NAME

fcntl — file descriptor control

LIBRARY

Standard C Library (libc, -lc)

SYNOPSIS

#include <fcntl.h>

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

DESCRIPTION

fcntl() 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:

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:

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.

RETURN VALUES

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

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

COMPATIBILITY

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.

ERRORS

fcntl() will fail if:

STANDARDS

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

HISTORY

The fcntl() function call appeared in 4.2BSD.

Manual browser: fcntl(2)