NAME
libexpect - programmed dialogue library with interactive programs
DESCRIPTION
This library contains functions that allow Expect to be used as a Tcl
extension or to be used directly from C or C++ (without Tcl). Adding
Expect as a Tcl extension is very short and simple, so that will be
covered first.
SYNOPSIS
#include expect_tcl.h
Expect_Init(interp);
cc files... -lexpect5.20 -ltcl7.5 -lm
Note: library versions may differ in the actual release.
The Expect_Init function adds expect commands to the named interpreter.
It avoids overwriting commands that already exist, however aliases
beginning with "exp_" are always created for expect commands. So for
example, "send" can be used as "exp_send".
Generally, you should only call Expect commands via Tcl_Eval. Certain
auxiliary functions may be called directly. They are summarized below.
They may be useful in constructing your own main. Look at the file
exp_main_exp.c in the Expect distribution as a prototype main. Another
prototype is tclAppInit.c in the Tcl source distribution. A prototype
for working with Tk is in exp_main_tk.c in the Expect distribution.
int exp_cmdlinecmds;
int exp_interactive;
FILE *exp_cmdfile;
char *exp_cmdfilename;
int exp_tcl_debugger_available;
void exp_parse_argv(Tcl_Interp *,int argc,char **argv);
int exp_interpreter(Tcl_Interp *);
void exp_interpret_cmdfile(Tcl_Interp *,FILE *);
void exp_interpret_cmdfilename(Tcl_Interp *,char *);
void exp_interpret_rcfiles(Tcl_Interp *,int my_rc,int sys_rc);
char * exp_cook(char *s,int *len);
void (*exp_app_exit)EXP_PROTO((Tcl_Interp *);
void exp_exit(Tcl_Interp *,int status);
void exp_exit_handlers(Tcl_Interp *);
void exp_error(Tcl_Interp,char *,...);
exp_cmdlinecmds is 1 if Expect has been invoked with commands on the
program command-line (using "-c" for example). exp_interactive is 1 if
Expect has been invoked with the -i flag or if no commands or script is
being invoked. exp_cmdfile is a stream from which Expect will read
commands. exp_cmdfilename is the name of a file which Expect will open
and read commands from. exp_tcl_debugger_available is 1 if the
debugger has been armed.
exp_parse_argv reads the representation of the command line. Based on
what is found, any of the other variables listed here are initialized
appropriately. exp_interpreter interactively prompts the user for
commands and evaluates them. exp_interpret_cmdfile reads the given
stream and evaluates any commands found. exp_interpret_cmdfilename
opens the named file and evaluates any commands found.
exp_interpret_rcfiles reads and evalutes the .rc files. If my_rc is
zero, then ~/.expectrc is skipped. If sys_rc is zero, then the system-
wide expectrc file is skipped. exp_cook returns a static buffer
containing the argument reproduced with newlines replaced by carriage-
return linefeed sequences. The primary purpose of this is to allow
messages to be produced without worrying about whether the terminal is
in raw mode or cooked mode. If length is zero, it is computed via
strlen. exp_error is a printf-like function that writes the result to
interp->result.
SYNOPSIS
#include <expect.h>
int
exp_spawnl(file, arg0 [, arg1, ..., argn] (char *)0);
char *file;
char *arg0, *arg1, ... *argn;
int
exp_spawnv(file,argv);
char *file, *argv[ ];
int
exp_spawnfd(fd);
int fd;
FILE *
exp_popen(command);
char *command;
extern int exp_pid;
extern int exp_ttyinit;
extern int exp_ttycopy;
extern int exp_console;
extern char *exp_stty_init;
extern void (*exp_close_in_child)();
extern void (*exp_child_exec_prelude)();
extern void exp_close_tcl_files();
cc files... -lexpect -ltcl -lm
DESCRIPTION
exp_spawnl and exp_spawnv fork a new process so that its stdin, stdout,
and stderr can be written and read by the current process. file is the
name of a file to be executed. The arg pointers are null-terminated
strings. Following the style of execve(), arg0 (or argv[0]) is
customarily a duplicate of the name of the file.
