FCNTL(2) System Calls Manual FCNTL(2)

fcntl — file descriptor control

Standard C Library (libc, -lc)

#include <fcntl.h>

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

() provides for control over descriptors. 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 it is interpreted as an int by some commands and ignored by others.

Commands are:

Return a new descriptor as follows:

  • Lowest numbered available descriptor greater than or equal to arg, which is 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 cleared to allow the file descriptor to remain open across execve(2) system calls.
  • The close-on-fork flag FD_CLOFORK associated with the new file descriptor is cleared, so the file descriptor will remain open across fork(2) system calls.
Same as F_DUPFD, but sets the close-on-exec property on the file descriptor created.
Same as F_DUPFD, but sets the close-on-fork property on the file descriptor created.
Same as F_DUPFD, but sets both the close-on-exec and close-on-fork properties on the file descriptor created.
Get the flags associated with the file descriptor fd (arg is ignored). If the returned value ANDed with FD_CLOEXEC is 0, the file will remain open across (), otherwise the file will be closed upon execution of exec(). If the returned value ANDed with FD_CLOFORK is not 0, the file will be closed upon execution of the () family of system calls.
Set flags associated with fd. The available flags, passed as arg (treated as an integer), are FD_CLOEXEC and FD_CLOFORK.
Get descriptor status flags, as described below (arg is ignored).
Set descriptor status flags to arg, which is 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 as negative, otherwise arg is interpreted as a process ID. The argument arg is interpreted as an int.
Close all file descriptors greater than or equal to fd, (arg is ignored).
Return the maximum file descriptor number currently open by the process. (arg is ignored).
Return 1 if the O_NOSIGPIPE flag is set in the file descriptor, otherwise 0 (arg is ignored).
If arg is 0, clear the O_NOSIGPIPE status in the file descriptor, otherwise set it.
Place a pathname corresponding to fd in the buffer pointed to by arg. arg should be pointing to a buffer of at least MAXPATHLEN bytes.
Add seals specified in arg to fd to restrict possible operations on fd as described below. Like flags, multiple seals can be specified at once. Additionally, specifying seals that are already associated with fd is a no-op.
Get the seals currently associated with fd as described below (arg is ignored).

The set of valid flags for the F_GETFL and F_SETFL flags are as follows: O_APPEND, O_ASYNC, O_SYNC, O_NONBLOCK, O_DSYNC, O_RSYNC, O_ALT_IO, O_DIRECT, O_NOSIGPIPE. These flags are described in 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 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 l_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). As specified by the value of l_type, 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 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 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.

Possible seals are:

Prevent any further seals from being added to fd.
Prevent the size of fd from decreasing.
Prevent the size of fd from increasing.
Prevent any write operations to fd. F_SEAL_WRITE cannot be applied if fd has any memory mappings.
Like F_SEAL_WRITE but allow any current memory mappings of fd to remain open, including those with PROT_WRITE.

For F_GETPATH:

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

A new file descriptor.
Value of flag (one or both of the bits FD_CLOEXEC and FD_CLOFORK).
Value of flags.
Value of file descriptor owner.
Value of the highest file descriptor open by the process.
Value of the seals currently associated with fd.
other
Value other than -1.

Otherwise, a value of -1 is returned and errno is set to indicate the error.

This interface follows the completely stupid semantics of AT&T System V UNIX 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) function. The flock(2) interface has much more rational last close semantics and allows locks to be inherited by child processes. Calling 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 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.

fcntl() will fail if:

[]
The argument cmd is F_GETPATH and read or search permission was denied for a component of the pathname.
[]
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.
[]
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.

[]
The argument cmd is F_ADD_SEALS, arg contains F_SEAL_WRITE and fd is currently mapped by mmap(2).
[]
The argument cmd is F_SETLKW, and a deadlock condition was detected.
[]
The argument cmd is F_SETLKW, and the function was interrupted by a signal.
[]
The argument cmd is invalid.

The argument 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 fildes refers to a file that does not support locking.

The argument cmd is F_ADD_SEALS or F_GET_SEALS and fd does not support seals.

The argument cmd is F_ADD_SEALS and arg contains set bits for unsupported seals.

[]
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.
[]
cmd is F_DUPFD and system-wide the maximum allowed number of file descriptors are currently open.
[]
The argument cmd is F_GETPATH and a component of the pathname no longer exists.
[]
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_GETPATH and insufficient memory is available.

The argument cmd is F_GETLK, F_SETLK, or F_SETLKW, and the file lock limit for the current unprivileged user has been reached. It can be modified using the kern.maxfiles sysctl(7).

[]
The argument cmd is F_ADD_SEALS and fd already has F_SEAL_SEAL.
[]
The argument cmd is F_GETPATH and the resulting path would be greater than MAXPATHLEN.
[]
cmd is F_SETOWN and the process ID given as argument is not in use.

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

The fcntl() function conforms to IEEE Std 1003.1-1990 (“POSIX.1”).

The fcntl() function call appeared in 4.2BSD. The F_DUPFD_CLOBOTH operation first appeared in NetBSD 11.0.

July 8, 2025 NetBSD 11.0