NAME
fcntl - file control
LIBRARY
Standard C Library (libc, -lc)
SYNOPSIS
#include <fcntl.h>
int
fcntl(int fd, int cmd, ...);
DESCRIPTION
The fcntl() system call provides for control over descriptors. The
argument fd is a descriptor to be operated on by cmd as described below.
Depending on the value of cmd, fcntl() can take an additional third
argument int arg.
F_DUPFD Return a new descriptor as follows:
· Lowest numbered available descriptor greater than or
equal to arg.
· Same object references as the original descriptor.
· New descriptor shares the same file offset if the
object was a file.
· Same access mode (read, write or read/write).
· Same file status flags (i.e., both file descriptors
share the same file status flags).
· The close-on-exec flag associated with the new file
descriptor is set to remain open across execve(2)
system calls.
F_DUP2FD It is functionally equivalent to
dup2(fd, arg)
The F_DUP2FD constant is not portable, so it should not be
used if portability is needed. Use dup2() instead.
F_GETFD Get the close-on-exec flag associated with the file descriptor
fd as FD_CLOEXEC. If the returned value ANDed with FD_CLOEXEC
is 0, the file will remain open across exec(), otherwise the
file will be closed upon execution of exec() (arg is ignored).
F_SETFD Set the close-on-exec flag associated with fd to arg, where
arg is either 0 or FD_CLOEXEC, as described above.
F_GETFL Get descriptor status flags, as described below (arg is
ignored).
F_SETFL Set descriptor status flags to arg.
F_GETOWN Get the process ID or process group currently receiving SIGIO
and SIGURG signals; process groups are returned as negative
values (arg is ignored).
F_SETOWN Set the process or process group to receive SIGIO and SIGURG
signals; process groups are specified by supplying arg as
negative, otherwise arg is interpreted as a process ID.
The flags for the F_GETFL and F_SETFL flags are as follows:
O_NONBLOCK Non-blocking I/O; if no data is available to a read(2)
system call, or if a write(2) operation would block, the
read or write call returns -1 with the error EAGAIN.
O_APPEND Force each write to append at the end of file; corresponds
to the O_APPEND flag of open(2).
O_DIRECT Minimize or eliminate the cache effects of reading and
writing. The system will attempt to avoid caching the data
you read or write. If it cannot avoid caching the data, it
will minimize the impact the data has on the cache. Use of
this flag can drastically reduce performance if not used
with care.
O_ASYNC Enable the SIGIO signal to be sent to the process group when
I/O is possible, e.g., upon availability of data to be read.
Several commands are available for doing advisory file locking; they all
operate on the following structure:
struct flock {
off_t l_start; /* starting offset */
off_t l_len; /* len = 0 means until end of file */
pid_t l_pid; /* lock owner */
short l_type; /* lock type: read/write, etc. */
short l_whence; /* type of l_start */
int l_sysid; /* remote system id or zero for local */
};
The commands available for advisory record locking are as follows:
F_GETLK Get the first lock that blocks the lock description pointed to
by the third argument, arg, taken as a pointer to a struct
flock (see above). The information retrieved overwrites the
information passed to fcntl() in the flock structure. If no
lock is found that would prevent this lock from being created,
the structure is left unchanged by this system call except for
the lock type which is set to F_UNLCK.
F_SETLK Set or clear a file segment lock according to the lock
description pointed to by the third argument, arg, taken as a
pointer to a struct flock (see above). F_SETLK is used to
establish shared (or read) locks (F_RDLCK) or exclusive (or
write) locks, (F_WRLCK), as well as remove either type of lock
(F_UNLCK). If a shared or exclusive lock cannot be set,
fcntl() returns immediately with EAGAIN.
F_SETLKW This command is the same as F_SETLK except that if a shared or
exclusive lock is blocked by other locks, the process waits
until the request can be satisfied. If a signal that is to be
caught is received while fcntl() is waiting for a region, the
fcntl() will be interrupted if the signal handler has not
specified the SA_RESTART (see sigaction(2)).
When a shared lock has been set on a segment of a file, other processes
can set shared locks on that segment or a portion of it. A shared lock
prevents any other process from setting an exclusive lock on any portion
of the protected area. A request for a shared lock fails if the file
descriptor was not opened with read access.
An exclusive lock prevents any other process from setting a shared lock
or an exclusive lock on any portion of the protected area. A request for
an exclusive lock fails if the file was not opened with write access.
The value of l_whence is SEEK_SET, SEEK_CUR, or SEEK_END to indicate that
the relative offset, l_start bytes, will be measured from the start of
the file, current position, or end of the file, respectively. The value
of l_len is the number of consecutive bytes to be locked. If l_len is
negative, l_start means end edge of the region. The l_pid and l_sysid
fields are only used with F_GETLK to return the process ID of the process
holding a blocking lock and the system ID of the system that owns that
process. Locks created by the local system will have a system ID of
zero. After a successful F_GETLK request, the value of l_whence is
SEEK_SET.
Locks may start and extend beyond the current end of a file, but may not
start or extend before the beginning of the file. A lock is set to
extend to the largest possible value of the file offset for that file if
l_len is set to zero. If l_whence and l_start point to the beginning of
the file, and l_len is zero, the entire file is locked. If an
application wishes only to do entire file locking, the flock(2) system
call is much more efficient.
