native format
D A long double-precision float in the native format.
(Long doubles are available only if your system supports long
double values _and_ if Perl has been compiled to support those.
Causes a fatal error otherwise.)
p A pointer to a null-terminated string.
P A pointer to a structure (fixed-length string).
u A uuencoded string.
U A Unicode character number. Encodes to a character in character mode
and UTF-8 (or UTF-EBCDIC in EBCDIC platforms) in byte mode.
w A BER compressed integer (not an ASN.1 BER, see perlpacktut for
details). Its bytes represent an unsigned integer in base 128,
most significant digit first, with as few digits as possible. Bit
eight (the high bit) is set on each byte except the last.
x A null byte.
X Back up a byte.
@ Null fill or truncate to absolute position, counted from the
start of the innermost ()-group.
. Null fill or truncate to absolute position specified by value.
( Start of a ()-group.
One or more of the modifiers below may optionally follow some
letters in the TEMPLATE (the second column lists the letters
for which the modifier is valid):
! sSlLiI Forces native (short, long, int) sizes instead
of fixed (16-/32-bit) sizes.
xX Make x and X act as alignment commands.
nNvV Treat integers as signed instead of unsigned.
@. Specify position as byte offset in the internal
representation of the packed string. Efficient but
dangerous.
> sSiIlLqQ Force big-endian byte-order on the type.
jJfFdDpP (The "big end" touches the construct.)
< sSiIlLqQ Force little-endian byte-order on the type.
jJfFdDpP (The "little end" touches the construct.)
The ">" and "<" modifiers can also be used on "()"-groups, in
which case they force a certain byte-order on all components of
that group, including subgroups.
The following rules apply:
· Each letter may optionally be followed by a number
giving a repeat count. With all types except "a", "A",
"Z", "b", "B", "h", "H", "@", ".", "x", "X" and "P" the
pack function will gobble up that many values from the
LIST. A "*" for the repeat count means to use however
many items are left, except for "@", "x", "X", where it
is equivalent to 0, for <.> where it means relative to
string start and "u", where it is equivalent to 1 (or
45, which is the same). A numeric repeat count may
optionally be enclosed in brackets, as in "pack
'C[80]', @arr".
One can replace the numeric repeat count by a template
enclosed in brackets; then the packed length of this
template in bytes is used as a count. For example,
"x[L]" skips a long (it skips the number of bytes in a
long); the template "$t X[$t] $t" unpack()s twice what
$t unpacks. If the template in brackets contains
alignment commands (such as "x![d]"), its packed length
is calculated as if the start of the template has the
maximal possible alignment.
When used with "Z", "*" results in the addition of a
trailing null byte (so the packed result will be one
longer than the byte "length" of the item).
When used with "@", the repeat count represents an
offset from the start of the innermost () group.
When used with ".", the repeat count is used to
determine the starting position from where the value
offset is calculated. If the repeat count is 0, it’s
relative to the current position. If the repeat count
is "*", the offset is relative to the start of the
packed string. And if its an integer "n" the offset is
relative to the start of the n-th innermost () group
(or the start of the string if "n" is bigger then the
group level).
The repeat count for "u" is interpreted as the maximal
number of bytes to encode per line of output, with 0, 1
and 2 replaced by 45. The repeat count should not be
more than 65.
· The "a", "A", and "Z" types gobble just one value, but
pack it as a string of length count, padding with nulls
or spaces as necessary. When unpacking, "A" strips
trailing whitespace and nulls, "Z" strips everything
after the first null, and "a" returns data verbatim.
If the value-to-pack is too long, it is truncated. If
too long and an explicit count is provided, "Z" packs
only "$count-1" bytes, followed by a null byte. Thus
"Z" always packs a trailing null (except when the count
is 0).
· Likewise, the "b" and "B" fields pack a string that
many bits long. Each character of the input field of
pack() generates 1 bit of the result. Each result bit
is based on the least-significant bit of the
corresponding input character, i.e., on "ord($char)%2".
In particular, characters "0" and "1" generate bits 0
and 1, as do characters "\0" and "\1".
Starting from the beginning of the input string of
pack(), each 8-tuple of characters is converted to 1
character of output. With format "b" the first
character of the 8-tuple determines the least-
significant bit of a character, and with format "B" it
determines the most-significant bit of a character.
If the length of the input string is not exactly
divisible by 8, the remainder is packed as if the input
string were padded by null characters at the end.
Similarly, during unpack()ing the "extra" bits are
ignored.
If the input string of pack() is longer than needed,
extra characters are ignored. A "*" for the repeat
count of pack() means to use all the characters of the
input field. On unpack()ing the bits are converted to
a string of "0"s and "1"s.
· The "h" and "H" fields pack a string that many nybbles
(4-bit groups, representable as hexadecimal digits,
0-9a-f) long.
Each character of the input field of pack() generates 4
bits of the result. For non-alphabetical characters
the result is based on the 4 least-significant bits of
the input character, i.e., on "ord($char)%16". In
particular, characters "0" and "1" generate nybbles 0
and 1, as do bytes "\0" and "\1". For characters
"a".."f" and "A".."F" the result is compatible with the
usual hexadecimal digits, so that "a" and "A" both
generate the nybble "0xa==10". The result for
characters "g".."z" and "G".."Z" is not well-defined.
Starting from the beginning of the input string of
pack(), each pair of characters is converted to 1
character of output. With format "h" the first
character of the pair determines the least-significant
nybble of the output character, and with format "H" it
determines the most-significant nybble.
If the length of the input string is not even, it
behaves as if padded by a null character at the end.
Similarly, during unpack()ing the "extra" nybbles are
ignored.
If the input string of pack() is longer than needed,
extra characters are ignored. A "*" for the repeat
count of pack() means to use all the characters of the
input field. On unpack()ing the nybbles are converted
to a string of hexadecimal digits.
· The "p" type packs a pointer to a null-terminated
string. You are responsible for ensuring the string is
not a temporary value (which can potentially get
deallocated before you get around to using the packed
result). The "P" type packs a pointer to a structure
of the size indicated by the length. A NULL pointer is
created if the corresponding value for "p" or "P" is
"undef", similarly for unpack().
If your system has a strange pointer size (i.e. a
pointer is neither as big as an int nor as big as a
long), it may not be possible to pack or unpack
pointers in big- or little-endian byte order.
Attempting to do so will result in a fatal error.
· The "/" template character allows packing and unpacking
of a sequence of items where the packed structure
contains a packed item count followed by the packed
items themselves.
For "pack" you write length-item"/"sequence-item and
the length-item describes how the length value is
packed. The ones likely to be of most use are integer-
packing ones like "n" (for Java strings), "w" (for
ASN.1 or SNMP) and "N" (for Sun XDR).
For "pack", the sequence-item may have a repeat count,
in which case the minimum of that and the number of
available items is used as argument for the length-
item. If it has no repeat count or uses a ’*’, the
number of available items is used.
For "unpack" an internal stack of integer arguments
unpacked so far is used. You write "/"sequence-item and
the repeat count is obtained by popping off the last
element from the stack. The sequence-item must not have
a repeat count.
If the sequence-item refers to a string type ("A", "a"
or "Z"), the length-item is a string length, not a
number of strings. If there is an explicit repeat count
for pack, the packed string will be adjusted to that
given length.
unpack 'W/a', "\04Gurusamy"; gives ('Guru')
unpack 'a3/A A*', '007 Bond J '; gives (' Bond', 'J')
unpack 'a3 x2 /A A*', '007: Bond, J.'; gives ('Bond, J', '.')
pack 'n/a* w/a','hello,','world'; gives "\000\006hello,\005world"
pack 'a/W2', ord('a') .. ord('z'); gives '2ab'
The length-item is not returned explicitly from
"unpack".
Adding a count to the length-item letter is unlikely to
do anything useful, unless that letter is "A", "a" or
"Z". Packing with a length-item of "a" or "Z" may
introduce "\000" characters, which Perl does not regard
as legal in numeric strings.
· The integer types "s", "S", "l", and "L" may be
followed by a "!" modifier to signify native shorts or
longs--as you can see from above for example a bare "l"
does mean exactly 32 bits, the native "long" (as seen
by the local C compiler) may be larger. This is an
issue mainly in 64-bit platforms. You can see whether
using "!" makes any difference by
print length(pack("s")), " ", length(pack("s!")), "\n";
print length(pack("l")), " ", length(pack("l!")), "\n";
"i!" and "I!" also work but only because of
completeness; they are identical to "i" and "I".
The actual sizes (in bytes) of native shorts, ints,
longs, and long longs on the platform where Perl was
built are also available via Config:
use Config;
print $Config{shortsize}, "\n";
print $Config{intsize}, "\n";
print $Config{longsize}, "\n";
print $Config{longlongsize}, "\n";
(The $Config{longlongsize} will be undefined if your
system does not support long longs.)
· The integer formats "s", "S", "i", "I", "l", "L", "j",
and "J" are inherently non-portable between processors
and operating systems because they obey the native
byteorder and endianness. For example a 4-byte integer
0x12345678 (305419896 decimal) would be ordered
natively (arranged in and handled by the CPU registers)
into bytes as
0x12 0x34 0x56 0x78 # big-endian
0x78 0x56 0x34 0x12 # little-endian
Basically, the Intel and VAX CPUs are little-endian,
while everybody else, for example Motorola m68k/88k,
PPC, Sparc, HP PA, Power, and Cray are big-endian.
Alpha and MIPS can be either: Digital/Compaq used/uses
them in little-endian mode; SGI/Cray uses them in big-
endian mode.
The names ‘big-endian’ and ‘little-endian’ are comic
references to the classic "Gulliver’s Travels" (via the
paper "On Holy Wars and a Plea for Peace" by Danny
Cohen, USC/ISI IEN 137, April 1, 1980) and the egg-
eating habits of the Lilliputians.
Some systems may have even weirder byte orders such as
0x56 0x78 0x12 0x34
0x34 0x12 0x78 0x56
You can see your system’s preference with
print join(" ", map { sprintf "%#02x", $_ }
unpack("W*",pack("L",0x12345678))), "\n";
The byteorder on the platform where Perl was built is
also available via Config:
use Config;
print $Config{byteorder}, "\n";
Byteorders '1234' and '12345678' are little-endian,
'4321' and '87654321' are big-endian.
If you want portable packed integers you can either use
the formats "n", "N", "v", and "V", or you can use the
">" and "<" modifiers. These modifiers are only
available as of perl 5.9.2. See also perlport.
· All integer and floating point formats as well as "p"
and "P" and "()"-groups may be followed by the ">" or
"<" modifiers to force big- or little- endian byte-
order, respectively. This is especially useful, since
"n", "N", "v" and "V" don’t cover signed integers,
64-bit integers and floating point values. However,
there are some things to keep in mind.
Exchanging signed integers between different platforms
only works if all platforms store them in the same
format. Most platforms store signed integers in two’s
complement, so usually this is not an issue.
The ">" or "<" modifiers can only be used on floating
point formats on big- or little-endian machines.
Otherwise, attempting to do so will result in a fatal
error.
Forcing big- or little-endian byte-order on floating
point values for data exchange can only work if all
platforms are using the same binary representation
(e.g. IEEE floating point format). Even if all
platforms are using IEEE, there may be subtle
differences. Being able to use ">" or "<" on floating
point values can be very useful, but also very
dangerous if you don’t know exactly what you’re doing.
It is definitely not a general way to portably store
floating point values.
When using ">" or "<" on an "()"-group, this will
affect all types inside the group that accept the byte-
order modifiers, including all subgroups. It will
silently be ignored for all other types. You are not
allowed to override the byte-order within a group that
already has a byte-order modifier suffix.
· Real numbers (floats and doubles) are in the native
machine format only; due to the multiplicity of
floating formats around, and the lack of a standard
"network" representation, no facility for interchange
has been made. This means that packed floating point
data written on one machine may not be readable on
another - even if both use IEEE floating point
arithmetic (as the endian-ness of the memory
representation is not part of the IEEE spec). See also
perlport.
If you know exactly what you’re doing, you can use the
">" or "<" modifiers to force big- or little-endian
byte-order on floating point values.
Note that Perl uses doubles (or long doubles, if
configured) internally for all numeric calculation, and
converting from double into float and thence back to
double again will lose precision (i.e., "unpack("f",
pack("f", $foo)") will not in general equal $foo).
· Pack and unpack can operate in two modes, character
mode ("C0" mode) where the packed string is processed
per character and UTF-8 mode ("U0" mode) where the
packed string is processed in its UTF-8-encoded Unicode
form on a byte by byte basis. Character mode is the
default unless the format string starts with an "U".
You can switch mode at any moment with an explicit "C0"
or "U0" in the format. A mode is in effect until the
next mode switch or until the end of the ()-group in
which it was entered.
· You must yourself do any alignment or padding by
inserting for example enough 'x'es while packing.
There is no way to pack() and unpack() could know where
the characters are going to or coming from. Therefore
"pack" (and "unpack") handle their output and input as
flat sequences of characters.
· A ()-group is a sub-TEMPLATE enclosed in parentheses.
A group may take a repeat count, both as postfix, and
for unpack() also via the "/" template character.
Within each repetition of a group, positioning with "@"
starts again at 0. Therefore, the result of
pack( '@1A((@2A)@3A)', 'a', 'b', 'c' )
is the string "\0a\0\0bc".
· "x" and "X" accept "!" modifier. In this case they act
as alignment commands: they jump forward/back to the
closest position aligned at a multiple of "count"
characters. For example, to pack() or unpack() C’s
"struct {char c; double d; char cc[2]}" one may need to
use the template "W x![d] d W[2]"; this assumes that
doubles must be aligned on the double’s size.
For alignment commands "count" of 0 is equivalent to
"count" of 1; both result in no-ops.
· "n", "N", "v" and "V" accept the "!" modifier. In this
case they will represent signed 16-/32-bit integers in
big-/little-endian order. This is only portable if all
platforms sharing the packed data use the same binary
representation for signed integers (e.g. all platforms
are using two’s complement representation).
· A comment in a TEMPLATE starts with "#" and goes to the
end of line. White space may be used to separate pack
codes from each other, but modifiers and a repeat count
must follow immediately.
· If TEMPLATE requires more arguments to pack() than
actually given, pack() assumes additional "" arguments.
If TEMPLATE requires fewer arguments to pack() than
actually given, extra arguments are ignored.
Examples:
$foo = pack("WWWW",65,66,67,68);
# foo eq "ABCD"
$foo = pack("W4",65,66,67,68);
# same thing
$foo = pack("W4",0x24b6,0x24b7,0x24b8,0x24b9);
# same thing with Unicode circled letters.
$foo = pack("U4",0x24b6,0x24b7,0x24b8,0x24b9);
# same thing with Unicode circled letters. You don't get the UTF-8
# bytes because the U at the start of the format caused a switch to
# U0-mode, so the UTF-8 bytes get joined into characters
$foo = pack("C0U4",0x24b6,0x24b7,0x24b8,0x24b9);
# foo eq "\xe2\x92\xb6\xe2\x92\xb7\xe2\x92\xb8\xe2\x92\xb9"
# This is the UTF-8 encoding of the string in the previous example
$foo = pack("ccxxcc",65,66,67,68);
# foo eq "AB\0\0CD"
# note: the above examples featuring "W" and "c" are true
# only on ASCII and ASCII-derived systems such as ISO Latin 1
# and UTF-8. In EBCDIC the first example would be
# $foo = pack("WWWW",193,194,195,196);
$foo = pack("s2",1,2);
# "\1\0\2\0" on little-endian
# "\0\1\0\2" on big-endian
$foo = pack("a4","abcd","x","y","z");
# "abcd"
$foo = pack("aaaa","abcd","x","y","z");
# "axyz"
$foo = pack("a14","abcdefg");
# "abcdefg\0\0\0\0\0\0\0"
$foo = pack("i9pl", gmtime);
# a real struct tm (on my system anyway)
$utmp_template = "Z8 Z8 Z16 L";
$utmp = pack($utmp_template, @utmp1);
# a struct utmp (BSDish)
@utmp2 = unpack($utmp_template, $utmp);
# "@utmp1" eq "@utmp2"
sub bintodec {
unpack("N", pack("B32", substr("0" x 32 . shift, -32)));
}
$foo = pack('sx2l', 12, 34);
# short 12, two zero bytes padding, long 34
$bar = pack('s@4l', 12, 34);
# short 12, zero fill to position 4, long 34
# $foo eq $bar
$baz = pack('s.l', 12, 4, 34);
# short 12, zero fill to position 4, long 34
$foo = pack('nN', 42, 4711);
# pack big-endian 16- and 32-bit unsigned integers
$foo = pack('S>L>', 42, 4711);
# exactly the same
$foo = pack('s<l<', -42, 4711);
# pack little-endian 16- and 32-bit signed integers
$foo = pack('(sl)<', -42, 4711);
# exactly the same
The same template may generally also be used in unpack().
package NAMESPACE
package Declares the compilation unit as being in the given namespace.
