fcntl - manipulate file descriptor
int fcntl(int fd, int cmd));
int fcntl(int fd, int cmd, long arg
fcntl performs one of various miscellaneous operations on
fd. The operation in question is determined by cmd:
F_DUPFD Makes arg be a copy of fd, closing fd first if
The same functionality can be more easily achieved
by using dup2(2).
The old and new descriptors may be used inter-
changeably. They share locks, file position
pointers and flags; for example, if the file posi-
tion is modified by using lseek on one of the
descriptors, the position is also changed for the
The two descriptors do not share the close-on-exec
flag, however. The close-on-exec flag of the copy
is off, meaning that it will be closed on exec.
On success, the new descriptor is returned.
F_GETFD Read the close-on-exec flag. If the low-order bit
is 0, the file will remain open across exec, other-
wise it will be closed.
F_SETFD Set the close-on-exec flag to the value specified
by arg (only the least significant bit is used).
F_GETFL Read the descriptor's flags (all flags (as set by
open(2)) are returned).
F_SETFL Set the descriptor's flags to the value specified
by arg. Only O_APPEND and O_NONBLOCK may be set.
The flags are shared between copies (made with dup
etc.) of the same file descriptor.
The flags and their semantics are described in
F_GETLK, F_SETLK and F_SETLKW
Manage discretionary file locks. The third argu-
ment arg is a pointer to a struct flock (that may
be overwritten by this call).
F_GETLK Return the flock structure that prevents us from
obtaining the lock, or set the l_type field of the
lock to F_UNLCK if there is no obstruction.
F_SETLK The lock is set (when l_type is F_RDLCK or F_WRLCK)
or cleared (when it is F_UNLCK). If the lock is
held by someone else, this call returns -1 and sets
errno to EACCES or EAGAIN.
F_SETLKW Like F_SETLK, but instead of returning an error we
wait for the lock to be released. If a signal that
is to be caught is received while fcntl() is wait-
ing, it is interrupted and returns immediately
(with return value -1 and errno set to EINTR).
F_GETOWN Get the process ID or process group currently
receiving SIGIO and SIGURG signals for events on
file descriptor fd. Process groups are returned as
F_SETOWN Set the process ID or process group that will
receive SIGIO and SIGURG signals for events on file
descriptor fd. Process groups are specified using
If you set the O_ASYNC status flag on a file
descriptor (either by providing this flag with the
open call, or by using the F_SETFL command of
fcntl), a SIGIO signal is sent whenever input or
output becomes possible on that file descriptor.
The process or process group to receive the signal
can be selected by using the F_SETOWN command to
the fcntl function. If the file descriptor is a
socket, this also selects the recipient of SIGURG
signals that are delivered when out-of-band data
arrives on that socket. (SIGURG is sent in any
situation where select would report the socket as
having an "exceptional condition".) If the file
descriptor corresponds to a terminal device, then
SIGIO signals are sent to the foreground process
group of the terminal.
The use of O_ASYNC, F_GETOWN, F_SETOWN is BSD-
specific. POSIX has asynchronous I/O and the
aio_sigevent structure to achieve similar things.
For a successful call, the return value depends on the
F_DUPFD The new descriptor.
F_GETFD Value of flag.
F_GETFL Value of flags.
F_GETOWN Value of descriptor owner.
F_SETFD, F_SETFL, F_GETLK, F_SETLK, F_SETLKW Some value dif-
ferent from -1.
On error, -1 is returned, and errno is set appropriately.
EACCES Operation is prohibited by locks held by other
EAGAIN Operation is prohibited because the file has been
memory-mapped by another process.
EDEADLK It was detected that the specified F_SETLKW command
would cause a deadlock.
EBADF fd is not an open file descriptor.
EINTR The F_SETLKW command was interrupted by a signal.
EINVAL For F_DUPFD, arg is negative or is greater than the
maximum allowable value.
EMFILE For F_DUPFD, the process already has the maximum
number of file descriptors open.
ENOLCK Too many segment locks open, lock table is full.
The errors returned by dup2 are different from those
returned by F_DUPFD.
SVr4, SVID, POSIX, X/OPEN, BSD 4.3. Only the operations
F_DUPFD, F_GETFD, F_SETFD, F_GETFL, F_SETFL, F_GETLK,
F_SETLK and F_SETLKW are specified in POSIX.1; F_GETOWN and
F_SETOWN are BSDisms not supported in SVr4. The flags legal
for F_GETFL/F_SETFL are those supported by open(2) and vary
between these systems; O_APPEND, O_NONBLOCK, O_RDONLY, and
O_RDWR are specified in POSIX.1. SVr4 supports several
other options and flags not documented here.
POSIX.1 documents an additional EINTR condition. SVr4 docu-
ments additional EFAULT, EINTR, EIO, ENOLINK and EOVERFLOW
dup2(2), open(2), socket(2), flock(2)