Four interfaces are available, exp_spawnl is useful when the number of
arguments is known at compile time. exp_spawnv is useful when the
number of arguments is not known at compile time. exp_spawnfd is
useful when an open file descriptor is already available as a source.
exp_popen is explained later on.
If the process is successfully created, a file descriptor is returned
which corresponds to the process’s stdin, stdout and stderr. A stream
may be associated with the file descriptor by using fdopen(). (This
should almost certainly be followed by setbuf() to unbuffer the I/O.)
Closing the file descriptor will typically be detected by the process
as an EOF. Once such a process exits, it should be waited upon (via
wait) in order to free up the kernel process slot. (Some systems allow
you to avoid this if you ignore the SIGCHLD signal).
exp_popen is yet another interface, styled after popen(). It takes a
Bourne shell command line, and returns a stream that corresponds to the
process’s stdin, stdout and stderr. The actual implementation of
exp_popen below demonstrates exp_spawnl.
FILE *
exp_popen(program)
char *program;
{
FILE *fp;
int ec;
if (0 > (ec = exp_spawnl("sh","sh","-c",program,(char *)0)))
return(0);
if (NULL == (fp = fdopen(ec,"r+")) return(0);
setbuf(fp,(char *)0);
return(fp);
}
After a process is started, the variable exp_pid is set to the process-
id of the new process. The variable exp_pty_slave_name is set to the
name of the slave side of the pty.
The spawn functions uses a pty to communicate with the process. By
default, the pty is initialized the same way as the user’s tty (if
possible, i.e., if the environment has a controlling terminal.) This
initialization can be skipped by setting exp_ttycopy to 0.
The pty is further initialized to some system wide defaults if
exp_ttyinit is non-zero. The default is generally comparable to "stty
sane".
The tty setting can be further modified by setting the variable
exp_stty_init. This variable is interpreted in the style of stty
arguments. For example, exp_stty_init = "sane"; repeats the default
initialization.
On some systems, it is possible to redirect console output to ptys. If
this is supported, you can force the next spawn to obtain the console
output by setting the variable exp_console to 1.
Between the time a process is started and the new program is given
control, the spawn functions can clean up the environment by closing
file descriptors. By default, the only file descriptors closed are
ones internal to Expect and any marked "close-on-exec".
If needed, you can close additional file descriptors by creating an
appropriate function and assigning it to exp_close_in_child. The
function will be called after the fork and before the exec. (This also
modifies the behavior of the spawn command in Expect.)
If you are also using Tcl, it may be convenient to use the function
exp_close_tcl_files which closes all files between the default standard
file descriptors and the highest descriptor known to Tcl. (Expect does
this.)
The function exp_child_exec_prelude is the last function called prior
to the actual exec in the child. You can redefine this for effects
such as manipulating the uid or the signals.
IF YOU WANT TO ALLOCATE YOUR OWN PTY
extern int exp_autoallocpty;
extern int exp_pty[2];
The spawn functions use a pty to communicate with the process. By
default, a pty is automatically allocated each time a process is
spawned. If you want to allocate ptys yourself, before calling one of
the spawn functions, set exp_autoallocpty to 0, exp_pty[0] to the
master pty file descriptor and exp_pty[1] to the slave pty file
descriptor. The expect library will not do any pty initializations
(e.g., exp_stty_init will not be used). The slave pty file descriptor
will be automatically closed when the process is spawned. After the
process is started, all further communication takes place with the
master pty file descriptor.
exp_spawnl and exp_spawnv duplicate the shell’s actions in searching
for an executable file in a list of directories. The directory list is
obtained from the environment.
