lockf

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



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

SYNOPSIS
       #include <unistd.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	nega-
       tive,  where pos is the current file position, and if len is zero, the
       section extends from the current file position to infinity, encompass-
       ing  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/Documen-
       tation.



Linux 2.0			  2002-04-22			     LOCKF(3)