lockf — apply, test or remove a POSIX lock on an open file
#include <unistd.h>
int
lockf( |
int | fd, |
int | cmd, | |
off_t | len) ; |
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.
On success, zero is returned. On error, −1 is
returned, and errno
is set
appropriately.
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.
fd
is not an
open file descriptor.
The command was T_LOCK
and this lock operation would cause a deadlock.
An invalid operation was specified in fd
.
Too many segment locks open, lock table is full.
There are also locks.txt
and mandatory.txt
in /usr/src/linux/Documentation
.
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