LOCKF(3)

NAME
lockf - apply, test or remove a POSIX lock on an open file

SYNOPSIS
#include <unistd.h>

int lockf(int fd, int cmd, off_t len);

Feature Test Macro Requirements for glibc (see feature_test_macros(7)):

lockf(): _BSD_SOURCE || _SVID_SOURCE || _XOPEN_SOURCE >= 500

DESCRIPTION
Apply, test or remove a POSIX lock on a section of an open file. The file is specified by fd, a file descriptor open for writing, the action by cmd, and the section consists of byte positions pos..pos+len-1 if len is positive, and pos-len..pos-1 if len is negative, where pos is the current file position, and if len is zero, the section extends from the current file position to infinity, encompassing the present and future end-of-file positions. In all cases, the section may extend past current end-of-file.

On Linux, this call is just an interface for fcntl(2). (In general, the relation between lockf() and fcntl(2) is unspecified.)

Valid operations are given below:

F_LOCK
    Set an exclusive lock on the specified section of the file. If (part of) this section is already locked, the call blocks until the previous lock is released. If this section overlaps an earlier locked section, both are merged. File locks are released as soon as the process holding the locks closes some file descriptor for the file. A child process does not inherit these locks.

F_TLOCK
    Same as F_LOCK but the call never blocks and returns an error instead if the file is already locked.

F_ULOCK
    Unlock the indicated section of the file. This may cause a locked section to be split into two locked sections.

F_TEST
    Test the lock: return 0 if the specified section is unlocked or locked by this process; return -1, set errno to EAGAIN (EACCES on some other systems), if another process holds a lock.

RETURN VALUE
On success, zero is returned. On error, -1 is returned, and errno is set appropriately.

ERRORS

EACCES or EAGAIN
    The file is locked and F_TLOCK or F_TEST was specified, or the operation is prohibited because the file has been memory-mapped by another process.

EBADF
    fd is not an open file descriptor.

EDEADLK
    The command was T_LOCK and this lock operation would cause a deadlock.

EINVAL
    An invalid operation was specified in fd.

ENOLCK
    Too many segment locks open, lock table is full.

CONFORMING TO
SVr4, POSIX.1-2001

SEE ALSO
fcntl(2), flock(2)
There are also locks.txt and mandatory.txt in /usr/src/linux/Documentation.