lockf(3) -- Linux man page
NAMElockf - apply, test or remove a POSIX lock on an open file
DESCRIPTIONApply, 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:
- 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.
- Same as F_LOCK but the call never blocks and returns an error instead if the file is already locked.
- Unlock the indicated section of the file. This may cause a locked section to be split into two locked sections.
- 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 VALUEOn 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.
CONFORMING TOSYSV, POSIX 1003.1-2001
SEE ALSOfcntl(2), flock(2)
There are also locks.txt and mandatory.txt in /usr/src/linux/Documentation.