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Manual page for LOCKF(3)

lockf - advisory record locking on files

SYNOPSIS

#include <sys/fcntl.h>

#define F_ULOCK 0 /* Unlock a previously locked section */ #define F_LOCK 1 /* Lock a section for exclusive use */ #define F_TLOCK 2 /* Test and lock a section (non-blocking) */ #define F_TEST 3 /* Test section for other process' locks */

int lockf(fd, cmd, size) int fd, cmd; long size;

DESCRIPTION

lockf() may be used to test, apply, or remove an advisory record lock on the file associated with the open descriptor fd. Advisory locks are non-mandatory: They allow cooperating processes to perform consistent operations on files, but do not prevent other processes from ignoring the locks and accessing the files.

A lock is obtained by specifying a cmd parameter of F_LOCK or F_TLOCK. To unlock an existing lock, the F_ULOCK cmd is used. F_TEST is used to detect if a lock by another process is present on the specified segment.

F_LOCK and F_TLOCK requests differ only by the action taken if the lock may not be immediately granted. F_TLOCK returns a -1 by the function and sets errno to EAGAIN if the section is already locked by another process. F_LOCK will cause the process to sleep until the lock may be granted or a signal is caught.

size is the number of contiguous bytes to be locked or unlocked. The lock starts at the current file offset in the file and extends forward for a positive size or backward for a negative size (preceding but not including the current offset). A segment need not be allocated to the file in order to be locked; however, a segment may not extend to a negative offset relative to the beginning of the file. If size is zero, the lock will extend from the current offset through the EOF. If such a lock starts at offset 0, then the entire file will be locked (regardless of future file extensions).

NOTES

The descriptor fd must have been opened with O_WRONLY or O_RDWR permission in order to establish locks with this function call.

All locks associated with a file for a given process are removed when the file is closed or the process terminates. Locks are not inherited by the child process in a fork.2 system call.

RETURN VALUE

Zero is returned on success, -1 on error, with an error code stored in errno.

ERRORS

lockf() will fail if one or more of the following are true:

EBADF: fd is not a valid open descriptor.
EBADF: cmd is F_LOCK or F_TLOCK and the process does not have write permission on the file.
EAGAIN: cmd is F_TLOCK or F_TEST and the section is already locked by another process.
EINTR: cmd is F_LOCK and a signal interrupted the process while it was waiting for the lock to be granted.
ENOLCK: cmd is F_LOCK, F_TLOCK, or F_ULOCK and there are no more file lock entries available.

BUGS

File locks obtained through the lockf() mechanism do not interact in any way with those acquired using flock.2 They do, however, work correctly with the locks claimed by fcntl.2