The scope of the package declaration is from the declaration
itself through the end of the enclosing block, file, or eval
(the same as the "my" operator). All further unqualified
dynamic identifiers will be in this namespace. A package
statement affects only dynamic variables--including those
you’ve used "local" on--but not lexical variables, which are
created with "my". Typically it would be the first declaration
in a file to be included by the "require" or "use" operator.
You can switch into a package in more than one place; it merely
influences which symbol table is used by the compiler for the
rest of that block. You can refer to variables and filehandles
in other packages by prefixing the identifier with the package
name and a double colon: $Package::Variable. If the package
name is null, the "main" package as assumed. That is, $::sail
is equivalent to $main::sail (as well as to "$main'sail", still
seen in older code).
See "Packages" in perlmod for more information about packages,
modules, and classes. See perlsub for other scoping issues.
pipe READHANDLE,WRITEHANDLE
Opens a pair of connected pipes like the corresponding system
call. Note that if you set up a loop of piped processes,
deadlock can occur unless you are very careful. In addition,
note that Perl’s pipes use IO buffering, so you may need to set
$| to flush your WRITEHANDLE after each command, depending on
the application.
See IPC::Open2, IPC::Open3, and "Bidirectional Communication"
in perlipc for examples of such things.
On systems that support a close-on-exec flag on files, the flag
will be set for the newly opened file descriptors as determined
by the value of $^F. See "$^F" in perlvar.
pop ARRAY
pop Pops and returns the last value of the array, shortening the
array by one element.
If there are no elements in the array, returns the undefined
value (although this may happen at other times as well). If
ARRAY is omitted, pops the @ARGV array in the main program, and
the @_ array in subroutines, just like "shift".
pos SCALAR
pos Returns the offset of where the last "m//g" search left off for
the variable in question ($_ is used when the variable is not
specified). Note that 0 is a valid match offset. "undef"
indicates that the search position is reset (usually due to
match failure, but can also be because no match has yet been
performed on the scalar). "pos" directly accesses the location
used by the regexp engine to store the offset, so assigning to
"pos" will change that offset, and so will also influence the
"\G" zero-width assertion in regular expressions. Because a
failed "m//gc" match doesn’t reset the offset, the return from
"pos" won’t change either in this case. See perlre and perlop.
print FILEHANDLE LIST
print LIST
print Prints a string or a list of strings. Returns true if
successful. FILEHANDLE may be a scalar variable name, in which
case the variable contains the name of or a reference to the
filehandle, thus introducing one level of indirection. (NOTE:
If FILEHANDLE is a variable and the next token is a term, it
may be misinterpreted as an operator unless you interpose a "+"
or put parentheses around the arguments.) If FILEHANDLE is
omitted, prints by default to standard output (or to the last
selected output channel--see "select"). If LIST is also
omitted, prints $_ to the currently selected output channel.
To set the default output channel to something other than
STDOUT use the select operation. The current value of $, (if
any) is printed between each LIST item. The current value of
"$\" (if any) is printed after the entire LIST has been
printed. Because print takes a LIST, anything in the LIST is
evaluated in list context, and any subroutine that you call
will have one or more of its expressions evaluated in list
context. Also be careful not to follow the print keyword with
a left parenthesis unless you want the corresponding right
parenthesis to terminate the arguments to the print--interpose
a "+" or put parentheses around all the arguments.
Note that if you’re storing FILEHANDLEs in an array, or if
you’re using any other expression more complex than a scalar
variable to retrieve it, you will have to use a block returning
the filehandle value instead:
print { $files[$i] } "stuff\n";
print { $OK ? STDOUT : STDERR } "stuff\n";
printf FILEHANDLE FORMAT, LIST
printf FORMAT, LIST
Equivalent to "print FILEHANDLE sprintf(FORMAT, LIST)", except
that "$\" (the output record separator) is not appended. The
first argument of the list will be interpreted as the "printf"
format. See "sprintf" for an explanation of the format
argument. If "use locale" is in effect, and POSIX::setlocale()
has been called, the character used for the decimal separator
in formatted floating point numbers is affected by the
LC_NUMERIC locale. See perllocale and POSIX.
Don’t fall into the trap of using a "printf" when a simple
"print" would do. The "print" is more efficient and less error
prone.
prototype FUNCTION
Returns the prototype of a function as a string (or "undef" if
the function has no prototype). FUNCTION is a reference to, or
the name of, the function whose prototype you want to retrieve.
If FUNCTION is a string starting with "CORE::", the rest is
taken as a name for Perl builtin. If the builtin is not
overridable (such as "qw//") or if its arguments cannot be
adequately expressed by a prototype (such as "system"),
prototype() returns "undef", because the builtin does not
really behave like a Perl function. Otherwise, the string
describing the equivalent prototype is returned.
push ARRAY,LIST
Treats ARRAY as a stack, and pushes the values of LIST onto the
end of ARRAY. The length of ARRAY increases by the length of
LIST. Has the same effect as
for $value (LIST) {
$ARRAY[++$#ARRAY] = $value;
}
but is more efficient. Returns the number of elements in the
array following the completed "push".
q/STRING/
qq/STRING/
qx/STRING/
qw/STRING/
Generalized quotes. See "Quote-Like Operators" in perlop.
qr/STRING/
Regexp-like quote. See "Regexp Quote-Like Operators" in
perlop.
quotemeta EXPR
quotemeta
Returns the value of EXPR with all non-"word" characters
backslashed. (That is, all characters not matching
"/[A-Za-z_0-9]/" will be preceded by a backslash in the
returned string, regardless of any locale settings.) This is
the internal function implementing the "\Q" escape in double-
quoted strings.
If EXPR is omitted, uses $_.
rand EXPR
rand Returns a random fractional number greater than or equal to 0
and less than the value of EXPR. (EXPR should be positive.)
If EXPR is omitted, the value 1 is used. Currently EXPR with
the value 0 is also special-cased as 1 - this has not been
documented before perl 5.8.0 and is subject to change in future
versions of perl. Automatically calls "srand" unless "srand"
has already been called. See also "srand".
Apply "int()" to the value returned by "rand()" if you want
random integers instead of random fractional numbers. For
example,
int(rand(10))
returns a random integer between 0 and 9, inclusive.
(Note: If your rand function consistently returns numbers that
are too large or too small, then your version of Perl was
probably compiled with the wrong number of RANDBITS.)
read FILEHANDLE,SCALAR,LENGTH,OFFSET
read FILEHANDLE,SCALAR,LENGTH
Attempts to read LENGTH characters of data into variable SCALAR
from the specified FILEHANDLE. Returns the number of
characters actually read, 0 at end of file, or undef if there
was an error (in the latter case $! is also set). SCALAR will
be grown or shrunk so that the last character actually read is
the last character of the scalar after the read.
An OFFSET may be specified to place the read data at some place
in the string other than the beginning. A negative OFFSET
specifies placement at that many characters counting backwards
from the end of the string. A positive OFFSET greater than the
length of SCALAR results in the string being padded to the
required size with "\0" bytes before the result of the read is
appended.
The call is actually implemented in terms of either Perl’s or
system’s fread() call. To get a true read(2) system call, see
"sysread".
Note the characters: depending on the status of the filehandle,
either (8-bit) bytes or characters are read. By default all
filehandles operate on bytes, but for example if the filehandle
has been opened with the ":utf8" I/O layer (see "open", and the
"open" pragma, open), the I/O will operate on UTF-8 encoded
Unicode characters, not bytes. Similarly for the ":encoding"
pragma: in that case pretty much any characters can be read.
readdir DIRHANDLE
Returns the next directory entry for a directory opened by
"opendir". If used in list context, returns all the rest of
the entries in the directory. If there are no more entries,
returns an undefined value in scalar context or a null list in
list context.
If you’re planning to filetest the return values out of a
"readdir", you’d better prepend the directory in question.
Otherwise, because we didn’t "chdir" there, it would have been
testing the wrong file.
opendir(my $dh, $some_dir) || die "can't opendir $some_dir: $!";
@dots = grep { /^\./ && -f "$some_dir/$_" } readdir($dh);
closedir $dh;
readline EXPR
readline
Reads from the filehandle whose typeglob is contained in EXPR
(or from *ARGV if EXPR is not provided). In scalar context,
each call reads and returns the next line, until end-of-file is
reached, whereupon the subsequent call returns undef. In list
context, reads until end-of-file is reached and returns a list
of lines. Note that the notion of "line" used here is however
you may have defined it with $/ or $INPUT_RECORD_SEPARATOR).
See "$/" in perlvar.
When $/ is set to "undef", when readline() is in scalar context
(i.e. file slurp mode), and when an empty file is read, it
returns '' the first time, followed by "undef" subsequently.
This is the internal function implementing the "<EXPR>"
operator, but you can use it directly. The "<EXPR>" operator
is discussed in more detail in "I/O Operators" in perlop.
$line = <STDIN>;
$line = readline(*STDIN); # same thing
If readline encounters an operating system error, $! will be
set with the corresponding error message. It can be helpful to
check $! when you are reading from filehandles you don’t trust,
such as a tty or a socket. The following example uses the
operator form of "readline", and takes the necessary steps to
ensure that "readline" was successful.
for (;;) {
undef $!;
unless (defined( $line = <> )) {
last if eof;
die $! if $!;
}
# ...
}
readlink EXPR
readlink
Returns the value of a symbolic link, if symbolic links are
implemented. If not, gives a fatal error. If there is some
system error, returns the undefined value and sets $! (errno).
If EXPR is omitted, uses $_.
readpipe EXPR
readpipe
EXPR is executed as a system command. The collected standard
output of the command is returned. In scalar context, it comes
back as a single (potentially multi-line) string. In list
context, returns a list of lines (however you’ve defined lines
with $/ or $INPUT_RECORD_SEPARATOR). This is the internal
function implementing the "qx/EXPR/" operator, but you can use
it directly. The "qx/EXPR/" operator is discussed in more
detail in "I/O Operators" in perlop. If EXPR is omitted, uses
$_.
recv SOCKET,SCALAR,LENGTH,FLAGS
Receives a message on a socket. Attempts to receive LENGTH
characters of data into variable SCALAR from the specified
SOCKET filehandle. SCALAR will be grown or shrunk to the
length actually read. Takes the same flags as the system call
of the same name. Returns the address of the sender if
SOCKET’s protocol supports this; returns an empty string
otherwise. If there’s an error, returns the undefined value.
This call is actually implemented in terms of recvfrom(2)
system call. See "UDP: Message Passing" in perlipc for
examples.
Note the characters: depending on the status of the socket,
either (8-bit) bytes or characters are received. By default
all sockets operate on bytes, but for example if the socket has
been changed using binmode() to operate with the
":encoding(utf8)" I/O layer (see the "open" pragma, open), the
I/O will operate on UTF-8 encoded Unicode characters, not
bytes. Similarly for the ":encoding" pragma: in that case
pretty much any characters can be read.
redo LABEL
redo The "redo" command restarts the loop block without evaluating
the conditional again. The "continue" block, if any, is not
executed. If the LABEL is omitted, the command refers to the
innermost enclosing loop. Programs that want to lie to
themselves about what was just input normally use this command:
# a simpleminded Pascal comment stripper
# (warning: assumes no { or } in strings)
LINE: while (<STDIN>) {
while (s|({.*}.*){.*}|$1 |) {}
s|{.*}| |;
if (s|{.*| |) {
$front = $_;
while (<STDIN>) {
if (/}/) { # end of comment?
s|^|$front\{|;
redo LINE;
}
}
}
print;
}
"redo" cannot be used to retry a block which returns a value
such as "eval {}", "sub {}" or "do {}", and should not be used
to exit a grep() or map() operation.
Note that a block by itself is semantically identical to a loop
that executes once. Thus "redo" inside such a block will
effectively turn it into a looping construct.
See also "continue" for an illustration of how "last", "next",
and "redo" work.
ref EXPR
ref Returns a non-empty string if EXPR is a reference, the empty
string otherwise. If EXPR is not specified, $_ will be used.
The value returned depends on the type of thing the reference
is a reference to. Builtin types include:
SCALAR
ARRAY
HASH
CODE
REF
GLOB
LVALUE
FORMAT
IO
VSTRING
Regexp
If the referenced object has been blessed into a package, then
that package name is returned instead. You can think of "ref"
as a "typeof" operator.
if (ref($r) eq "HASH") {
print "r is a reference to a hash.\n";
}
unless (ref($r)) {
print "r is not a reference at all.\n";
}
The return value "LVALUE" indicates a reference to an lvalue
that is not a variable. You get this from taking the reference
of function calls like "pos()" or "substr()". "VSTRING" is
returned if the reference points to a version string.
The result "Regexp" indicates that the argument is a regular
expression resulting from "qr//".
See also perlref.
rename OLDNAME,NEWNAME
Changes the name of a file; an existing file NEWNAME will be
clobbered. Returns true for success, false otherwise.
Behavior of this function varies wildly depending on your
system implementation. For example, it will usually not work
across file system boundaries, even though the system mv
command sometimes compensates for this. Other restrictions
include whether it works on directories, open files, or pre-
existing files. Check perlport and either the rename(2)
manpage or equivalent system documentation for details.
For a platform independent "move" function look at the
File::Copy module.
require VERSION
require EXPR
require Demands a version of Perl specified by VERSION, or demands some
semantics specified by EXPR or by $_ if EXPR is not supplied.
VERSION may be either a numeric argument such as 5.006, which
will be compared to $], or a literal of the form v5.6.1, which
will be compared to $^V (aka $PERL_VERSION). A fatal error is
produced at run time if VERSION is greater than the version of
the current Perl interpreter. Compare with "use", which can do
a similar check at compile time.
Specifying VERSION as a literal of the form v5.6.1 should
generally be avoided, because it leads to misleading error
messages under earlier versions of Perl that do not support
this syntax. The equivalent numeric version should be used
instead.
require v5.6.1; # run time version check
require 5.6.1; # ditto
require 5.006_001; # ditto; preferred for backwards compatibility
Otherwise, "require" demands that a library file be included if
it hasn’t already been included. The file is included via the
do-FILE mechanism, which is essentially just a variety of
"eval" with the caveat that lexical variables in the invoking
script will be invisible to the included code. Has semantics
similar to the following subroutine:
sub require {
my ($filename) = @_;
if (exists $INC{$filename}) {
return 1 if $INC{$filename};
die "Compilation failed in require";
}
my ($realfilename,$result);
ITER: {
foreach $prefix (@INC) {
$realfilename = "$prefix/$filename";
if (-f $realfilename) {
$INC{$filename} = $realfilename;
$result = do $realfilename;
last ITER;
}
}
die "Can't find $filename in \@INC";
}
if ($@) {
$INC{$filename} = undef;
die $@;
} elsif (!$result) {
delete $INC{$filename};
die "$filename did not return true value";
} else {
return $result;
}
}
Note that the file will not be included twice under the same
specified name.
The file must return true as the last statement to indicate
successful execution of any initialization code, so it’s
customary to end such a file with "1;" unless you’re sure it’ll
return true otherwise. But it’s better just to put the "1;",
in case you add more statements.
If EXPR is a bareword, the require assumes a ".pm" extension
and replaces "::" with "/" in the filename for you, to make it
easy to load standard modules. This form of loading of modules
does not risk altering your namespace.
In other words, if you try this:
require Foo::Bar; # a splendid bareword
The require function will actually look for the "Foo/Bar.pm"
file in the directories specified in the @INC array.
But if you try this:
$class = 'Foo::Bar';
require $class; # $class is not a bareword
#or
require "Foo::Bar"; # not a bareword because of the ""
The require function will look for the "Foo::Bar" file in the
@INC array and will complain about not finding "Foo::Bar"
there. In this case you can do:
eval "require $class";
Now that you understand how "require" looks for files in the
case of a bareword argument, there is a little extra
functionality going on behind the scenes. Before "require"
looks for a ".pm" extension, it will first look for a similar
filename with a ".pmc" extension. If this file is found, it
will be loaded in place of any file ending in a ".pm"
extension.
You can also insert hooks into the import facility, by putting
directly Perl code into the @INC array. There are three forms
of hooks: subroutine references, array references and blessed
objects.
Subroutine references are the simplest case. When the
inclusion system walks through @INC and encounters a
subroutine, this subroutine gets called with two parameters,
the first being a reference to itself, and the second the name
of the file to be included (e.g. "Foo/Bar.pm"). The subroutine
should return nothing, or a list of up to three values in the
following order:
1. A filehandle, from which the file will be read.
2. A reference to a subroutine. If there is no filehandle
(previous item), then this subroutine is expected to
generate one line of source code per call, writing the line
into $_ and returning 1, then returning 0 at "end of file".
