FCNTL(2) System Calls FCNTL(2)
NAME
fcntl - file control
SYNOPSIS
#include <fcntl.h>
int fcntl (int fd, int cmd, ... /* int arg */ );
DESCRIPTION
See the BUGS section for caveats on the GNO implementation.
Fcntl provides for control over descriptors. The argument fd is a de‐
scriptor to be operated on by cmd as follows:
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 de‐
scriptor is set to remain open across execve(2) system
calls.
F_GETFD
Get the close-on-exec flag associated with the file de‐
scriptor fd. If the low-order bit of the returned value
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 the low
order bit of arg (0 or 1 as 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)
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; corre‐
sponds to the O_APPEND flag of open(2).
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 */
};
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 function 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 ex‐
clusive (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 EAC
CES.
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 por‐
tion 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 ac‐
cess.
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, the result is undefined. The l_pid field is only
used with F_GETLK to return the process ID of the process holding a
blocking lock. 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 ap‐
plication 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. Be‐
fore a successful return from an F_SETLK or an F_SETLKW request when
the calling process has previously existing locks on bytes in the re‐
gion 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 an‐
other 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
POSIX 1003.1-88 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 re‐
lease 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 inher‐
ited by a child process created using the fork(2) function. The
flock(2) interface has much more rational last close semantics and al‐
lows locks to be inherited by child processes. Flock(2) 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. Note
that flock(2) and fcntl(2) locks may be safely used concurrently.
All locks associated with a file for a given process are removed when
the process terminates.
A potential for deadlock occurs if a process controlling a locked re‐
gion is put to sleep by attempting to lock the locked region of another
process. This implementation detects that sleeping until a locked re‐
gion is unlocked would cause a deadlock and fails with an EDEADLK er‐
ror.
IMPLEMENTATION NOTES
In the non-threaded library fcntl is implemented as the fcntl syscall.
In the threaded library, the fcntl syscall is assembled to
_thread_sys_fcntl and fcntl is implemented as a function which disables
thread rescheduling, locks fd for read and write, then calls
_thread_sys_fcntl. Before returning, fcntl unlocks fd and enables
thread rescheduling.
RETURN VALUES
Upon successful completion, the value returned depends on cmd as fol‐
lows:
F_DUPFD
A new file descriptor.
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
Fcntl will fail if:
EACCES The argument arg 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 Fildes is not a valid open file descriptor.
The argument cmd is F_SETLK or F_SETLKW, the type of lock
(l_type) is a shared lock (F_RDLCK), and fildes 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 fildes 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 function was inter‐
rupted by a signal.
EINVAL Cmd 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, or fildes refers to a file
that does not support locking.
EMFILE The argument cmd is F_DUPFD 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.
ESRCH Cmd 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).
BUGS
Currently, the only cmds used by the GNO implementation are F_DUPFD and
F_GETFL. Any other values will result in an error and set errno to
EINVAL.
SEE ALSO
close(2), execve(2), flock(2), getdtablesize(2), open(2), sigvec(2),
tcgetpgrp(3), tcsetpgrp(3)
HISTORY
The fcntl function call appeared in 4.2BSD.
GNO 15 September 1997 FCNTL(2)
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