EXPECT PROCESSING
While it is possible to use read() to read information from a process
spawned by exp_spawnl or exp_spawnv, more convenient functions are
provided. They are as follows:
int
exp_expectl(fd,type1,pattern1,[re1,],value1,type2,...,exp_end);
int fd;
enum exp_type type;
char *pattern1, *pattern2, ...;
regexp *re1, *re2, ...;
int value1, value2, ...;
int
exp_fexpectl(fp,type1,pattern1,[re1,]value1,type2,...,exp_end);
FILE *fp;
enum exp_type type;
char *pattern1, *pattern2, ...;
regexp *re1, *re2, ...;
int value1, value2, ...;
enum exp_type {
exp_end,
exp_glob,
exp_exact,
exp_regexp,
exp_compiled,
exp_null,
};
struct exp_case {
char *pattern;
regexp *re;
enum exp_type type;
int value;
};
int
exp_expectv(fd,cases);
int fd;
struct exp_case *cases;
int
exp_fexpectv(fp,cases);
FILE *fp;
struct exp_case *cases;
extern int exp_timeout;
extern char *exp_match;
extern char *exp_match_end;
extern char *exp_buffer;
extern char *exp_buffer_end;
extern int exp_match_max;
extern int exp_full_buffer;
extern int exp_remove_nulls;
The functions wait until the output from a process matches one of the
patterns, a specified time period has passed, or an EOF is seen.
The first argument to each function is either a file descriptor or a
stream. Successive sets of arguments describe patterns and associated
integer values to return when the pattern matches.
The type argument is one of four values. exp_end indicates that no
more patterns appear. exp_glob indicates that the pattern is a glob-
style string pattern. exp_exact indicates that the pattern is an exact
string. exp_regexp indicates that the pattern is a regexp-style string
pattern. exp_compiled indicates that the pattern is a regexp-style
string pattern, and that its compiled form is also provided. exp_null
indicates that the pattern is a null (for debugging purposes, a string
pattern must also follow).
If the compiled form is not provided with the functions exp_expectl and
exp_fexpectl, any pattern compilation done internally is thrown away
after the function returns. The functions exp_expectv and exp_fexpectv
will automatically compile patterns and will not throw them away.
Instead, they must be discarded by the user, by calling free on each
pattern. It is only necessary to discard them, the last time the cases
are used.
Regexp subpatterns matched are stored in the compiled regexp. Assuming
"re" contains a compiled regexp, the matched string can be found in
re->startp[0]. The match substrings (according to the parentheses) in
the original pattern can be found in re->startp[1], re->startp[2], and
so on, up to re->startp[9]. The corresponding strings ends are
re->endp[x] where x is that same index as for the string start.
The type exp_null matches if a null appears in the input. The variable
exp_remove_nulls must be set to 0 to prevent nulls from being
automatically stripped. By default, exp_remove_nulls is set to 1 and
nulls are automatically stripped.
exp_expectv and exp_fexpectv are useful when the number of patterns is
not known in advance. In this case, the sets are provided in an array.
The end of the array is denoted by a struct exp_case with type exp_end.
For the rest of this discussion, these functions will be referred to
generically as expect.
If a pattern matches, then the corresponding integer value is returned.
Values need not be unique, however they should be positive to avoid
being mistaken for EXP_EOF, EXP_TIMEOUT, or EXP_FULLBUFFER. Upon EOF
or timeout, the value EXP_EOF or EXP_TIMEOUT is returned. The default
timeout period is 10 seconds but may be changed by setting the variable
exp_timeout. A value of -1 disables a timeout from occurring. A value
of 0 causes the expect function to return immediately (i.e., poll)
after one read(). However it must be preceded by a function such as
select, poll, or an event manager callback to guarantee that there is
data to be read.
If the variable exp_full_buffer is 1, then EXP_FULLBUFFER is returned
if exp_buffer fills with no pattern having matched.
When the expect function returns, exp_buffer points to the buffer of
characters that was being considered for matching. exp_buffer_end
points to one past the last character in exp_buffer. If a match
occurred, exp_match points into exp_buffer where the match began.
exp_match_end points to one character past where the match ended.
Each time new input arrives, it is compared to each pattern in the
order they are listed. Thus, you may test for absence of a match by
making the last pattern something guaranteed to appear, such as a
prompt. In situations where there is no prompt, you must check for
EXP_TIMEOUT (just like you would if you were interacting manually).