If there is a filehandle, then the subroutine will be
called to act as a simple source filter, with the line as
read in $_. Again, return 1 for each valid line, and 0
after all lines have been returned.
3. Optional state for the subroutine. The state is passed in
as $_[1]. A reference to the subroutine itself is passed in
as $_[0].
If an empty list, "undef", or nothing that matches the first 3
values above is returned then "require" will look at the
remaining elements of @INC. Note that this file handle must be
a real file handle (strictly a typeglob, or reference to a
typeglob, blessed or unblessed) - tied file handles will be
ignored and return value processing will stop there.
If the hook is an array reference, its first element must be a
subroutine reference. This subroutine is called as above, but
the first parameter is the array reference. This enables to
pass indirectly some arguments to the subroutine.
In other words, you can write:
push @INC, \&my_sub;
sub my_sub {
my ($coderef, $filename) = @_; # $coderef is \&my_sub
...
}
or:
push @INC, [ \&my_sub, $x, $y, ... ];
sub my_sub {
my ($arrayref, $filename) = @_;
# Retrieve $x, $y, ...
my @parameters = @$arrayref[1..$#$arrayref];
...
}
If the hook is an object, it must provide an INC method that
will be called as above, the first parameter being the object
itself. (Note that you must fully qualify the sub’s name, as
unqualified "INC" is always forced into package "main".) Here
is a typical code layout:
# In Foo.pm
package Foo;
sub new { ... }
sub Foo::INC {
my ($self, $filename) = @_;
...
}
# In the main program
push @INC, Foo->new(...);
Note that these hooks are also permitted to set the %INC entry
corresponding to the files they have loaded. See "%INC" in
perlvar.
For a yet-more-powerful import facility, see "use" and perlmod.
reset EXPR
reset Generally used in a "continue" block at the end of a loop to
clear variables and reset "??" searches so that they work
again. The expression is interpreted as a list of single
characters (hyphens allowed for ranges). All variables and
arrays beginning with one of those letters are reset to their
pristine state. If the expression is omitted, one-match
searches ("?pattern?") are reset to match again. Resets only
variables or searches in the current package. Always returns
1. Examples:
reset 'X'; # reset all X variables
reset 'a-z'; # reset lower case variables
reset; # just reset ?one-time? searches
Resetting "A-Z" is not recommended because you’ll wipe out your
@ARGV and @INC arrays and your %ENV hash. Resets only package
variables--lexical variables are unaffected, but they clean
themselves up on scope exit anyway, so you’ll probably want to
use them instead. See "my".
return EXPR
return Returns from a subroutine, "eval", or "do FILE" with the value
given in EXPR. Evaluation of EXPR may be in list, scalar, or
void context, depending on how the return value will be used,
and the context may vary from one execution to the next (see
"wantarray"). If no EXPR is given, returns an empty list in
list context, the undefined value in scalar context, and (of
course) nothing at all in a void context.
(Note that in the absence of an explicit "return", a
subroutine, eval, or do FILE will automatically return the
value of the last expression evaluated.)
reverse LIST
In list context, returns a list value consisting of the
elements of LIST in the opposite order. In scalar context,
concatenates the elements of LIST and returns a string value
with all characters in the opposite order.
print join(", ", reverse "world", "Hello"); # Hello, world
print scalar reverse "dlrow ,", "olleH"; # Hello, world
Used without arguments in scalar context, reverse() reverses
$_.
$_ = "dlrow ,olleH";
print reverse; # No output, list context
print scalar reverse; # Hello, world
This operator is also handy for inverting a hash, although
there are some caveats. If a value is duplicated in the
original hash, only one of those can be represented as a key in
the inverted hash. Also, this has to unwind one hash and build
a whole new one, which may take some time on a large hash, such
as from a DBM file.
%by_name = reverse %by_address; # Invert the hash
rewinddir DIRHANDLE
Sets the current position to the beginning of the directory for
the "readdir" routine on DIRHANDLE.
rindex STR,SUBSTR,POSITION
rindex STR,SUBSTR
Works just like index() except that it returns the position of
the last occurrence of SUBSTR in STR. If POSITION is
specified, returns the last occurrence beginning at or before
that position.
rmdir FILENAME
rmdir Deletes the directory specified by FILENAME if that directory
is empty. If it succeeds it returns true, otherwise it returns
false and sets $! (errno). If FILENAME is omitted, uses $_.
To remove a directory tree recursively ("rm -rf" on unix) look
at the "rmtree" function of the File::Path module.
s/// The substitution operator. See "Regexp Quote-Like Operators"
in perlop.
say FILEHANDLE LIST
say LIST
say Just like "print", but implicitly appends a newline. "say
LIST" is simply an abbreviation for "{ local $\ = "\n"; print
LIST }".
This keyword is only available when the "say" feature is
enabled: see feature.
scalar EXPR
Forces EXPR to be interpreted in scalar context and returns the
value of EXPR.
@counts = ( scalar @a, scalar @b, scalar @c );
There is no equivalent operator to force an expression to be
interpolated in list context because in practice, this is never
needed. If you really wanted to do so, however, you could use
the construction "@{[ (some expression) ]}", but usually a
simple "(some expression)" suffices.
Because "scalar" is unary operator, if you accidentally use for
EXPR a parenthesized list, this behaves as a scalar comma
expression, evaluating all but the last element in void context
and returning the final element evaluated in scalar context.
This is seldom what you want.
The following single statement:
print uc(scalar(&foo,$bar)),$baz;
is the moral equivalent of these two:
&foo;
print(uc($bar),$baz);
See perlop for more details on unary operators and the comma
operator.
seek FILEHANDLE,POSITION,WHENCE
Sets FILEHANDLE’s position, just like the "fseek" call of
"stdio". FILEHANDLE may be an expression whose value gives the
name of the filehandle. The values for WHENCE are 0 to set the
new position in bytes to POSITION, 1 to set it to the current
position plus POSITION, and 2 to set it to EOF plus POSITION
(typically negative). For WHENCE you may use the constants
"SEEK_SET", "SEEK_CUR", and "SEEK_END" (start of the file,
current position, end of the file) from the Fcntl module.
Returns 1 upon success, 0 otherwise.
Note the in bytes: even if the filehandle has been set to
operate on characters (for example by using the
":encoding(utf8)" open layer), tell() will return byte offsets,
not character offsets (because implementing that would render
seek() and tell() rather slow).
If you want to position file for "sysread" or "syswrite", don’t
use "seek"--buffering makes its effect on the file’s system
position unpredictable and non-portable. Use "sysseek"
instead.
Due to the rules and rigors of ANSI C, on some systems you have
to do a seek whenever you switch between reading and writing.
Amongst other things, this may have the effect of calling
stdio’s clearerr(3). A WHENCE of 1 ("SEEK_CUR") is useful for
not moving the file position:
seek(TEST,0,1);
This is also useful for applications emulating "tail -f". Once
you hit EOF on your read, and then sleep for a while, you might
have to stick in a seek() to reset things. The "seek" doesn’t
change the current position, but it does clear the end-of-file
condition on the handle, so that the next "<FILE>" makes Perl
try again to read something. We hope.
If that doesn’t work (some IO implementations are particularly
cantankerous), then you may need something more like this:
for (;;) {
for ($curpos = tell(FILE); $_ = <FILE>;
$curpos = tell(FILE)) {
# search for some stuff and put it into files
}
sleep($for_a_while);
seek(FILE, $curpos, 0);
}
seekdir DIRHANDLE,POS
Sets the current position for the "readdir" routine on
DIRHANDLE. POS must be a value returned by "telldir".
"seekdir" also has the same caveats about possible directory
compaction as the corresponding system library routine.
select FILEHANDLE
select Returns the currently selected filehandle. If FILEHANDLE is
supplied, sets the new current default filehandle for output.
This has two effects: first, a "write" or a "print" without a
filehandle will default to this FILEHANDLE. Second, references
to variables related to output will refer to this output
channel. For example, if you have to set the top of form
format for more than one output channel, you might do the
following:
select(REPORT1);
$^ = 'report1_top';
select(REPORT2);
$^ = 'report2_top';
FILEHANDLE may be an expression whose value gives the name of
the actual filehandle. Thus:
$oldfh = select(STDERR); $| = 1; select($oldfh);
Some programmers may prefer to think of filehandles as objects
with methods, preferring to write the last example as:
use IO::Handle;
STDERR->autoflush(1);
select RBITS,WBITS,EBITS,TIMEOUT
This calls the select(2) system call with the bit masks
specified, which can be constructed using "fileno" and "vec",
along these lines:
$rin = $win = $ein = '';
vec($rin,fileno(STDIN),1) = 1;
vec($win,fileno(STDOUT),1) = 1;
$ein = $rin | $win;
If you want to select on many filehandles you might wish to
write a subroutine:
sub fhbits {
my(@fhlist) = split(' ',$_[0]);
my($bits);
for (@fhlist) {
vec($bits,fileno($_),1) = 1;
}
$bits;
}
$rin = fhbits('STDIN TTY SOCK');
The usual idiom is:
($nfound,$timeleft) =
select($rout=$rin, $wout=$win, $eout=$ein, $timeout);
or to block until something becomes ready just do this
$nfound = select($rout=$rin, $wout=$win, $eout=$ein, undef);
Most systems do not bother to return anything useful in
$timeleft, so calling select() in scalar context just returns
$nfound.
Any of the bit masks can also be undef. The timeout, if
specified, is in seconds, which may be fractional. Note: not
all implementations are capable of returning the $timeleft. If
not, they always return $timeleft equal to the supplied
$timeout.
You can effect a sleep of 250 milliseconds this way:
select(undef, undef, undef, 0.25);
Note that whether "select" gets restarted after signals (say,
SIGALRM) is implementation-dependent. See also perlport for
notes on the portability of "select".
On error, "select" behaves like the select(2) system call : it
returns -1 and sets $!.
Note: on some Unixes, the select(2) system call may report a
socket file descriptor as "ready for reading", when actually no
data is available, thus a subsequent read blocks. It can be
avoided using always the O_NONBLOCK flag on the socket. See
select(2) and fcntl(2) for further details.
WARNING: One should not attempt to mix buffered I/O (like
"read" or <FH>) with "select", except as permitted by POSIX,
and even then only on POSIX systems. You have to use "sysread"
instead.
semctl ID,SEMNUM,CMD,ARG
Calls the System V IPC function "semctl". You’ll probably have
to say
use IPC::SysV;
first to get the correct constant definitions. If CMD is
IPC_STAT or GETALL, then ARG must be a variable that will hold
the returned semid_ds structure or semaphore value array.
Returns like "ioctl": the undefined value for error, ""0 but
true"" for zero, or the actual return value otherwise. The ARG
must consist of a vector of native short integers, which may be
created with "pack("s!",(0)x$nsem)". See also "SysV IPC" in
perlipc, "IPC::SysV", "IPC::Semaphore" documentation.
semget KEY,NSEMS,FLAGS
Calls the System V IPC function semget. Returns the semaphore
id, or the undefined value if there is an error. See also
"SysV IPC" in perlipc, "IPC::SysV", "IPC::SysV::Semaphore"
documentation.
semop KEY,OPSTRING
Calls the System V IPC function semop to perform semaphore
operations such as signalling and waiting. OPSTRING must be a
packed array of semop structures. Each semop structure can be
generated with "pack("s!3", $semnum, $semop, $semflag)". The
length of OPSTRING implies the number of semaphore operations.
Returns true if successful, or false if there is an error. As
an example, the following code waits on semaphore $semnum of
semaphore id $semid:
$semop = pack("s!3", $semnum, -1, 0);
die "Semaphore trouble: $!\n" unless semop($semid, $semop);
To signal the semaphore, replace "-1" with 1. See also "SysV
IPC" in perlipc, "IPC::SysV", and "IPC::SysV::Semaphore"
documentation.
send SOCKET,MSG,FLAGS,TO
send SOCKET,MSG,FLAGS
Sends a message on a socket. Attempts to send the scalar MSG
to the SOCKET filehandle. Takes the same flags as the system
call of the same name. On unconnected sockets you must specify
a destination to send TO, in which case it does a C "sendto".
Returns the number of characters sent, or the undefined value
if there is an error. The C system call sendmsg(2) is
currently unimplemented. See "UDP: Message Passing" in perlipc
for examples.
Note the characters: depending on the status of the socket,
either (8-bit) bytes or characters are sent. By default all
sockets operate on bytes, but for example if the socket has
been changed using binmode() to operate with the
":encoding(utf8)" I/O layer (see "open", or the "open" pragma,
open), the I/O will operate on UTF-8 encoded Unicode
characters, not bytes. Similarly for the ":encoding" pragma:
in that case pretty much any characters can be sent.
setpgrp PID,PGRP
Sets the current process group for the specified PID, 0 for the
current process. Will produce a fatal error if used on a
machine that doesn’t implement POSIX setpgid(2) or BSD
setpgrp(2). If the arguments are omitted, it defaults to
"0,0". Note that the BSD 4.2 version of "setpgrp" does not
accept any arguments, so only "setpgrp(0,0)" is portable. See
also "POSIX::setsid()".
setpriority WHICH,WHO,PRIORITY
Sets the current priority for a process, a process group, or a
user. (See setpriority(2).) Will produce a fatal error if
used on a machine that doesn’t implement setpriority(2).
setsockopt SOCKET,LEVEL,OPTNAME,OPTVAL
Sets the socket option requested. Returns undefined if there
is an error. Use integer constants provided by the "Socket"
module for LEVEL and OPNAME. Values for LEVEL can also be
obtained from getprotobyname. OPTVAL might either be a packed
string or an integer. An integer OPTVAL is shorthand for
pack("i", OPTVAL).
An example disabling the Nagle’s algorithm for a socket:
use Socket qw(IPPROTO_TCP TCP_NODELAY);
setsockopt($socket, IPPROTO_TCP, TCP_NODELAY, 1);
shift ARRAY
shift Shifts the first value of the array off and returns it,
shortening the array by 1 and moving everything down. If there
are no elements in the array, returns the undefined value. If
ARRAY is omitted, shifts the @_ array within the lexical scope
of subroutines and formats, and the @ARGV array outside of a
subroutine and also within the lexical scopes established by
the "eval STRING", "BEGIN {}", "INIT {}", "CHECK {}",
"UNITCHECK {}" and "END {}" constructs.
See also "unshift", "push", and "pop". "shift" and "unshift"
do the same thing to the left end of an array that "pop" and
"push" do to the right end.
shmctl ID,CMD,ARG
Calls the System V IPC function shmctl. You’ll probably have
to say
use IPC::SysV;
first to get the correct constant definitions. If CMD is
"IPC_STAT", then ARG must be a variable that will hold the
returned "shmid_ds" structure. Returns like ioctl: the
undefined value for error, "0 but true" for zero, or the actual
return value otherwise. See also "SysV IPC" in perlipc and
"IPC::SysV" documentation.
shmget KEY,SIZE,FLAGS
Calls the System V IPC function shmget. Returns the shared
memory segment id, or the undefined value if there is an error.
See also "SysV IPC" in perlipc and "IPC::SysV" documentation.
shmread ID,VAR,POS,SIZE
shmwrite ID,STRING,POS,SIZE
Reads or writes the System V shared memory segment ID starting
at position POS for size SIZE by attaching to it, copying
in/out, and detaching from it. When reading, VAR must be a
variable that will hold the data read. When writing, if STRING
is too long, only SIZE bytes are used; if STRING is too short,
nulls are written to fill out SIZE bytes. Return true if
successful, or false if there is an error. shmread() taints
the variable. See also "SysV IPC" in perlipc, "IPC::SysV"
documentation, and the "IPC::Shareable" module from CPAN.
shutdown SOCKET,HOW
Shuts down a socket connection in the manner indicated by HOW,
which has the same interpretation as in the system call of the
same name.
shutdown(SOCKET, 0); # I/we have stopped reading data
shutdown(SOCKET, 1); # I/we have stopped writing data
shutdown(SOCKET, 2); # I/we have stopped using this socket
This is useful with sockets when you want to tell the other
side you’re done writing but not done reading, or vice versa.
It’s also a more insistent form of close because it also
disables the file descriptor in any forked copies in other
processes.
Returns 1 for success. In the case of error, returns "undef" if
the first argument is not a valid filehandle, or returns 0 and
sets $! for any other failure.
sin EXPR
sin Returns the sine of EXPR (expressed in radians). If EXPR is
omitted, returns sine of $_.
