LOCKF(3) Linux Programmer's Manual LOCKF(3)
lockf - apply, test or remove a POSIX lock on an open file
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 posi-
tions. 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 sec-
tion 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.
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.
On success, zero is returned. On error, -1 is returned, and errno is set appropriately.
EAGAIN The file is locked and F_TLOCK or F_TEST was specified, or the operation is prohib-
ited because the file has been memory-mapped by another process.
EBADF fd is not an open file descriptor.
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.
SYSV, POSIX 1003.1-2001
There are also locks.txt and mandatory.txt in /usr/src/linux/Documentation.
Linux 2.0 2002-04-22 LOCKF(3)