More philosophy and strategies on specifying expect patterns can be
found in the documentation on the expect program itself. See SEE ALSO
below.
Patterns are the usual C-shell-style regular expressions. For example,
the following fragment looks for a successful login, such as from a
telnet dialogue.
switch (exp_expectl(
exp_glob,"connected",CONN,
exp_glob,"busy",BUSY,
exp_glob,"failed",ABORT,
exp_glob,"invalid password",ABORT,
exp_end)) {
case CONN: /* logged in successfully */
break;
case BUSY: /* couldn’t log in at the moment */
break;
case EXP_TIMEOUT:
case ABORT: /* can’t log in at any moment! */
break;
default: /* problem with expect */
}
Asterisks (as in the example above) are a useful shorthand for omitting
line-termination characters and other detail. Patterns must match the
entire output of the current process (since the previous read on the
descriptor or stream). More than 2000 bytes of output can force
earlier bytes to be "forgotten". This may be changed by setting the
variable exp_match_max. Note that excessively large values can slow
down the pattern matcher.
RUNNING IN THE BACKGROUND
extern int exp_disconnected;
int exp_disconnect();
It is possible to move a process into the background after it has begun
running. A typical use for this is to read passwords and then go into
the background to sleep before using the passwords to do real work.
To move a process into the background, fork, call exp_disconnect() in
the child process and exit() in the parent process. This disassociates
your process from the controlling terminal. If you wish to move a
process into the background in a different way, you must set the
variable exp_disconnected to 1. This allows processes spawned after
this point to be started correctly.
MULTIPLEXING
By default, the expect functions block inside of a read on a single
file descriptor. If you want to wait on patterns from multiple file
descriptors, use select, poll, or an event manager. They will tell you
what file descriptor is ready to read.
When a file descriptor is ready to read, you can use the expect
functions to do one and only read by setting timeout to 0.
SLAVE CONTROL
void
exp_slave_control(fd,enable)
int fd;
int enable;
Pty trapping is normally done automatically by the expect functions.
However, if you want to issue an ioctl, for example, directly on the
slave device, you should temporary disable trapping.
Pty trapping can be controlled with exp_slave_control. The first
argument is the file descriptor corresponding to the spawned process.
The second argument is a 0 if trapping is to be disabled and 1 if it is
to be enabled.
ERRORS
All functions indicate errors by returning -1 and setting errno.
Errors that occur after the spawn functions fork (e.g., attempting to
spawn a non-existent program) are written to the process’s stderr, and
will be read by the first expect.
SIGNALS
extern int exp_reading;
extern jmp_buf exp_readenv;
expect uses alarm() to timeout, thus if you generate alarms during
expect, it will timeout prematurely.
Internally, expect calls read() which can be interrupted by signals.
If you define signal handlers, you can choose to restart or abort
expect’s internal read. The variable, exp_reading, is true if (and
only if) expect’s read has been interrupted.
longjmp(exp_readenv,EXP_ABORT) will abort the read.
longjmp(exp_readenv,EXP_RESTART) will restart the read.
LOGGING
extern int exp_loguser;
extern int exp_logfile_all
extern FILE *exp_logfile;
If exp_loguser is nonzero, expect sends any output from the spawned
process to stdout. Since interactive programs typically echo their
input, this usually suffices to show both sides of the conversation.
If exp_logfile is also nonzero, this same output is written to the
stream defined by exp_logfile. If exp_logfile_all is non-zero,
exp_logfile is written regardless of the value of exp_loguser.
DEBUGGING
While I consider the library to be easy to use, I think that the
standalone expect program is much, much, easier to use than working
with the C compiler and its usual edit, compile, debug cycle. Unlike
typical C programs, most of the debugging isn’t getting the C compiler
to accept your programs - rather, it is getting the dialogue correct.
Also, translating scripts from expect to C is usually not necessary.