For the inverse sine operation, you may use the
"Math::Trig::asin" function, or use this relation:
sub asin { atan2($_[0], sqrt(1 - $_[0] * $_[0])) }
sleep EXPR
sleep Causes the script to sleep for EXPR seconds, or forever if no
EXPR. Returns the number of seconds actually slept.
May be interrupted if the process receives a signal such as
"SIGALRM".
eval {
local $SIG{ALARM} = sub { die "Alarm!\n" };
sleep;
};
die $@ unless $@ eq "Alarm!\n";
You probably cannot mix "alarm" and "sleep" calls, because
"sleep" is often implemented using "alarm".
On some older systems, it may sleep up to a full second less
than what you requested, depending on how it counts seconds.
Most modern systems always sleep the full amount. They may
appear to sleep longer than that, however, because your process
might not be scheduled right away in a busy multitasking
system.
For delays of finer granularity than one second, the
Time::HiRes module (from CPAN, and starting from Perl 5.8 part
of the standard distribution) provides usleep(). You may also
use Perl’s four-argument version of select() leaving the first
three arguments undefined, or you might be able to use the
"syscall" interface to access setitimer(2) if your system
supports it. See perlfaq8 for details.
See also the POSIX module’s "pause" function.
socket SOCKET,DOMAIN,TYPE,PROTOCOL
Opens a socket of the specified kind and attaches it to
filehandle SOCKET. DOMAIN, TYPE, and PROTOCOL are specified
the same as for the system call of the same name. You should
"use Socket" first to get the proper definitions imported. See
the examples in "Sockets: Client/Server Communication" in
perlipc.
On systems that support a close-on-exec flag on files, the flag
will be set for the newly opened file descriptor, as determined
by the value of $^F. See "$^F" in perlvar.
socketpair SOCKET1,SOCKET2,DOMAIN,TYPE,PROTOCOL
Creates an unnamed pair of sockets in the specified domain, of
the specified type. DOMAIN, TYPE, and PROTOCOL are specified
the same as for the system call of the same name. If
unimplemented, yields a fatal error. Returns true if
successful.
On systems that support a close-on-exec flag on files, the flag
will be set for the newly opened file descriptors, as
determined by the value of $^F. See "$^F" in perlvar.
Some systems defined "pipe" in terms of "socketpair", in which
a call to "pipe(Rdr, Wtr)" is essentially:
use Socket;
socketpair(Rdr, Wtr, AF_UNIX, SOCK_STREAM, PF_UNSPEC);
shutdown(Rdr, 1); # no more writing for reader
shutdown(Wtr, 0); # no more reading for writer
See perlipc for an example of socketpair use. Perl 5.8 and
later will emulate socketpair using IP sockets to localhost if
your system implements sockets but not socketpair.
sort SUBNAME LIST
sort BLOCK LIST
sort LIST
In list context, this sorts the LIST and returns the sorted
list value. In scalar context, the behaviour of "sort()" is
undefined.
If SUBNAME or BLOCK is omitted, "sort"s in standard string
comparison order. If SUBNAME is specified, it gives the name
of a subroutine that returns an integer less than, equal to, or
greater than 0, depending on how the elements of the list are
to be ordered. (The "<=>" and "cmp" operators are extremely
useful in such routines.) SUBNAME may be a scalar variable
name (unsubscripted), in which case the value provides the name
of (or a reference to) the actual subroutine to use. In place
of a SUBNAME, you can provide a BLOCK as an anonymous, in-line
sort subroutine.
If the subroutine’s prototype is "($$)", the elements to be
compared are passed by reference in @_, as for a normal
subroutine. This is slower than unprototyped subroutines,
where the elements to be compared are passed into the
subroutine as the package global variables $a and $b (see
example below). Note that in the latter case, it is usually
counter-productive to declare $a and $b as lexicals.
The values to be compared are always passed by reference and
should not be modified.
You also cannot exit out of the sort block or subroutine using
any of the loop control operators described in perlsyn or with
"goto".
When "use locale" is in effect, "sort LIST" sorts LIST
according to the current collation locale. See perllocale.
sort() returns aliases into the original list, much as a for
loop’s index variable aliases the list elements. That is,
modifying an element of a list returned by sort() (for example,
in a "foreach", "map" or "grep") actually modifies the element
in the original list. This is usually something to be avoided
when writing clear code.
Perl 5.6 and earlier used a quicksort algorithm to implement
sort. That algorithm was not stable, and could go quadratic.
(A stable sort preserves the input order of elements that
compare equal. Although quicksort’s run time is O(NlogN) when
averaged over all arrays of length N, the time can be O(N**2),
quadratic behavior, for some inputs.) In 5.7, the quicksort
implementation was replaced with a stable mergesort algorithm
whose worst-case behavior is O(NlogN). But benchmarks
indicated that for some inputs, on some platforms, the original
quicksort was faster. 5.8 has a sort pragma for limited
control of the sort. Its rather blunt control of the
underlying algorithm may not persist into future Perls, but the
ability to characterize the input or output in implementation
independent ways quite probably will. See the sort pragma.
Examples:
# sort lexically
@articles = sort @files;
# same thing, but with explicit sort routine
@articles = sort {$a cmp $b} @files;
# now case-insensitively
@articles = sort {uc($a) cmp uc($b)} @files;
# same thing in reversed order
@articles = sort {$b cmp $a} @files;
# sort numerically ascending
@articles = sort {$a <=> $b} @files;
# sort numerically descending
@articles = sort {$b <=> $a} @files;
# this sorts the %age hash by value instead of key
# using an in-line function
@eldest = sort { $age{$b} <=> $age{$a} } keys %age;
# sort using explicit subroutine name
sub byage {
$age{$a} <=> $age{$b}; # presuming numeric
}
@sortedclass = sort byage @class;
sub backwards { $b cmp $a }
@harry = qw(dog cat x Cain Abel);
@george = qw(gone chased yz Punished Axed);
print sort @harry;
# prints AbelCaincatdogx
print sort backwards @harry;
# prints xdogcatCainAbel
print sort @george, 'to', @harry;
# prints AbelAxedCainPunishedcatchaseddoggonetoxyz
# inefficiently sort by descending numeric compare using
# the first integer after the first = sign, or the
# whole record case-insensitively otherwise
@new = sort {
($b =~ /=(\d+)/)[0] <=> ($a =~ /=(\d+)/)[0]
||
uc($a) cmp uc($b)
} @old;
# same thing, but much more efficiently;
# we'll build auxiliary indices instead
# for speed
@nums = @caps = ();
for (@old) {
push @nums, /=(\d+)/;
push @caps, uc($_);
}
@new = @old[ sort {
$nums[$b] <=> $nums[$a]
||
$caps[$a] cmp $caps[$b]
} 0..$#old
];
# same thing, but without any temps
@new = map { $_->[0] }
sort { $b->[1] <=> $a->[1]
||
$a->[2] cmp $b->[2]
} map { [$_, /=(\d+)/, uc($_)] } @old;
# using a prototype allows you to use any comparison subroutine
# as a sort subroutine (including other package's subroutines)
package other;
sub backwards ($$) { $_[1] cmp $_[0]; } # $a and $b are not set here
package main;
@new = sort other::backwards @old;
# guarantee stability, regardless of algorithm
use sort 'stable';
@new = sort { substr($a, 3, 5) cmp substr($b, 3, 5) } @old;
# force use of mergesort (not portable outside Perl 5.8)
use sort '_mergesort'; # note discouraging _
@new = sort { substr($a, 3, 5) cmp substr($b, 3, 5) } @old;
Warning: syntactical care is required when sorting the list
returned from a function. If you want to sort the list returned
by the function call "find_records(@key)", you can use:
@contact = sort { $a cmp $b } find_records @key;
@contact = sort +find_records(@key);
@contact = sort &find_records(@key);
@contact = sort(find_records(@key));
If instead you want to sort the array @key with the comparison
routine "find_records()" then you can use:
@contact = sort { find_records() } @key;
@contact = sort find_records(@key);
@contact = sort(find_records @key);
@contact = sort(find_records (@key));
If you’re using strict, you must not declare $a and $b as
lexicals. They are package globals. That means that if you’re
in the "main" package and type
@articles = sort {$b <=> $a} @files;
then $a and $b are $main::a and $main::b (or $::a and $::b),
but if you’re in the "FooPack" package, it’s the same as typing
@articles = sort {$FooPack::b <=> $FooPack::a} @files;
The comparison function is required to behave. If it returns
inconsistent results (sometimes saying $x[1] is less than $x[2]
and sometimes saying the opposite, for example) the results are
not well-defined.
Because "<=>" returns "undef" when either operand is "NaN"
(not-a-number), and because "sort" will trigger a fatal error
unless the result of a comparison is defined, when sorting with
a comparison function like "$a <=> $b", be careful about lists
that might contain a "NaN". The following example takes
advantage of the fact that "NaN != NaN" to eliminate any "NaN"s
from the input.
@result = sort { $a <=> $b } grep { $_ == $_ } @input;
splice ARRAY,OFFSET,LENGTH,LIST
splice ARRAY,OFFSET,LENGTH
splice ARRAY,OFFSET
splice ARRAY
Removes the elements designated by OFFSET and LENGTH from an
array, and replaces them with the elements of LIST, if any. In
list context, returns the elements removed from the array. In
scalar context, returns the last element removed, or "undef" if
no elements are removed. The array grows or shrinks as
necessary. If OFFSET is negative then it starts that far from
the end of the array. If LENGTH is omitted, removes everything
from OFFSET onward. If LENGTH is negative, removes the
elements from OFFSET onward except for -LENGTH elements at the
end of the array. If both OFFSET and LENGTH are omitted,
removes everything. If OFFSET is past the end of the array,
perl issues a warning, and splices at the end of the array.
The following equivalences hold (assuming "$[ == 0 and $#a >=
$i" )
push(@a,$x,$y) splice(@a,@a,0,$x,$y)
pop(@a) splice(@a,-1)
shift(@a) splice(@a,0,1)
unshift(@a,$x,$y) splice(@a,0,0,$x,$y)
$a[$i] = $y splice(@a,$i,1,$y)
Example, assuming array lengths are passed before arrays:
sub aeq { # compare two list values
my(@a) = splice(@_,0,shift);
my(@b) = splice(@_,0,shift);
return 0 unless @a == @b; # same len?
while (@a) {
return 0 if pop(@a) ne pop(@b);
}
return 1;
}
if (&aeq($len,@foo[1..$len],0+@bar,@bar)) { ... }
split /PATTERN/,EXPR,LIMIT
split /PATTERN/,EXPR
split /PATTERN/
split Splits the string EXPR into a list of strings and returns that
list. By default, empty leading fields are preserved, and
empty trailing ones are deleted. (If all fields are empty,
they are considered to be trailing.)
In scalar context, returns the number of fields found. In
scalar and void context it splits into the @_ array. Use of
split in scalar and void context is deprecated, however,
because it clobbers your subroutine arguments.
If EXPR is omitted, splits the $_ string. If PATTERN is also
omitted, splits on whitespace (after skipping any leading
whitespace). Anything matching PATTERN is taken to be a
delimiter separating the fields. (Note that the delimiter may
be longer than one character.)
If LIMIT is specified and positive, it represents the maximum
number of fields the EXPR will be split into, though the actual
number of fields returned depends on the number of times
PATTERN matches within EXPR. If LIMIT is unspecified or zero,
trailing null fields are stripped (which potential users of
"pop" would do well to remember). If LIMIT is negative, it is
treated as if an arbitrarily large LIMIT had been specified.
Note that splitting an EXPR that evaluates to the empty string
always returns the empty list, regardless of the LIMIT
specified.
A pattern matching the null string (not to be confused with a
null pattern "//", which is just one member of the set of
patterns matching a null string) will split the value of EXPR
into separate characters at each point it matches that way.
For example:
print join(':', split(/ */, 'hi there')), "\n";
produces the output ’h:i:t:h:e:r:e’.
As a special case for "split", using the empty pattern "//"
specifically matches only the null string, and is not be
confused with the regular use of "//" to mean "the last
successful pattern match". So, for "split", the following:
print join(':', split(//, 'hi there')), "\n";
produces the output ’h:i: :t:h:e:r:e’.
Empty leading fields are produced when there are positive-width
matches at the beginning of the string; a zero-width match at
the beginning of the string does not produce an empty field.
For example:
print join(':', split(/(?=\w)/, 'hi there!'));
produces the output ’h:i :t:h:e:r:e!’. Empty trailing fields,
on the other hand, are produced when there is a match at the
end of the string (and when LIMIT is given and is not 0),
regardless of the length of the match. For example:
print join(':', split(//, 'hi there!', -1)), "\n";
print join(':', split(/\W/, 'hi there!', -1)), "\n";
produce the output ’h:i: :t:h:e:r:e:!:’ and ’hi:there:’,
respectively, both with an empty trailing field.
The LIMIT parameter can be used to split a line partially
($login, $passwd, $remainder) = split(/:/, $_, 3);
When assigning to a list, if LIMIT is omitted, or zero, Perl
supplies a LIMIT one larger than the number of variables in the
list, to avoid unnecessary work. For the list above LIMIT
would have been 4 by default. In time critical applications it
behooves you not to split into more fields than you really
need.
If the PATTERN contains parentheses, additional list elements
are created from each matching substring in the delimiter.
split(/([,-])/, "1-10,20", 3);
produces the list value
(1, '-', 10, ',', 20)
If you had the entire header of a normal Unix email message in
$header, you could split it up into fields and their values
this way:
$header =~ s/\n(?=\s)//g; # fix continuation lines
%hdrs = (UNIX_FROM => split /^(\S*?):\s*/m, $header);
The pattern "/PATTERN/" may be replaced with an expression to
specify patterns that vary at runtime. (To do runtime
compilation only once, use "/$variable/o".)
As a special case, specifying a PATTERN of space (' ') will
split on white space just as "split" with no arguments does.
Thus, "split(' ')" can be used to emulate awk’s default
behavior, whereas "split(/ /)" will give you as many null
initial fields as there are leading spaces. A "split" on
"/\s+/" is like a "split(' ')" except that any leading
whitespace produces a null first field. A "split" with no
arguments really does a "split(' ', $_)" internally.
A PATTERN of "/^/" is treated as if it were "/^/m", since it
isn’t much use otherwise.
Example:
open(PASSWD, '/etc/passwd');
while (<PASSWD>) {
chomp;
($login, $passwd, $uid, $gid,
$gcos, $home, $shell) = split(/:/);
#...
}
As with regular pattern matching, any capturing parentheses
that are not matched in a "split()" will be set to "undef" when
returned:
@fields = split /(A)|B/, "1A2B3";
# @fields is (1, 'A', 2, undef, 3)
sprintf FORMAT, LIST
Returns a string formatted by the usual "printf" conventions of
the C library function "sprintf". See below for more details
and see sprintf(3) or printf(3) on your system for an
explanation of the general principles.
For example:
# Format number with up to 8 leading zeroes
$result = sprintf("%08d", $number);
# Round number to 3 digits after decimal point
$rounded = sprintf("%.3f", $number);
Perl does its own "sprintf" formatting--it emulates the C
function "sprintf", but it doesn’t use it (except for floating-
point numbers, and even then only the standard modifiers are
allowed). As a result, any non-standard extensions in your
local "sprintf" are not available from Perl.
Unlike "printf", "sprintf" does not do what you probably mean
when you pass it an array as your first argument. The array is
given scalar context, and instead of using the 0th element of
the array as the format, Perl will use the count of elements in
the array as the format, which is almost never useful.
Perl’s "sprintf" permits the following universally-known
conversions:
%% a percent sign
%c a character with the given number
%s a string
%d a signed integer, in decimal
%u an unsigned integer, in decimal
%o an unsigned integer, in octal
%x an unsigned integer, in hexadecimal
%e a floating-point number, in scientific notation
%f a floating-point number, in fixed decimal notation
%g a floating-point number, in %e or %f notation
In addition, Perl permits the following widely-supported
conversions:
%X like %x, but using upper-case letters
%E like %e, but using an upper-case "E"
%G like %g, but with an upper-case "E" (if applicable)
%b an unsigned integer, in binary
%B like %b, but using an upper-case "B" with the # flag
%p a pointer (outputs the Perl value's address in hexadecimal)
%n special: *stores* the number of characters output so far
into the next variable in the parameter list
Finally, for backward (and we do mean "backward")
compatibility, Perl permits these unnecessary but widely-
supported conversions:
%i a synonym for %d
%D a synonym for %ld
%U a synonym for %lu
%O a synonym for %lo
%F a synonym for %f
Note that the number of exponent digits in the scientific
notation produced by %e, %E, %g and %G for numbers with the
modulus of the exponent less than 100 is system-dependent: it
may be three or less (zero-padded as necessary). In other
words, 1.23 times ten to the 99th may be either "1.23e99" or
"1.23e099".
