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
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.