For example, the speed of interactive dialogues is virtually never an
issue. So please try the standalone ’expect’ program first. I suspect
it is a more appropriate solution for most people than the library.
Nonetheless, if you feel compelled to debug in C, here are some tools
to help you.
extern int exp_is_debugging;
extern FILE *exp_debugfile;
While expect dialogues seem very intuitive, trying to codify them in a
program can reveal many surprises in a program’s interface. Therefore
a variety of debugging aids are available. They are controlled by the
above variables, all 0 by default.
Debugging information internal to expect is sent to stderr when
exp_is_debugging is non-zero. The debugging information includes every
character received, and every attempt made to match the current input
against the patterns. In addition, non-printable characters are
translated to a printable form. For example, a control-C appears as a
caret followed by a C. If exp_logfile is non-zero, this information is
also written to that stream.
If exp_debugfile is non-zero, all normal and debugging information is
written to that stream, regardless of the value of exp_is_debugging.
CAVEATS
The stream versions of the expect functions are much slower than the
file descriptor versions because there is no way to portably read an
unknown number of bytes without the potential of timing out. Thus,
characters are read one at a time. You are therefore strongly
encouraged to use the file descriptor versions of expect (although,
automated versions of interactive programs don’t usually demand high
speed anyway).
You can actually get the best of both worlds, writing with the usual
stream functions and reading with the file descriptor versions of
expect as long as you don’t attempt to intermix other stream input
functions (e.g., fgetc). To do this, pass fileno(stream) as the file
descriptor each time. Fortunately, there is little reason to use
anything but the expect functions when reading from interactive
programs.
There is no matching exp_pclose to exp_popen (unlike popen and pclose).
It only takes two functions to close down a connection (fclose()
followed by waiting on the pid), but it is not uncommon to separate
these two actions by large time intervals, so the function seems of
little value.
If you are running on a Cray running Unicos (all I know for sure from
experience), you must run your compiled program as root or setuid. The
problem is that the Cray only allows root processes to open ptys. You
should observe as much precautions as possible: If you don’t need
permissions, setuid(0) only immediately before calling one of the spawn
functions and immediately set it back afterwards.
Normally, spawn takes little time to execute. If you notice spawn
taking a significant amount of time, it is probably encountering ptys
that are wedged. A number of tests are run on ptys to avoid
entanglements with errant processes. (These take 10 seconds per wedged
pty.) Running expect with the -d option will show if expect is
encountering many ptys in odd states. If you cannot kill the processes
to which these ptys are attached, your only recourse may be to reboot.
BUGS
The exp_fexpect functions don’t work at all under HP-UX - it appears to
be a bug in getc. Follow the advice (above) about using the exp_expect
functions (which doesn’t need to call getc). If you fix the problem
(before I do - please check the latest release) let me know.
SEE ALSO
An alternative to this library is the expect program. expect
interprets scripts written in a high-level language which direct the
dialogue. In addition, the user can take control and interact directly
when desired. If it is not absolutely necessary to write your own C
program, it is much easier to use expect to perform the entire
interaction. It is described further in the following references:
"expect: Curing Those Uncontrollable Fits of Interactivity" by Don
Libes, Proceedings of the Summer 1990 USENIX Conference, Anaheim,
California, June 11-15, 1990.
"Using expect to Automate System Administration Tasks" by Don Libes,
Proceedings of the 1990 USENIX Large Installation Systems
Administration Conference, Colorado Springs, Colorado, October 17-19,
1990.
expect(1), alarm(3), read(2), write(2), fdopen(3), execve(2),
execvp(3), longjmp(3), pty(4).
There are several examples C programs in the test directory of expect’s
source distribution which use the expect library.
AUTHOR
Don Libes, libes@nist.gov, National Institute of Standards and
Technology
ACKNOWLEDGEMENTS
Thanks to John Ousterhout (UCBerkeley) for supplying the pattern
matcher.
Design and implementation of the expect library was paid for by the
U.S. government and is therefore in the public domain. However the
author and NIST would like credit if this program and documentation or
portions of them are used.
12 December 1991