Between the "%" and the format letter, you may specify a number
of additional attributes controlling the interpretation of the
format. In order, these are:
format parameter index
An explicit format parameter index, such as "2$". By
default sprintf will format the next unused argument in the
list, but this allows you to take the arguments out of
order, e.g.:
printf '%2$d %1$d', 12, 34; # prints "34 12"
printf '%3$d %d %1$d', 1, 2, 3; # prints "3 1 1"
flags
one or more of:
space prefix non-negative number with a space
+ prefix non-negative number with a plus sign
- left-justify within the field
0 use zeros, not spaces, to right-justify
# ensure the leading "0" for any octal,
prefix non-zero hexadecimal with "0x" or "0X",
prefix non-zero binary with "0b" or "0B"
For example:
printf '<% d>', 12; # prints "< 12>"
printf '<%+d>', 12; # prints "<+12>"
printf '<%6s>', 12; # prints "< 12>"
printf '<%-6s>', 12; # prints "<12 >"
printf '<%06s>', 12; # prints "<000012>"
printf '<%#o>', 12; # prints "<014>"
printf '<%#x>', 12; # prints "<0xc>"
printf '<%#X>', 12; # prints "<0XC>"
printf '<%#b>', 12; # prints "<0b1100>"
printf '<%#B>', 12; # prints "<0B1100>"
When a space and a plus sign are given as the flags at
once, a plus sign is used to prefix a positive number.
printf '<%+ d>', 12; # prints "<+12>"
printf '<% +d>', 12; # prints "<+12>"
When the # flag and a precision are given in the %o
conversion, the precision is incremented if it’s necessary
for the leading "0".
printf '<%#.5o>', 012; # prints "<00012>"
printf '<%#.5o>', 012345; # prints "<012345>"
printf '<%#.0o>', 0; # prints "<0>"
vector flag
This flag tells perl to interpret the supplied string as a
vector of integers, one for each character in the string.
Perl applies the format to each integer in turn, then joins
the resulting strings with a separator (a dot "." by
default). This can be useful for displaying ordinal values
of characters in arbitrary strings:
printf "%vd", "AB\x{100}"; # prints "65.66.256"
printf "version is v%vd\n", $^V; # Perl's version
Put an asterisk "*" before the "v" to override the string
to use to separate the numbers:
printf "address is %*vX\n", ":", $addr; # IPv6 address
printf "bits are %0*v8b\n", " ", $bits; # random bitstring
You can also explicitly specify the argument number to use
for the join string using e.g. "*2$v":
printf '%*4$vX %*4$vX %*4$vX', @addr[1..3], ":"; # 3 IPv6 addresses
(minimum) width
Arguments are usually formatted to be only as wide as
required to display the given value. You can override the
width by putting a number here, or get the width from the
next argument (with "*") or from a specified argument (with
e.g. "*2$"):
printf '<%s>', "a"; # prints "<a>"
printf '<%6s>', "a"; # prints "< a>"
printf '<%*s>', 6, "a"; # prints "< a>"
printf '<%*2$s>', "a", 6; # prints "< a>"
printf '<%2s>', "long"; # prints "<long>" (does not truncate)
If a field width obtained through "*" is negative, it has
the same effect as the "-" flag: left-justification.
precision, or maximum width
You can specify a precision (for numeric conversions) or a
maximum width (for string conversions) by specifying a "."
followed by a number. For floating point formats, with the
exception of ’g’ and ’G’, this specifies the number of
decimal places to show (the default being 6), e.g.:
# these examples are subject to system-specific variation
printf '<%f>', 1; # prints "<1.000000>"
printf '<%.1f>', 1; # prints "<1.0>"
printf '<%.0f>', 1; # prints "<1>"
printf '<%e>', 10; # prints "<1.000000e+01>"
printf '<%.1e>', 10; # prints "<1.0e+01>"
For ’g’ and ’G’, this specifies the maximum number of
digits to show, including prior to the decimal point as
well as after it, e.g.:
# these examples are subject to system-specific variation
printf '<%g>', 1; # prints "<1>"
printf '<%.10g>', 1; # prints "<1>"
printf '<%g>', 100; # prints "<100>"
printf '<%.1g>', 100; # prints "<1e+02>"
printf '<%.2g>', 100.01; # prints "<1e+02>"
printf '<%.5g>', 100.01; # prints "<100.01>"
printf '<%.4g>', 100.01; # prints "<100>"
For integer conversions, specifying a precision implies
that the output of the number itself should be zero-padded
to this width, where the 0 flag is ignored:
printf '<%.6d>', 1; # prints "<000001>"
printf '<%+.6d>', 1; # prints "<+000001>"
printf '<%-10.6d>', 1; # prints "<000001 >"
printf '<%10.6d>', 1; # prints "< 000001>"
printf '<%010.6d>', 1; # prints "< 000001>"
printf '<%+10.6d>', 1; # prints "< +000001>"
printf '<%.6x>', 1; # prints "<000001>"
printf '<%#.6x>', 1; # prints "<0x000001>"
printf '<%-10.6x>', 1; # prints "<000001 >"
printf '<%10.6x>', 1; # prints "< 000001>"
printf '<%010.6x>', 1; # prints "< 000001>"
printf '<%#10.6x>', 1; # prints "< 0x000001>"
For string conversions, specifying a precision truncates
the string to fit in the specified width:
printf '<%.5s>', "truncated"; # prints "<trunc>"
printf '<%10.5s>', "truncated"; # prints "< trunc>"
You can also get the precision from the next argument using
".*":
printf '<%.6x>', 1; # prints "<000001>"
printf '<%.*x>', 6, 1; # prints "<000001>"
If a precision obtained through "*" is negative, it has the
same effect as no precision.
printf '<%.*s>', 7, "string"; # prints "<string>"
printf '<%.*s>', 3, "string"; # prints "<str>"
printf '<%.*s>', 0, "string"; # prints "<>"
printf '<%.*s>', -1, "string"; # prints "<string>"
printf '<%.*d>', 1, 0; # prints "<0>"
printf '<%.*d>', 0, 0; # prints "<>"
printf '<%.*d>', -1, 0; # prints "<0>"
You cannot currently get the precision from a specified
number, but it is intended that this will be possible in
the future using e.g. ".*2$":
printf '<%.*2$x>', 1, 6; # INVALID, but in future will print "<000001>"
size
For numeric conversions, you can specify the size to
interpret the number as using "l", "h", "V", "q", "L", or
"ll". For integer conversions ("d u o x X b i D U O"),
numbers are usually assumed to be whatever the default
integer size is on your platform (usually 32 or 64 bits),
but you can override this to use instead one of the
standard C types, as supported by the compiler used to
build Perl:
l interpret integer as C type "long" or "unsigned long"
h interpret integer as C type "short" or "unsigned short"
q, L or ll interpret integer as C type "long long", "unsigned long long".
or "quads" (typically 64-bit integers)
The last will produce errors if Perl does not understand
"quads" in your installation. (This requires that either
the platform natively supports quads or Perl was
specifically compiled to support quads.) You can find out
whether your Perl supports quads via Config:
use Config;
($Config{use64bitint} eq 'define' || $Config{longsize} >= 8) &&
print "quads\n";
For floating point conversions ("e f g E F G"), numbers are
usually assumed to be the default floating point size on
your platform (double or long double), but you can force
’long double’ with "q", "L", or "ll" if your platform
supports them. You can find out whether your Perl supports
long doubles via Config:
use Config;
$Config{d_longdbl} eq 'define' && print "long doubles\n";
You can find out whether Perl considers ’long double’ to be
the default floating point size to use on your platform via
Config:
use Config;
($Config{uselongdouble} eq 'define') &&
print "long doubles by default\n";
It can also be the case that long doubles and doubles are
the same thing:
use Config;
($Config{doublesize} == $Config{longdblsize}) &&
print "doubles are long doubles\n";
The size specifier "V" has no effect for Perl code, but it
is supported for compatibility with XS code; it means ’use
the standard size for a Perl integer (or floating-point
number)’, which is already the default for Perl code.
order of arguments
Normally, sprintf takes the next unused argument as the
value to format for each format specification. If the
format specification uses "*" to require additional
arguments, these are consumed from the argument list in the
order in which they appear in the format specification
before the value to format. Where an argument is specified
using an explicit index, this does not affect the normal
order for the arguments (even when the explicitly specified
index would have been the next argument in any case).
So:
printf '<%*.*s>', $a, $b, $c;
would use $a for the width, $b for the precision and $c as
the value to format, while:
printf '<%*1$.*s>', $a, $b;
would use $a for the width and the precision, and $b as the
value to format.
Here are some more examples - beware that when using an
explicit index, the "$" may need to be escaped:
printf "%2\$d %d\n", 12, 34; # will print "34 12\n"
printf "%2\$d %d %d\n", 12, 34; # will print "34 12 34\n"
printf "%3\$d %d %d\n", 12, 34, 56; # will print "56 12 34\n"
printf "%2\$*3\$d %d\n", 12, 34, 3; # will print " 34 12\n"
If "use locale" is in effect, and POSIX::setlocale() has been
called, the character used for the decimal separator in
formatted floating point numbers is affected by the LC_NUMERIC
locale. See perllocale and POSIX.
sqrt EXPR
sqrt Return the square root of EXPR. If EXPR is omitted, returns
square root of $_. Only works on non-negative operands, unless
you’ve loaded the standard Math::Complex module.
use Math::Complex;
print sqrt(-2); # prints 1.4142135623731i
srand EXPR
srand Sets the random number seed for the "rand" operator.
The point of the function is to "seed" the "rand" function so
that "rand" can produce a different sequence each time you run
your program.
If srand() is not called explicitly, it is called implicitly at
the first use of the "rand" operator. However, this was not
the case in versions of Perl before 5.004, so if your script
will run under older Perl versions, it should call "srand".
Most programs won’t even call srand() at all, except those that
need a cryptographically-strong starting point rather than the
generally acceptable default, which is based on time of day,
process ID, and memory allocation, or the /dev/urandom device,
if available.
You can call srand($seed) with the same $seed to reproduce the
same sequence from rand(), but this is usually reserved for
generating predictable results for testing or debugging.
Otherwise, don’t call srand() more than once in your program.
Do not call srand() (i.e. without an argument) more than once
in a script. The internal state of the random number generator
should contain more entropy than can be provided by any seed,
so calling srand() again actually loses randomness.
Most implementations of "srand" take an integer and will
silently truncate decimal numbers. This means "srand(42)" will
usually produce the same results as "srand(42.1)". To be safe,
always pass "srand" an integer.
In versions of Perl prior to 5.004 the default seed was just
the current "time". This isn’t a particularly good seed, so
many old programs supply their own seed value (often "time ^
$$" or "time ^ ($$ + ($$ << 15))"), but that isn’t necessary
any more.
For cryptographic purposes, however, you need something much
more random than the default seed. Checksumming the compressed
output of one or more rapidly changing operating system status
programs is the usual method. For example:
srand (time ^ $$ ^ unpack "%L*", `ps axww | gzip -f`);
If you’re particularly concerned with this, see the
"Math::TrulyRandom" module in CPAN.
Frequently called programs (like CGI scripts) that simply use
time ^ $$
for a seed can fall prey to the mathematical property that
a^b == (a+1)^(b+1)
one-third of the time. So don’t do that.
stat FILEHANDLE
stat EXPR
stat DIRHANDLE
stat Returns a 13-element list giving the status info for a file,
either the file opened via FILEHANDLE or DIRHANDLE, or named by
EXPR. If EXPR is omitted, it stats $_. Returns a null list if
the stat fails. Typically used as follows:
($dev,$ino,$mode,$nlink,$uid,$gid,$rdev,$size,
$atime,$mtime,$ctime,$blksize,$blocks)
= stat($filename);
Not all fields are supported on all filesystem types. Here are
the meanings of the fields:
0 dev device number of filesystem
1 ino inode number
2 mode file mode (type and permissions)
3 nlink number of (hard) links to the file
4 uid numeric user ID of file's owner
5 gid numeric group ID of file's owner
6 rdev the device identifier (special files only)
7 size total size of file, in bytes
8 atime last access time in seconds since the epoch
9 mtime last modify time in seconds since the epoch
10 ctime inode change time in seconds since the epoch (*)
11 blksize preferred block size for file system I/O
12 blocks actual number of blocks allocated
(The epoch was at 00:00 January 1, 1970 GMT.)
(*) Not all fields are supported on all filesystem types.
Notably, the ctime field is non-portable. In particular, you
cannot expect it to be a "creation time", see "Files and
Filesystems" in perlport for details.
If "stat" is passed the special filehandle consisting of an
underline, no stat is done, but the current contents of the
stat structure from the last "stat", "lstat", or filetest are
returned. Example:
if (-x $file && (($d) = stat(_)) && $d < 0) {
print "$file is executable NFS file\n";
}
(This works on machines only for which the device number is
negative under NFS.)
Because the mode contains both the file type and its
permissions, you should mask off the file type portion and
(s)printf using a "%o" if you want to see the real permissions.
$mode = (stat($filename))[2];
printf "Permissions are %04o\n", $mode & 07777;
In scalar context, "stat" returns a boolean value indicating
success or failure, and, if successful, sets the information
associated with the special filehandle "_".
The File::stat module provides a convenient, by-name access
mechanism:
use File::stat;
$sb = stat($filename);
printf "File is %s, size is %s, perm %04o, mtime %s\n",
$filename, $sb->size, $sb->mode & 07777,
scalar localtime $sb->mtime;
You can import symbolic mode constants ("S_IF*") and functions
("S_IS*") from the Fcntl module:
use Fcntl ':mode';
$mode = (stat($filename))[2];
$user_rwx = ($mode & S_IRWXU) >> 6;
$group_read = ($mode & S_IRGRP) >> 3;
$other_execute = $mode & S_IXOTH;
printf "Permissions are %04o\n", S_IMODE($mode), "\n";
$is_setuid = $mode & S_ISUID;
$is_directory = S_ISDIR($mode);
You could write the last two using the "-u" and "-d" operators.
The commonly available "S_IF*" constants are
# Permissions: read, write, execute, for user, group, others.
S_IRWXU S_IRUSR S_IWUSR S_IXUSR
S_IRWXG S_IRGRP S_IWGRP S_IXGRP
S_IRWXO S_IROTH S_IWOTH S_IXOTH
# Setuid/Setgid/Stickiness/SaveText.
# Note that the exact meaning of these is system dependent.
S_ISUID S_ISGID S_ISVTX S_ISTXT
# File types. Not necessarily all are available on your system.
S_IFREG S_IFDIR S_IFLNK S_IFBLK S_IFCHR S_IFIFO S_IFSOCK S_IFWHT S_ENFMT
# The following are compatibility aliases for S_IRUSR, S_IWUSR, S_IXUSR.
S_IREAD S_IWRITE S_IEXEC
and the "S_IF*" functions are
S_IMODE($mode) the part of $mode containing the permission bits
and the setuid/setgid/sticky bits
S_IFMT($mode) the part of $mode containing the file type
which can be bit-anded with e.g. S_IFREG
or with the following functions
# The operators -f, -d, -l, -b, -c, -p, and -S.
S_ISREG($mode) S_ISDIR($mode) S_ISLNK($mode)
S_ISBLK($mode) S_ISCHR($mode) S_ISFIFO($mode) S_ISSOCK($mode)
# No direct -X operator counterpart, but for the first one
# the -g operator is often equivalent. The ENFMT stands for
# record flocking enforcement, a platform-dependent feature.
S_ISENFMT($mode) S_ISWHT($mode)
See your native chmod(2) and stat(2) documentation for more
details about the "S_*" constants. To get status info for a
symbolic link instead of the target file behind the link, use
the "lstat" function.
state EXPR
state TYPE EXPR
state EXPR : ATTRS
state TYPE EXPR : ATTRS
"state" declares a lexically scoped variable, just like "my"
does. However, those variables will never be reinitialized,
contrary to lexical variables that are reinitialized each time
their enclosing block is entered.
"state" variables are only enabled when the "feature 'state'"
pragma is in effect. See feature.
study SCALAR
study Takes extra time to study SCALAR ($_ if unspecified) in
anticipation of doing many pattern matches on the string before
it is next modified. This may or may not save time, depending
on the nature and number of patterns you are searching on, and
on the distribution of character frequencies in the string to
be searched--you probably want to compare run times with and
without it to see which runs faster. Those loops that scan for
many short constant strings (including the constant parts of
more complex patterns) will benefit most. You may have only
one "study" active at a time--if you study a different scalar
the first is "unstudied". (The way "study" works is this: a
linked list of every character in the string to be searched is
made, so we know, for example, where all the 'k' characters
are. From each search string, the rarest character is
selected, based on some static frequency tables constructed
from some C programs and English text. Only those places that
contain this "rarest" character are examined.)