There is at most one type of lock set for each byte in the file. Before
a successful return from an F_SETLK or an F_SETLKW request when the
calling process has previously existing locks on bytes in the region
specified by the request, the previous lock type for each byte in the
specified region is replaced by the new lock type. As specified above
under the descriptions of shared locks and exclusive locks, an F_SETLK or
an F_SETLKW request fails or blocks respectively when another process has
existing locks on bytes in the specified region and the type of any of
those locks conflicts with the type specified in the request.
This interface follows the completely stupid semantics of System V and
IEEE Std 1003.1-1988 (“POSIX.1”) that require that all locks associated
with a file for a given process are removed when any file descriptor for
that file is closed by that process. This semantic means that
applications must be aware of any files that a subroutine library may
access. For example if an application for updating the password file
locks the password file database while making the update, and then calls
getpwnam(3) to retrieve a record, the lock will be lost because
getpwnam(3) opens, reads, and closes the password database. The database
close will release all locks that the process has associated with the
database, even if the library routine never requested a lock on the
database. Another minor semantic problem with this interface is that
locks are not inherited by a child process created using the fork(2)
system call. The flock(2) interface has much more rational last close
semantics and allows locks to be inherited by child processes. The
flock(2) system call is recommended for applications that want to ensure
the integrity of their locks when using library routines or wish to pass
locks to their children.
The fcntl(), flock(2), and lockf(3) locks are compatible. Processes
using different locking interfaces can cooperate over the same file
safely. However, only one of such interfaces should be used within the
same process. If a file is locked by a process through flock(2), any
record within the file will be seen as locked from the viewpoint of
another process using fcntl() or lockf(3), and vice versa. Note that
fcntl(F_GETLK) returns -1 in l_pid if the process holding a blocking lock
previously locked the file descriptor by flock(2).
All locks associated with a file for a given process are removed when the
process terminates.
All locks obtained before a call to execve(2) remain in effect until the
new program releases them. If the new program does not know about the
locks, they will not be released until the program exits.
A potential for deadlock occurs if a process controlling a locked region
is put to sleep by attempting to lock the locked region of another
process. This implementation detects that sleeping until a locked region
is unlocked would cause a deadlock and fails with an EDEADLK error.
RETURN VALUES
Upon successful completion, the value returned depends on cmd as follows:
F_DUPFD A new file descriptor.
F_DUP2FD A file descriptor equal to arg.
F_GETFD Value of flag (only the low-order bit is defined).
F_GETFL Value of flags.
F_GETOWN Value of file descriptor owner.
other Value other than -1.
Otherwise, a value of -1 is returned and errno is set to indicate the
error.
ERRORS
The fcntl() system call will fail if:
[EAGAIN] The argument cmd is F_SETLK, the type of lock (l_type)
is a shared lock (F_RDLCK) or exclusive lock
(F_WRLCK), and the segment of a file to be locked is
already exclusive-locked by another process; or the
type is an exclusive lock and some portion of the
segment of a file to be locked is already shared-
locked or exclusive-locked by another process.
[EBADF] The fd argument is not a valid open file descriptor.
The argument cmd is F_DUP2FD, and arg is not a valid
file descriptor.
The argument cmd is F_SETLK or F_SETLKW, the type of
lock (l_type) is a shared lock (F_RDLCK), and fd is
not a valid file descriptor open for reading.
The argument cmd is F_SETLK or F_SETLKW, the type of
lock (l_type) is an exclusive lock (F_WRLCK), and fd
is not a valid file descriptor open for writing.
[EDEADLK] The argument cmd is F_SETLKW, and a deadlock condition
was detected.
[EINTR] The argument cmd is F_SETLKW, and the system call was
interrupted by a signal.
[EINVAL] The cmd argument is F_DUPFD and arg is negative or
greater than the maximum allowable number (see
getdtablesize(2)).
The argument cmd is F_GETLK, F_SETLK or F_SETLKW and
the data to which arg points is not valid.
[EMFILE] The argument cmd is F_DUPFD or F_DUP2FD and the
maximum number of file descriptors permitted for the
process are already in use, or no file descriptors
greater than or equal to arg are available.
[ENOLCK] The argument cmd is F_SETLK or F_SETLKW, and
satisfying the lock or unlock request would result in
the number of locked regions in the system exceeding a
system-imposed limit.
[EOPNOTSUPP] The argument cmd is F_GETLK, F_SETLK or F_SETLKW and
fd refers to a file for which locking is not
supported.
[EOVERFLOW] The argument cmd is F_GETLK, F_SETLK or F_SETLKW and
an off_t calculation overflowed.
[EPERM] The cmd argument is F_SETOWN and the process ID or
process group given as an argument is in a different
session than the caller.
[ESRCH] The cmd argument is F_SETOWN and the process ID given
as argument is not in use.
In addition, if fd refers to a descriptor open on a terminal device (as
opposed to a descriptor open on a socket), a cmd of F_SETOWN can fail for
the same reasons as in tcsetpgrp(3), and a cmd of F_GETOWN for the
reasons as stated in tcgetpgrp(3).
SEE ALSO
close(2), dup2(2), execve(2), flock(2), getdtablesize(2), open(2),
sigvec(2), lockf(3), tcgetpgrp(3), tcsetpgrp(3)
STANDARDS
The F_DUP2FD constant is non portable. It is provided for compatibility
with AIX and Solaris.
HISTORY
The fcntl() system call appeared in 4.2BSD.
The F_DUP2FD constant first appeared in FreeBSD 7.1.