LOCKF(3)						     Linux Programmer's Manual							  LOCKF(3)

NAME

       lockf - apply, test or remove a POSIX lock on an open file

SYNOPSIS

       #include <sys/file.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.

Linux 2.0							    2002-04-22								  LOCKF(3)