FCNTL(2) System Calls Manual FCNTL(2)

fcntl — file control

#include <fcntl.h>

int
fcntl(int fd, int cmd, ...);

The () system call provides control over the properties of a file that is already open. The argument fd is a descriptor to be operated on by cmd as described below. The third parameter is called arg and is technically a pointer to void, but is interpreted as an int by some commands, a pointer to a struct flock by others (see below), and ignored by the rest.

The commands are:

Return a new descriptor as follows:

  • Lowest numbered available descriptor greater than or equal to arg (interpreted as an int).
  • 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) calls.
Like F_DUPFD, but the FD_CLOEXEC flag associated with the new file descriptor is set, so the file descriptor is closed when execve(2) is called.
Like F_DUPFD, but the FD_CLOFORK flag associated with the new file descriptor is set, so the file descriptor is closed in the child when fork(2) or vfork(2) is called.
Get the close-on-exec and close-on-fork flags associated with the file descriptor fd as FD_CLOEXEC and FD_CLOFORK. If the returned value ANDed with FD_CLOEXEC is 0, the file will remain open across execve(2), otherwise the file will be closed upon execution of execve(2); if the returned value ANDed with FD_CLOFORK is 0, the file will remain open across fork(2) and vfork(2), otherwise the file will be closed upon execution of fork(2) or vfork(2) (arg is ignored).
Set the close-on-exec and close-on-fork flags associated with fd to arg, where arg (interpreted as an int) is the bitwise OR of zero or more of FD_CLOEXEC and FD_CLOFORK, as described above.
Get file status flags associated with the file descriptor fd, as described below (arg is ignored).
Set file status flags associated with the file descriptor fd to arg (interpreted as an int).
Get the process ID or process group currently receiving SIGIO and SIGURG signals; process groups are returned as negative values (arg is ignored).
Set the process or process group to receive SIGIO and SIGURG signals; process groups are specified by supplying arg (interpreted as an int) as negative, otherwise arg is taken as a process ID.

The flags for the F_GETFL and F_SETFL commands are as follows:

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.
Force each write to append at the end of file; corresponds to the O_APPEND flag of open(2).
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.
Cause writes to be synchronous. Data will be written to the physical device instead of just being stored in the buffer cache; corresponds to the O_SYNC flag of open(2).

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:

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 () 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.
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.
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, the area starting at l_start+l_len and ending at l_start-1 is locked. 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 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 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) function. The flock(2) interface has much more rational last close semantics and allows 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 () 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 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.

Upon successful completion, the value returned depends on cmd as follows:

A new file descriptor.
A new file descriptor.
A new file descriptor.
Value of file descriptor flags (only the FD_CLOEXEC and FD_CLOFORK bits are defined).
Value of flags.
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.

fcntl() will fail if:

[]
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.
[]
fd 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 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.

[]
The argument cmd is F_SETLKW, and a deadlock condition was detected.
[]
The argument cmd is F_SETLK or F_SETLKW, and the function was interrupted by a signal.
[]
The argument cmd is invalid.

cmd is F_DUPFD and arg is negative or greater than the maximum allowable number (see getdtablesize(3)).

The argument cmd is F_GETLK, F_SETLK, or F_SETLKW and the data to which arg points is not valid, or fd refers to a file that does not support locking.

[]
The argument cmd is F_DUPFD and the maximum number of open file descriptors permitted for the process are already in use, or no file descriptors greater than or equal to arg are available.
[]
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.
[]
The argument cmd is F_GETLK, F_SETLK or F_SETLKW and the segment length of a file to be locked is too large to be represented by an off_t.
[]
cmd is F_SETOWN and the process ID given in arg is not in use.

close(2), execve(2), flock(2), open(2), sigaction(2), getdtablesize(3)

The fcntl() function conforms to IEEE Std 1003.1-2008 (“POSIX.1”).

The fcntl() system call first appeared in AT&T System III UNIX and was reimplemented for 4.2BSD.

August 4, 2025 Debian