For example, here is a loop that inserts index producing
entries before any line containing a certain pattern:
while (<>) {
study;
print ".IX foo\n" if /\bfoo\b/;
print ".IX bar\n" if /\bbar\b/;
print ".IX blurfl\n" if /\bblurfl\b/;
# ...
print;
}
In searching for "/\bfoo\b/", only those locations in $_ that
contain "f" will be looked at, because "f" is rarer than "o".
In general, this is a big win except in pathological cases.
The only question is whether it saves you more time than it
took to build the linked list in the first place.
Note that if you have to look for strings that you don’t know
till runtime, you can build an entire loop as a string and
"eval" that to avoid recompiling all your patterns all the
time. Together with undefining $/ to input entire files as one
record, this can be very fast, often faster than specialized
programs like fgrep(1). The following scans a list of files
(@files) for a list of words (@words), and prints out the names
of those files that contain a match:
$search = 'while (<>) { study;';
foreach $word (@words) {
$search .= "++\$seen{\$ARGV} if /\\b$word\\b/;\n";
}
$search .= "}";
@ARGV = @files;
undef $/;
eval $search; # this screams
$/ = "\n"; # put back to normal input delimiter
foreach $file (sort keys(%seen)) {
print $file, "\n";
}
sub NAME BLOCK
sub NAME (PROTO) BLOCK
sub NAME : ATTRS BLOCK
sub NAME (PROTO) : ATTRS BLOCK
This is subroutine definition, not a real function per se.
Without a BLOCK it’s just a forward declaration. Without a
NAME, it’s an anonymous function declaration, and does actually
return a value: the CODE ref of the closure you just created.
See perlsub and perlref for details about subroutines and
references, and attributes and Attribute::Handlers for more
information about attributes.
substr EXPR,OFFSET,LENGTH,REPLACEMENT
substr EXPR,OFFSET,LENGTH
substr EXPR,OFFSET
Extracts a substring out of EXPR and returns it. First
character is at offset 0, or whatever you’ve set $[ to (but
don’t do that). If OFFSET is negative (or more precisely, less
than $[), starts that far from the end of the string. If
LENGTH is omitted, returns everything to the end of the string.
If LENGTH is negative, leaves that many characters off the end
of the string.
my $s = "The black cat climbed the green tree";
my $color = substr $s, 4, 5; # black
my $middle = substr $s, 4, -11; # black cat climbed the
my $end = substr $s, 14; # climbed the green tree
my $tail = substr $s, -4; # tree
my $z = substr $s, -4, 2; # tr
You can use the substr() function as an lvalue, in which case
EXPR must itself be an lvalue. If you assign something shorter
than LENGTH, the string will shrink, and if you assign
something longer than LENGTH, the string will grow to
accommodate it. To keep the string the same length you may
need to pad or chop your value using "sprintf".
If OFFSET and LENGTH specify a substring that is partly outside
the string, only the part within the string is returned. If
the substring is beyond either end of the string, substr()
returns the undefined value and produces a warning. When used
as an lvalue, specifying a substring that is entirely outside
the string is a fatal error. Here’s an example showing the
behavior for boundary cases:
my $name = 'fred';
substr($name, 4) = 'dy'; # $name is now 'freddy'
my $null = substr $name, 6, 2; # returns '' (no warning)
my $oops = substr $name, 7; # returns undef, with warning
substr($name, 7) = 'gap'; # fatal error
An alternative to using substr() as an lvalue is to specify the
replacement string as the 4th argument. This allows you to
replace parts of the EXPR and return what was there before in
one operation, just as you can with splice().
my $s = "The black cat climbed the green tree";
my $z = substr $s, 14, 7, "jumped from"; # climbed
# $s is now "The black cat jumped from the green tree"
Note that the lvalue returned by the 3-arg version of substr()
acts as a ’magic bullet’; each time it is assigned to, it
remembers which part of the original string is being modified;
for example:
$x = '1234';
for (substr($x,1,2)) {
$_ = 'a'; print $x,"\n"; # prints 1a4
$_ = 'xyz'; print $x,"\n"; # prints 1xyz4
$x = '56789';
$_ = 'pq'; print $x,"\n"; # prints 5pq9
}
Prior to Perl version 5.9.1, the result of using an lvalue
multiple times was unspecified.
symlink OLDFILE,NEWFILE
Creates a new filename symbolically linked to the old filename.
Returns 1 for success, 0 otherwise. On systems that don’t
support symbolic links, produces a fatal error at run time. To
check for that, use eval:
$symlink_exists = eval { symlink("",""); 1 };
syscall NUMBER, LIST
Calls the system call specified as the first element of the
list, passing the remaining elements as arguments to the system
call. If unimplemented, produces a fatal error. The arguments
are interpreted as follows: if a given argument is numeric, the
argument is passed as an int. If not, the pointer to the
string value is passed. You are responsible to make sure a
string is pre-extended long enough to receive any result that
might be written into a string. You can’t use a string literal
(or other read-only string) as an argument to "syscall" because
Perl has to assume that any string pointer might be written
through. If your integer arguments are not literals and have
never been interpreted in a numeric context, you may need to
add 0 to them to force them to look like numbers. This
emulates the "syswrite" function (or vice versa):
require 'syscall.ph'; # may need to run h2ph
$s = "hi there\n";
syscall(&SYS_write, fileno(STDOUT), $s, length $s);
Note that Perl supports passing of up to only 14 arguments to
your system call, which in practice should usually suffice.
Syscall returns whatever value returned by the system call it
calls. If the system call fails, "syscall" returns "-1" and
sets $! (errno). Note that some system calls can legitimately
return "-1". The proper way to handle such calls is to assign
"$!=0;" before the call and check the value of $! if syscall
returns "-1".
There’s a problem with "syscall(&SYS_pipe)": it returns the
file number of the read end of the pipe it creates. There is
no way to retrieve the file number of the other end. You can
avoid this problem by using "pipe" instead.
sysopen FILEHANDLE,FILENAME,MODE
sysopen FILEHANDLE,FILENAME,MODE,PERMS
Opens the file whose filename is given by FILENAME, and
associates it with FILEHANDLE. If FILEHANDLE is an expression,
its value is used as the name of the real filehandle wanted.
This function calls the underlying operating system’s "open"
function with the parameters FILENAME, MODE, PERMS.
The possible values and flag bits of the MODE parameter are
system-dependent; they are available via the standard module
"Fcntl". See the documentation of your operating system’s
"open" to see which values and flag bits are available. You
may combine several flags using the "|"-operator.
Some of the most common values are "O_RDONLY" for opening the
file in read-only mode, "O_WRONLY" for opening the file in
write-only mode, and "O_RDWR" for opening the file in read-
write mode.
For historical reasons, some values work on almost every system
supported by perl: zero means read-only, one means write-only,
and two means read/write. We know that these values do not
work under OS/390 & VM/ESA Unix and on the Macintosh; you
probably don’t want to use them in new code.
If the file named by FILENAME does not exist and the "open"
call creates it (typically because MODE includes the "O_CREAT"
flag), then the value of PERMS specifies the permissions of the
newly created file. If you omit the PERMS argument to
"sysopen", Perl uses the octal value 0666. These permission
values need to be in octal, and are modified by your process’s
current "umask".
In many systems the "O_EXCL" flag is available for opening
files in exclusive mode. This is not locking: exclusiveness
means here that if the file already exists, sysopen() fails.
"O_EXCL" may not work on network filesystems, and has no effect
unless the "O_CREAT" flag is set as well. Setting
"O_CREAT|O_EXCL" prevents the file from being opened if it is a
symbolic link. It does not protect against symbolic links in
the file’s path.
Sometimes you may want to truncate an already-existing file.
This can be done using the "O_TRUNC" flag. The behavior of
"O_TRUNC" with "O_RDONLY" is undefined.
You should seldom if ever use 0644 as argument to "sysopen",
because that takes away the user’s option to have a more
permissive umask. Better to omit it. See the perlfunc(1)
entry on "umask" for more on this.
Note that "sysopen" depends on the fdopen() C library function.
On many UNIX systems, fdopen() is known to fail when file
descriptors exceed a certain value, typically 255. If you need
more file descriptors than that, consider rebuilding Perl to
use the "sfio" library, or perhaps using the POSIX::open()
function.
See perlopentut for a kinder, gentler explanation of opening
files.
sysread FILEHANDLE,SCALAR,LENGTH,OFFSET
sysread FILEHANDLE,SCALAR,LENGTH
Attempts to read LENGTH bytes of data into variable SCALAR from
the specified FILEHANDLE, using the system call read(2). It
bypasses buffered IO, so mixing this with other kinds of reads,
"print", "write", "seek", "tell", or "eof" can cause confusion
because the perlio or stdio layers usually buffers data.
Returns the number of bytes actually read, 0 at end of file, or
undef if there was an error (in the latter case $! is also
set). SCALAR will be grown or shrunk so that the last byte
actually read is the last byte of the scalar after the read.
An OFFSET may be specified to place the read data at some place
in the string other than the beginning. A negative OFFSET
specifies placement at that many characters counting backwards
from the end of the string. A positive OFFSET greater than the
length of SCALAR results in the string being padded to the
required size with "\0" bytes before the result of the read is
appended.
There is no syseof() function, which is ok, since eof() doesn’t
work very well on device files (like ttys) anyway. Use
sysread() and check for a return value for 0 to decide whether
you’re done.
Note that if the filehandle has been marked as ":utf8" Unicode
characters are read instead of bytes (the LENGTH, OFFSET, and
the return value of sysread() are in Unicode characters). The
":encoding(...)" layer implicitly introduces the ":utf8" layer.
See "binmode", "open", and the "open" pragma, open.
sysseek FILEHANDLE,POSITION,WHENCE
Sets FILEHANDLE’s system position in bytes using the system
call lseek(2). FILEHANDLE may be an expression whose value
gives the name of the filehandle. The values for WHENCE are 0
to set the new position to POSITION, 1 to set the it to the
current position plus POSITION, and 2 to set it to EOF plus
POSITION (typically negative).
Note the in bytes: even if the filehandle has been set to
operate on characters (for example by using the
":encoding(utf8)" I/O layer), tell() will return byte offsets,
not character offsets (because implementing that would render
sysseek() very slow).
sysseek() bypasses normal buffered IO, so mixing this with
reads (other than "sysread", for example "<>" or read())
"print", "write", "seek", "tell", or "eof" may cause confusion.
For WHENCE, you may also use the constants "SEEK_SET",
"SEEK_CUR", and "SEEK_END" (start of the file, current
position, end of the file) from the Fcntl module. Use of the
constants is also more portable than relying on 0, 1, and 2.
For example to define a "systell" function:
use Fcntl 'SEEK_CUR';
sub systell { sysseek($_[0], 0, SEEK_CUR) }
Returns the new position, or the undefined value on failure. A
position of zero is returned as the string "0 but true"; thus
"sysseek" returns true on success and false on failure, yet you
can still easily determine the new position.
system LIST
system PROGRAM LIST
Does exactly the same thing as "exec LIST", except that a fork
is done first, and the parent process waits for the child
process to complete. Note that argument processing varies
depending on the number of arguments. If there is more than
one argument in LIST, or if LIST is an array with more than one
value, starts the program given by the first element of the
list with arguments given by the rest of the list. If there is
only one scalar argument, the argument is checked for shell
metacharacters, and if there are any, the entire argument is
passed to the system’s command shell for parsing (this is
"/bin/sh -c" on Unix platforms, but varies on other platforms).
If there are no shell metacharacters in the argument, it is
split into words and passed directly to "execvp", which is more
efficient.
Beginning with v5.6.0, Perl will attempt to flush all files
opened for output before any operation that may do a fork, but
this may not be supported on some platforms (see perlport). To
be safe, you may need to set $| ($AUTOFLUSH in English) or call
the "autoflush()" method of "IO::Handle" on any open handles.
The return value is the exit status of the program as returned
by the "wait" call. To get the actual exit value, shift right
by eight (see below). See also "exec". This is not what you
want to use to capture the output from a command, for that you
should use merely backticks or "qx//", as described in
"‘STRING‘" in perlop. Return value of -1 indicates a failure
to start the program or an error of the wait(2) system call
(inspect $! for the reason).
If you’d like to make "system" (and many other bits of Perl)
die on error, have a look at the autodie pragma.
Like "exec", "system" allows you to lie to a program about its
name if you use the "system PROGRAM LIST" syntax. Again, see
"exec".
Since "SIGINT" and "SIGQUIT" are ignored during the execution
of "system", if you expect your program to terminate on receipt
of these signals you will need to arrange to do so yourself
based on the return value.
@args = ("command", "arg1", "arg2");
system(@args) == 0
or die "system @args failed: $?"
If you’d like to manually inspect "system"’s failure, you can
check all possible failure modes by inspecting $? like this:
if ($? == -1) {
print "failed to execute: $!\n";
}
elsif ($? & 127) {
printf "child died with signal %d, %s coredump\n",
($? & 127), ($? & 128) ? 'with' : 'without';
}
else {
printf "child exited with value %d\n", $? >> 8;
}
Alternatively you might inspect the value of
"${^CHILD_ERROR_NATIVE}" with the W*() calls of the POSIX
extension.
When the arguments get executed via the system shell, results
and return codes will be subject to its quirks and
capabilities. See "‘STRING‘" in perlop and "exec" for details.
syswrite FILEHANDLE,SCALAR,LENGTH,OFFSET
syswrite FILEHANDLE,SCALAR,LENGTH
syswrite FILEHANDLE,SCALAR
Attempts to write LENGTH bytes of data from variable SCALAR to
the specified FILEHANDLE, using the system call write(2). If
LENGTH is not specified, writes whole SCALAR. It bypasses
buffered IO, so mixing this with reads (other than sysread()),
"print", "write", "seek", "tell", or "eof" may cause confusion
because the perlio and stdio layers usually buffers data.
Returns the number of bytes actually written, or "undef" if
there was an error (in this case the errno variable $! is also
set). If the LENGTH is greater than the available data in the
SCALAR after the OFFSET, only as much data as is available will
be written.
An OFFSET may be specified to write the data from some part of
the string other than the beginning. A negative OFFSET
specifies writing that many characters counting backwards from
the end of the string. In the case the SCALAR is empty you can
use OFFSET but only zero offset.
Note that if the filehandle has been marked as ":utf8", Unicode
characters are written instead of bytes (the LENGTH, OFFSET,
and the return value of syswrite() are in UTF-8 encoded Unicode
characters). The ":encoding(...)" layer implicitly introduces
the ":utf8" layer. See "binmode", "open", and the "open"
pragma, open.
tell FILEHANDLE
tell Returns the current position in bytes for FILEHANDLE, or -1 on
error. FILEHANDLE may be an expression whose value gives the
name of the actual filehandle. If FILEHANDLE is omitted,
assumes the file last read.
Note the in bytes: even if the filehandle has been set to
operate on characters (for example by using the
":encoding(utf8)" open layer), tell() will return byte offsets,
not character offsets (because that would render seek() and
tell() rather slow).
The return value of tell() for the standard streams like the
STDIN depends on the operating system: it may return -1 or
something else. tell() on pipes, fifos, and sockets usually
returns -1.
There is no "systell" function. Use "sysseek(FH, 0, 1)" for
that.
Do not use tell() (or other buffered I/O operations) on a file
handle that has been manipulated by sysread(), syswrite() or
sysseek(). Those functions ignore the buffering, while tell()
does not.
telldir DIRHANDLE
Returns the current position of the "readdir" routines on
DIRHANDLE. Value may be given to "seekdir" to access a
particular location in a directory. "telldir" has the same
caveats about possible directory compaction as the
corresponding system library routine.
tie VARIABLE,CLASSNAME,LIST
This function binds a variable to a package class that will
provide the implementation for the variable. VARIABLE is the
name of the variable to be enchanted. CLASSNAME is the name of
a class implementing objects of correct type. Any additional
arguments are passed to the "new" method of the class (meaning
"TIESCALAR", "TIEHANDLE", "TIEARRAY", or "TIEHASH"). Typically
these are arguments such as might be passed to the "dbm_open()"
function of C. The object returned by the "new" method is also
returned by the "tie" function, which would be useful if you
want to access other methods in CLASSNAME.
Note that functions such as "keys" and "values" may return huge
lists when used on large objects, like DBM files. You may
prefer to use the "each" function to iterate over such.
