LOCKF(3)		   Linux Programmers Manual		     LOCKF(3)



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

SYNOPSIS
       #include 

       int lockf(int fd, int cmd, off_t len);

   Feature Test Macro Requirements for glibc (see feature_test_macros(7)):

       lockf(): _BSD_SOURCE || _SVID_SOURCE || _XOPEN_SOURCE >= 500

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(2) 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 ear
	      lier 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  EAGAIN  (EACCES
	      on some other systems), if another process holds a lock.

RETURN VALUE
       On  success,  zero is returned.	On error, -1 is returned, and errno is
       set appropriately.

ERRORS
       EACCES or 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
       SVr4, POSIX.1-2001.

SEE ALSO
       fcntl(2), flock(2)
       There are also locks.txt and mandatory.txt in /usr/src/linux/Documenta
       tion.

COLOPHON
       This page is part of release 3.05 of the Linux  man-pages  project.   A
       description  of	the project, and information about reporting bugs, can
       be found at http://www.kernel.org/doc/man-pages/.



GNU				  2007-07-26			      LOCKF(3)