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 );
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 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
EACCES , if another process holds a lock.
RETURN VALUE
On success, zero is returned. On error, -1 is returned, and
errno is set appropriately.
ERRORS
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
SYSV, POSIX 1003.1-2001
SEE ALSO
fcntl(2) ,flock(2) There are also
locks.txt and
mandatory.txt in
/usr/src/linux/Documentation .