Example:
# print out history file offsets
use NDBM_File;
tie(%HIST, 'NDBM_File', '/usr/lib/news/history', 1, 0);
while (($key,$val) = each %HIST) {
print $key, ' = ', unpack('L',$val), "\n";
}
untie(%HIST);
A class implementing a hash should have the following methods:
TIEHASH classname, LIST
FETCH this, key
STORE this, key, value
DELETE this, key
CLEAR this
EXISTS this, key
FIRSTKEY this
NEXTKEY this, lastkey
SCALAR this
DESTROY this
UNTIE this
A class implementing an ordinary array should have the
following methods:
TIEARRAY classname, LIST
FETCH this, key
STORE this, key, value
FETCHSIZE this
STORESIZE this, count
CLEAR this
PUSH this, LIST
POP this
SHIFT this
UNSHIFT this, LIST
SPLICE this, offset, length, LIST
EXTEND this, count
DESTROY this
UNTIE this
A class implementing a file handle should have the following
methods:
TIEHANDLE classname, LIST
READ this, scalar, length, offset
READLINE this
GETC this
WRITE this, scalar, length, offset
PRINT this, LIST
PRINTF this, format, LIST
BINMODE this
EOF this
FILENO this
SEEK this, position, whence
TELL this
OPEN this, mode, LIST
CLOSE this
DESTROY this
UNTIE this
A class implementing a scalar should have the following
methods:
TIESCALAR classname, LIST
FETCH this,
STORE this, value
DESTROY this
UNTIE this
Not all methods indicated above need be implemented. See
perltie, Tie::Hash, Tie::Array, Tie::Scalar, and Tie::Handle.
Unlike "dbmopen", the "tie" function will not use or require a
module for you--you need to do that explicitly yourself. See
DB_File or the Config module for interesting "tie"
implementations.
For further details see perltie, "tied VARIABLE".
tied VARIABLE
Returns a reference to the object underlying VARIABLE (the same
value that was originally returned by the "tie" call that bound
the variable to a package.) Returns the undefined value if
VARIABLE isn’t tied to a package.
time Returns the number of non-leap seconds since whatever time the
system considers to be the epoch, suitable for feeding to
"gmtime" and "localtime". On most systems the epoch is 00:00:00
UTC, January 1, 1970; a prominent exception being Mac OS
Classic which uses 00:00:00, January 1, 1904 in the current
local time zone for its epoch.
For measuring time in better granularity than one second, you
may use either the Time::HiRes module (from CPAN, and starting
from Perl 5.8 part of the standard distribution), or if you
have gettimeofday(2), you may be able to use the "syscall"
interface of Perl. See perlfaq8 for details.
For date and time processing look at the many related modules
on CPAN. For a comprehensive date and time representation look
at the DateTime module.
times Returns a four-element list giving the user and system times,
in seconds, for this process and the children of this process.
($user,$system,$cuser,$csystem) = times;
In scalar context, "times" returns $user.
Note that times for children are included only after they
terminate.
tr/// The transliteration operator. Same as "y///". See "Quote and
Quote-like Operators" in perlop.
truncate FILEHANDLE,LENGTH
truncate EXPR,LENGTH
Truncates the file opened on FILEHANDLE, or named by EXPR, to
the specified length. Produces a fatal error if truncate isn’t
implemented on your system. Returns true if successful, the
undefined value otherwise.
The behavior is undefined if LENGTH is greater than the length
of the file.
The position in the file of FILEHANDLE is left unchanged. You
may want to call seek before writing to the file.
uc EXPR
uc Returns an uppercased version of EXPR. This is the internal
function implementing the "\U" escape in double-quoted strings.
Respects current LC_CTYPE locale if "use locale" in force. See
perllocale and perlunicode for more details about locale and
Unicode support. It does not attempt to do titlecase mapping
on initial letters. See "ucfirst" for that.
If EXPR is omitted, uses $_.
ucfirst EXPR
ucfirst Returns the value of EXPR with the first character in uppercase
(titlecase in Unicode). This is the internal function
implementing the "\u" escape in double-quoted strings.
Respects current LC_CTYPE locale if "use locale" in force. See
perllocale and perlunicode for more details about locale and
Unicode support.
If EXPR is omitted, uses $_.
umask EXPR
umask Sets the umask for the process to EXPR and returns the previous
value. If EXPR is omitted, merely returns the current umask.
The Unix permission "rwxr-x---" is represented as three sets of
three bits, or three octal digits: 0750 (the leading 0
indicates octal and isn’t one of the digits). The "umask"
value is such a number representing disabled permissions bits.
The permission (or "mode") values you pass "mkdir" or "sysopen"
are modified by your umask, so even if you tell "sysopen" to
create a file with permissions 0777, if your umask is 0022 then
the file will actually be created with permissions 0755. If
your "umask" were 0027 (group can’t write; others can’t read,
write, or execute), then passing "sysopen" 0666 would create a
file with mode 0640 ("0666 &~ 027" is 0640).
Here’s some advice: supply a creation mode of 0666 for regular
files (in "sysopen") and one of 0777 for directories (in
"mkdir") and executable files. This gives users the freedom of
choice: if they want protected files, they might choose process
umasks of 022, 027, or even the particularly antisocial mask of
077. Programs should rarely if ever make policy decisions
better left to the user. The exception to this is when writing
files that should be kept private: mail files, web browser
cookies, .rhosts files, and so on.
If umask(2) is not implemented on your system and you are
trying to restrict access for yourself (i.e., (EXPR & 0700) >
0), produces a fatal error at run time. If umask(2) is not
implemented and you are not trying to restrict access for
yourself, returns "undef".
Remember that a umask is a number, usually given in octal; it
is not a string of octal digits. See also "oct", if all you
have is a string.
undef EXPR
undef Undefines the value of EXPR, which must be an lvalue. Use only
on a scalar value, an array (using "@"), a hash (using "%"), a
subroutine (using "&"), or a typeglob (using "*"). (Saying
"undef $hash{$key}" will probably not do what you expect on
most predefined variables or DBM list values, so don’t do that;
see delete.) Always returns the undefined value. You can omit
the EXPR, in which case nothing is undefined, but you still get
an undefined value that you could, for instance, return from a
subroutine, assign to a variable or pass as a parameter.
Examples:
undef $foo;
undef $bar{'blurfl'}; # Compare to: delete $bar{'blurfl'};
undef @ary;
undef %hash;
undef &mysub;
undef *xyz; # destroys $xyz, @xyz, %xyz, &xyz, etc.
return (wantarray ? (undef, $errmsg) : undef) if $they_blew_it;
select undef, undef, undef, 0.25;
($a, $b, undef, $c) = &foo; # Ignore third value returned
Note that this is a unary operator, not a list operator.
unlink LIST
unlink Deletes a list of files. Returns the number of files
successfully deleted.
$cnt = unlink 'a', 'b', 'c';
unlink @goners;
unlink <*.bak>;
Note: "unlink" will not attempt to delete directories unless
you are superuser and the -U flag is supplied to Perl. Even if
these conditions are met, be warned that unlinking a directory
can inflict damage on your filesystem. Finally, using "unlink"
on directories is not supported on many operating systems. Use
"rmdir" instead.
If LIST is omitted, uses $_.
unpack TEMPLATE,EXPR
unpack TEMPLATE
"unpack" does the reverse of "pack": it takes a string and
expands it out into a list of values. (In scalar context, it
returns merely the first value produced.)
If EXPR is omitted, unpacks the $_ string.
The string is broken into chunks described by the TEMPLATE.
Each chunk is converted separately to a value. Typically,
either the string is a result of "pack", or the characters of
the string represent a C structure of some kind.
The TEMPLATE has the same format as in the "pack" function.
Here’s a subroutine that does substring:
sub substr {
my($what,$where,$howmuch) = @_;
unpack("x$where a$howmuch", $what);
}
and then there’s
sub ordinal { unpack("W",$_[0]); } # same as ord()
In addition to fields allowed in pack(), you may prefix a field
with a %<number> to indicate that you want a <number>-bit
checksum of the items instead of the items themselves. Default
is a 16-bit checksum. Checksum is calculated by summing
numeric values of expanded values (for string fields the sum of
"ord($char)" is taken, for bit fields the sum of zeroes and
ones).
For example, the following computes the same number as the
System V sum program:
$checksum = do {
local $/; # slurp!
unpack("%32W*",<>) % 65535;
};
The following efficiently counts the number of set bits in a
bit vector:
$setbits = unpack("%32b*", $selectmask);
The "p" and "P" formats should be used with care. Since Perl
has no way of checking whether the value passed to "unpack()"
corresponds to a valid memory location, passing a pointer value
that’s not known to be valid is likely to have disastrous
consequences.
If there are more pack codes or if the repeat count of a field
or a group is larger than what the remainder of the input
string allows, the result is not well defined: in some cases,
the repeat count is decreased, or "unpack()" will produce null
strings or zeroes, or terminate with an error. If the input
string is longer than one described by the TEMPLATE, the rest
is ignored.
See "pack" for more examples and notes.
untie VARIABLE
Breaks the binding between a variable and a package. (See
"tie".) Has no effect if the variable is not tied.
unshift ARRAY,LIST
Does the opposite of a "shift". Or the opposite of a "push",
depending on how you look at it. Prepends list to the front of
the array, and returns the new number of elements in the array.
unshift(@ARGV, '-e') unless $ARGV[0] =~ /^-/;
Note the LIST is prepended whole, not one element at a time, so
the prepended elements stay in the same order. Use "reverse"
to do the reverse.
use Module VERSION LIST
use Module VERSION
use Module LIST
use Module
use VERSION
Imports some semantics into the current package from the named
module, generally by aliasing certain subroutine or variable
names into your package. It is exactly equivalent to
BEGIN { require Module; Module->import( LIST ); }
except that Module must be a bareword.
In the peculiar "use VERSION" form, VERSION may be either a
numeric argument such as 5.006, which will be compared to $],
or a literal of the form v5.6.1, which will be compared to $^V
(aka $PERL_VERSION). A fatal error is produced if VERSION is
greater than the version of the current Perl interpreter; Perl
will not attempt to parse the rest of the file. Compare with
"require", which can do a similar check at run time.
Symmetrically, "no VERSION" allows you to specify that you want
a version of perl older than the specified one.
Specifying VERSION as a literal of the form v5.6.1 should
generally be avoided, because it leads to misleading error
messages under earlier versions of Perl (that is, prior to
5.6.0) that do not support this syntax. The equivalent numeric
version should be used instead.
use v5.6.1; # compile time version check
use 5.6.1; # ditto
use 5.006_001; # ditto; preferred for backwards compatibility
This is often useful if you need to check the current Perl
version before "use"ing library modules that won’t work with
older versions of Perl. (We try not to do this more than we
have to.)
Also, if the specified perl version is greater than or equal to
5.9.5, "use VERSION" will also load the "feature" pragma and
enable all features available in the requested version. See
feature.
The "BEGIN" forces the "require" and "import" to happen at
compile time. The "require" makes sure the module is loaded
into memory if it hasn’t been yet. The "import" is not a
builtin--it’s just an ordinary static method call into the
"Module" package to tell the module to import the list of
features back into the current package. The module can
implement its "import" method any way it likes, though most
modules just choose to derive their "import" method via
inheritance from the "Exporter" class that is defined in the
"Exporter" module. See Exporter. If no "import" method can be
found then the call is skipped, even if there is an AUTOLOAD
method.
If you do not want to call the package’s "import" method (for
instance, to stop your namespace from being altered),
explicitly supply the empty list:
use Module ();
That is exactly equivalent to
BEGIN { require Module }
If the VERSION argument is present between Module and LIST,
then the "use" will call the VERSION method in class Module
with the given version as an argument. The default VERSION
method, inherited from the UNIVERSAL class, croaks if the given
version is larger than the value of the variable
$Module::VERSION.
Again, there is a distinction between omitting LIST ("import"
called with no arguments) and an explicit empty LIST "()"
("import" not called). Note that there is no comma after
VERSION!
Because this is a wide-open interface, pragmas (compiler
directives) are also implemented this way. Currently
implemented pragmas are:
use constant;
use diagnostics;
use integer;
use sigtrap qw(SEGV BUS);
use strict qw(subs vars refs);
use subs qw(afunc blurfl);
use warnings qw(all);
use sort qw(stable _quicksort _mergesort);
Some of these pseudo-modules import semantics into the current
block scope (like "strict" or "integer", unlike ordinary
modules, which import symbols into the current package (which
are effective through the end of the file).
There’s a corresponding "no" command that unimports meanings
imported by "use", i.e., it calls "unimport Module LIST"
instead of "import". It behaves exactly as "import" does with
respect to VERSION, an omitted LIST, empty LIST, or no unimport
method being found.
no integer;
no strict 'refs';
no warnings;
See perlmodlib for a list of standard modules and pragmas. See
perlrun for the "-M" and "-m" command-line options to perl that
give "use" functionality from the command-line.
utime LIST
Changes the access and modification times on each file of a
list of files. The first two elements of the list must be the
NUMERICAL access and modification times, in that order.
Returns the number of files successfully changed. The inode
change time of each file is set to the current time. For
example, this code has the same effect as the Unix touch(1)
command when the files already exist and belong to the user
running the program:
#!/usr/bin/perl
$atime = $mtime = time;
utime $atime, $mtime, @ARGV;
Since perl 5.7.2, if the first two elements of the list are
"undef", then the utime(2) function in the C library will be
called with a null second argument. On most systems, this will
set the file’s access and modification times to the current
time (i.e. equivalent to the example above) and will even work
on other users’ files where you have write permission:
utime undef, undef, @ARGV;
Under NFS this will use the time of the NFS server, not the
time of the local machine. If there is a time synchronization
problem, the NFS server and local machine will have different
times. The Unix touch(1) command will in fact normally use
this form instead of the one shown in the first example.
Note that only passing one of the first two elements as "undef"
will be equivalent of passing it as 0 and will not have the
same effect as described when they are both "undef". This case
will also trigger an uninitialized warning.
On systems that support futimes, you might pass file handles
among the files. On systems that don’t support futimes,
passing file handles produces a fatal error at run time. The
file handles must be passed as globs or references to be
recognized. Barewords are considered file names.
values HASH
Returns a list consisting of all the values of the named hash.
(In a scalar context, returns the number of values.)
The values are returned in an apparently random order. The
actual random order is subject to change in future versions of
perl, but it is guaranteed to be the same order as either the
"keys" or "each" function would produce on the same
(unmodified) hash. Since Perl 5.8.1 the ordering is different
even between different runs of Perl for security reasons (see
"Algorithmic Complexity Attacks" in perlsec).
As a side effect, calling values() resets the HASH’s internal
iterator, see "each". (In particular, calling values() in void
context resets the iterator with no other overhead.)
Note that the values are not copied, which means modifying them
will modify the contents of the hash:
for (values %hash) { s/foo/bar/g } # modifies %hash values
for (@hash{keys %hash}) { s/foo/bar/g } # same
See also "keys", "each", and "sort".
vec EXPR,OFFSET,BITS
Treats the string in EXPR as a bit vector made up of elements
of width BITS, and returns the value of the element specified
by OFFSET as an unsigned integer. BITS therefore specifies the
number of bits that are reserved for each element in the bit
vector. This must be a power of two from 1 to 32 (or 64, if
your platform supports that).
If BITS is 8, "elements" coincide with bytes of the input
string.
If BITS is 16 or more, bytes of the input string are grouped
into chunks of size BITS/8, and each group is converted to a
number as with pack()/unpack() with big-endian formats "n"/"N"
(and analogously for BITS==64). See "pack" for details.
If bits is 4 or less, the string is broken into bytes, then the
bits of each byte are broken into 8/BITS groups. Bits of a
byte are numbered in a little-endian-ish way, as in 0x01, 0x02,
0x04, 0x08, 0x10, 0x20, 0x40, 0x80. For example, breaking the
single input byte "chr(0x36)" into two groups gives a list
"(0x6, 0x3)"; breaking it into 4 groups gives "(0x2, 0x1, 0x3,
0x0)".
"vec" may also be assigned to, in which case parentheses are
needed to give the expression the correct precedence as in
vec($image, $max_x * $x + $y, 8) = 3;
If the selected element is outside the string, the value 0 is
returned. If an element off the end of the string is written
to, Perl will first extend the string with sufficiently many
zero bytes. It is an error to try to write off the beginning
of the string (i.e. negative OFFSET).
If the string happens to be encoded as UTF-8 internally (and
thus has the UTF8 flag set), this is ignored by "vec", and it
operates on the internal byte string, not the conceptual
character string, even if you only have characters with values
less than 256.
Strings created with "vec" can also be manipulated with the
logical operators "|", "&", "^", and "~". These operators will
assume a bit vector operation is desired when both operands are
strings. See "Bitwise String Operators" in perlop.
The following code will build up an ASCII string saying
'PerlPerlPerl'. The comments show the string after each step.
Note that this code works in the same way on big-endian or
little-endian machines.
my $foo = '';
vec($foo, 0, 32) = 0x5065726C; # 'Perl'
# $foo eq "Perl" eq "\x50\x65\x72\x6C", 32 bits
print vec($foo, 0, 8); # prints 80 == 0x50 == ord('P')
vec($foo, 2, 16) = 0x5065; # 'PerlPe'
vec($foo, 3, 16) = 0x726C; # 'PerlPerl'
vec($foo, 8, 8) = 0x50; # 'PerlPerlP'
vec($foo, 9, 8) = 0x65; # 'PerlPerlPe'
vec($foo, 20, 4) = 2; # 'PerlPerlPe' . "\x02"
vec($foo, 21, 4) = 7; # 'PerlPerlPer'
# 'r' is "\x72"
vec($foo, 45, 2) = 3; # 'PerlPerlPer' . "\x0c"
vec($foo, 93, 1) = 1; # 'PerlPerlPer' . "\x2c"
vec($foo, 94, 1) = 1; # 'PerlPerlPerl'
# 'l' is "\x6c"
To transform a bit vector into a string or list of 0’s and 1’s,
use these:
$bits = unpack("b*", $vector);
@bits = split(//, unpack("b*", $vector));
If you know the exact length in bits, it can be used in place
of the "*".
Here is an example to illustrate how the bits actually fall in
place:
#!/usr/bin/perl -wl
print <<'EOT';
0 1 2 3
unpack("V",$_) 01234567890123456789012345678901
------------------------------------------------------------------
EOT
for $w (0..3) {
$width = 2**$w;
for ($shift=0; $shift < $width; ++$shift) {
for ($off=0; $off < 32/$width; ++$off) {
$str = pack("B*", "0"x32);
$bits = (1<<$shift);
vec($str, $off, $width) = $bits;
$res = unpack("b*",$str);
$val = unpack("V", $str);
write;
}
}
}
format STDOUT =
vec($_,@#,@#) = @<< == @######### @>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>
$off, $width, $bits, $val, $res
.
__END__
Regardless of the machine architecture on which it is run, the
above example should print the following table:
0 1 2 3
unpack("V",$_) 01234567890123456789012345678901
------------------------------------------------------------------
vec($_, 0, 1) = 1 == 1 10000000000000000000000000000000
vec($_, 1, 1) = 1 == 2 01000000000000000000000000000000
vec($_, 2, 1) = 1 == 4 00100000000000000000000000000000
vec($_, 3, 1) = 1 == 8 00010000000000000000000000000000
vec($_, 4, 1) = 1 == 16 00001000000000000000000000000000
vec($_, 5, 1) = 1 == 32 00000100000000000000000000000000
vec($_, 6, 1) = 1 == 64 00000010000000000000000000000000
vec($_, 7, 1) = 1 == 128 00000001000000000000000000000000
vec($_, 8, 1) = 1 == 256 00000000100000000000000000000000
vec($_, 9, 1) = 1 == 512 00000000010000000000000000000000
vec($_,10, 1) = 1 == 1024 00000000001000000000000000000000
vec($_,11, 1) = 1 == 2048 00000000000100000000000000000000
vec($_,12, 1) = 1 == 4096 00000000000010000000000000000000
vec($_,13, 1) = 1 == 8192 00000000000001000000000000000000
vec($_,14, 1) = 1 == 16384 00000000000000100000000000000000
vec($_,15, 1) = 1 == 32768 00000000000000010000000000000000
vec($_,16, 1) = 1 == 65536 00000000000000001000000000000000
vec($_,17, 1) = 1 == 131072 00000000000000000100000000000000
vec($_,18, 1) = 1 == 262144 00000000000000000010000000000000
vec($_,19, 1) = 1 == 524288 00000000000000000001000000000000
vec($_,20, 1) = 1 == 1048576 00000000000000000000100000000000
vec($_,21, 1) = 1 == 2097152 00000000000000000000010000000000
vec($_,22, 1) = 1 == 4194304 00000000000000000000001000000000
vec($_,23, 1) = 1 == 8388608 00000000000000000000000100000000
vec($_,24, 1) = 1 == 16777216 00000000000000000000000010000000
vec($_,25, 1) = 1 == 33554432 00000000000000000000000001000000
vec($_,26, 1) = 1 == 67108864 00000000000000000000000000100000
vec($_,27, 1) = 1 == 134217728 00000000000000000000000000010000
vec($_,28, 1) = 1 == 268435456 00000000000000000000000000001000
vec($_,29, 1) = 1 == 536870912 00000000000000000000000000000100
vec($_,30, 1) = 1 == 1073741824 00000000000000000000000000000010
vec($_,31, 1) = 1 == 2147483648 00000000000000000000000000000001
vec($_, 0, 2) = 1 == 1 10000000000000000000000000000000
vec($_, 1, 2) = 1 == 4 00100000000000000000000000000000
vec($_, 2, 2) = 1 == 16 00001000000000000000000000000000
vec($_, 3, 2) = 1 == 64 00000010000000000000000000000000
vec($_, 4, 2) = 1 == 256 00000000100000000000000000000000
vec($_, 5, 2) = 1 == 1024 00000000001000000000000000000000
vec($_, 6, 2) = 1 == 4096 00000000000010000000000000000000
vec($_, 7, 2) = 1 == 16384 00000000000000100000000000000000
vec($_, 8, 2) = 1 == 65536 00000000000000001000000000000000
vec($_, 9, 2) = 1 == 262144 00000000000000000010000000000000
vec($_,10, 2) = 1 == 1048576 00000000000000000000100000000000
vec($_,11, 2) = 1 == 4194304 00000000000000000000001000000000
vec($_,12, 2) = 1 == 16777216 00000000000000000000000010000000
vec($_,13, 2) = 1 == 67108864 00000000000000000000000000100000
vec($_,14, 2) = 1 == 268435456 00000000000000000000000000001000
vec($_,15, 2) = 1 == 1073741824 00000000000000000000000000000010
vec($_, 0, 2) = 2 == 2 01000000000000000000000000000000
vec($_, 1, 2) = 2 == 8 00010000000000000000000000000000
vec($_, 2, 2) = 2 == 32 00000100000000000000000000000000
vec($_, 3, 2) = 2 == 128 00000001000000000000000000000000
vec($_, 4, 2) = 2 == 512 00000000010000000000000000000000
vec($_, 5, 2) = 2 == 2048 00000000000100000000000000000000
vec($_, 6, 2) = 2 == 8192 00000000000001000000000000000000
vec($_, 7, 2) = 2 == 32768 00000000000000010000000000000000
vec($_, 8, 2) = 2 == 131072 00000000000000000100000000000000
vec($_, 9, 2) = 2 == 524288 00000000000000000001000000000000
vec($_,10, 2) = 2 == 2097152 00000000000000000000010000000000
vec($_,11, 2) = 2 == 8388608 00000000000000000000000100000000
vec($_,12, 2) = 2 == 33554432 00000000000000000000000001000000
vec($_,13, 2) = 2 == 134217728 00000000000000000000000000010000
vec($_,14, 2) = 2 == 536870912 00000000000000000000000000000100
vec($_,15, 2) = 2 == 2147483648 00000000000000000000000000000001
vec($_, 0, 4) = 1 == 1 10000000000000000000000000000000
vec($_, 1, 4) = 1 == 16 00001000000000000000000000000000
vec($_, 2, 4) = 1 == 256 00000000100000000000000000000000
vec($_, 3, 4) = 1 == 4096 00000000000010000000000000000000
vec($_, 4, 4) = 1 == 65536 00000000000000001000000000000000
vec($_, 5, 4) = 1 == 1048576 00000000000000000000100000000000
vec($_, 6, 4) = 1 == 16777216 00000000000000000000000010000000
vec($_, 7, 4) = 1 == 268435456 00000000000000000000000000001000
vec($_, 0, 4) = 2 == 2 01000000000000000000000000000000
vec($_, 1, 4) = 2 == 32 00000100000000000000000000000000
vec($_, 2, 4) = 2 == 512 00000000010000000000000000000000
vec($_, 3, 4) = 2 == 8192 00000000000001000000000000000000
vec($_, 4, 4) = 2 == 131072 00000000000000000100000000000000
vec($_, 5, 4) = 2 == 2097152 00000000000000000000010000000000
vec($_, 6, 4) = 2 == 33554432 00000000000000000000000001000000
vec($_, 7, 4) = 2 == 536870912 00000000000000000000000000000100
vec($_, 0, 4) = 4 == 4 00100000000000000000000000000000
vec($_, 1, 4) = 4 == 64 00000010000000000000000000000000
vec($_, 2, 4) = 4 == 1024 00000000001000000000000000000000
vec($_, 3, 4) = 4 == 16384 00000000000000100000000000000000
vec($_, 4, 4) = 4 == 262144 00000000000000000010000000000000
vec($_, 5, 4) = 4 == 4194304 00000000000000000000001000000000
vec($_, 6, 4) = 4 == 67108864 00000000000000000000000000100000
vec($_, 7, 4) = 4 == 1073741824 00000000000000000000000000000010
vec($_, 0, 4) = 8 == 8 00010000000000000000000000000000
vec($_, 1, 4) = 8 == 128 00000001000000000000000000000000
vec($_, 2, 4) = 8 == 2048 00000000000100000000000000000000
vec($_, 3, 4) = 8 == 32768 00000000000000010000000000000000
vec($_, 4, 4) = 8 == 524288 00000000000000000001000000000000
vec($_, 5, 4) = 8 == 8388608 00000000000000000000000100000000
vec($_, 6, 4) = 8 == 134217728 00000000000000000000000000010000
vec($_, 7, 4) = 8 == 2147483648 00000000000000000000000000000001
vec($_, 0, 8) = 1 == 1 10000000000000000000000000000000
vec($_, 1, 8) = 1 == 256 00000000100000000000000000000000
vec($_, 2, 8) = 1 == 65536 00000000000000001000000000000000
vec($_, 3, 8) = 1 == 16777216 00000000000000000000000010000000
vec($_, 0, 8) = 2 == 2 01000000000000000000000000000000
vec($_, 1, 8) = 2 == 512 00000000010000000000000000000000
vec($_, 2, 8) = 2 == 131072 00000000000000000100000000000000
vec($_, 3, 8) = 2 == 33554432 00000000000000000000000001000000
vec($_, 0, 8) = 4 == 4 00100000000000000000000000000000
vec($_, 1, 8) = 4 == 1024 00000000001000000000000000000000
vec($_, 2, 8) = 4 == 262144 00000000000000000010000000000000
vec($_, 3, 8) = 4 == 67108864 00000000000000000000000000100000
vec($_, 0, 8) = 8 == 8 00010000000000000000000000000000
vec($_, 1, 8) = 8 == 2048 00000000000100000000000000000000
vec($_, 2, 8) = 8 == 524288 00000000000000000001000000000000
vec($_, 3, 8) = 8 == 134217728 00000000000000000000000000010000
vec($_, 0, 8) = 16 == 16 00001000000000000000000000000000
vec($_, 1, 8) = 16 == 4096 00000000000010000000000000000000
vec($_, 2, 8) = 16 == 1048576 00000000000000000000100000000000
vec($_, 3, 8) = 16 == 268435456 00000000000000000000000000001000
vec($_, 0, 8) = 32 == 32 00000100000000000000000000000000
vec($_, 1, 8) = 32 == 8192 00000000000001000000000000000000
vec($_, 2, 8) = 32 == 2097152 00000000000000000000010000000000
vec($_, 3, 8) = 32 == 536870912 00000000000000000000000000000100
vec($_, 0, 8) = 64 == 64 00000010000000000000000000000000
vec($_, 1, 8) = 64 == 16384 00000000000000100000000000000000
vec($_, 2, 8) = 64 == 4194304 00000000000000000000001000000000
vec($_, 3, 8) = 64 == 1073741824 00000000000000000000000000000010
vec($_, 0, 8) = 128 == 128 00000001000000000000000000000000
vec($_, 1, 8) = 128 == 32768 00000000000000010000000000000000
vec($_, 2, 8) = 128 == 8388608 00000000000000000000000100000000
vec($_, 3, 8) = 128 == 2147483648 00000000000000000000000000000001
wait Behaves like the wait(2) system call on your system: it waits
for a child process to terminate and returns the pid of the
deceased process, or "-1" if there are no child processes. The
status is returned in $? and "${^CHILD_ERROR_NATIVE}". Note
that a return value of "-1" could mean that child processes are
being automatically reaped, as described in perlipc.
waitpid PID,FLAGS
Waits for a particular child process to terminate and returns
the pid of the deceased process, or "-1" if there is no such
child process. On some systems, a value of 0 indicates that
there are processes still running. The status is returned in
$? and "${^CHILD_ERROR_NATIVE}". If you say
use POSIX ":sys_wait_h";
#...
do {
$kid = waitpid(-1, WNOHANG);
} while $kid > 0;
then you can do a non-blocking wait for all pending zombie
processes. Non-blocking wait is available on machines
supporting either the waitpid(2) or wait4(2) system calls.
However, waiting for a particular pid with FLAGS of 0 is
implemented everywhere. (Perl emulates the system call by
remembering the status values of processes that have exited but
have not been harvested by the Perl script yet.)
Note that on some systems, a return value of "-1" could mean
that child processes are being automatically reaped. See
perlipc for details, and for other examples.
wantarray
Returns true if the context of the currently executing
subroutine or "eval" is looking for a list value. Returns
false if the context is looking for a scalar. Returns the
undefined value if the context is looking for no value (void
context).
return unless defined wantarray; # don't bother doing more
my @a = complex_calculation();
return wantarray ? @a : "@a";
"wantarray()"’s result is unspecified in the top level of a
file, in a "BEGIN", "UNITCHECK", "CHECK", "INIT" or "END"
block, or in a "DESTROY" method.
This function should have been named wantlist() instead.
warn LIST
Prints the value of LIST to STDERR. If the last element of
LIST does not end in a newline, it appends the same file/line
number text as "die" does.
If LIST is empty and $@ already contains a value (typically
from a previous eval) that value is used after appending
"\t...caught" to $@. This is useful for staying almost, but
not entirely similar to "die".
If $@ is empty then the string "Warning: Something's wrong" is
used.
No message is printed if there is a $SIG{__WARN__} handler
installed. It is the handler’s responsibility to deal with the
message as it sees fit (like, for instance, converting it into
a "die"). Most handlers must therefore make arrangements to
actually display the warnings that they are not prepared to
deal with, by calling "warn" again in the handler. Note that
this is quite safe and will not produce an endless loop, since
"__WARN__" hooks are not called from inside one.
You will find this behavior is slightly different from that of
$SIG{__DIE__} handlers (which don’t suppress the error text,
but can instead call "die" again to change it).
Using a "__WARN__" handler provides a powerful way to silence
all warnings (even the so-called mandatory ones). An example:
# wipe out *all* compile-time warnings
BEGIN { $SIG{'__WARN__'} = sub { warn $_[0] if $DOWARN } }
my $foo = 10;
my $foo = 20; # no warning about duplicate my $foo,
# but hey, you asked for it!
# no compile-time or run-time warnings before here
$DOWARN = 1;
# run-time warnings enabled after here
warn "\$foo is alive and $foo!"; # does show up
See perlvar for details on setting %SIG entries, and for more
examples. See the Carp module for other kinds of warnings
using its carp() and cluck() functions.
write FILEHANDLE
write EXPR
write Writes a formatted record (possibly multi-line) to the
specified FILEHANDLE, using the format associated with that
file. By default the format for a file is the one having the
same name as the filehandle, but the format for the current
output channel (see the "select" function) may be set
explicitly by assigning the name of the format to the $~
variable.
Top of form processing is handled automatically: if there is
insufficient room on the current page for the formatted record,
the page is advanced by writing a form feed, a special top-of-
page format is used to format the new page header, and then the
record is written. By default the top-of-page format is the
name of the filehandle with "_TOP" appended, but it may be
dynamically set to the format of your choice by assigning the
name to the $^ variable while the filehandle is selected. The
number of lines remaining on the current page is in variable
"$-", which can be set to 0 to force a new page.
If FILEHANDLE is unspecified, output goes to the current
default output channel, which starts out as STDOUT but may be
changed by the "select" operator. If the FILEHANDLE is an
EXPR, then the expression is evaluated and the resulting string
is used to look up the name of the FILEHANDLE at run time. For
more on formats, see perlform.
Note that write is not the opposite of "read". Unfortunately.
y/// The transliteration operator. Same as "tr///". See "Quote and
Quote-like Operators" in perlop.