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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);
   Feature Test Macro Requirements for glibc (see feature_test_macros(7)):
       lockf():
           _BSD_SOURCE || _SVID_SOURCE || _XOPEN_SOURCE >= 500 ||
           _XOPEN_SOURCE && _XOPEN_SOURCE_EXTENDED
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,  lockf()  is  just  an interface on top of fcntl(2) locking.
       Many other systems  implement  lockf()  in  this  way,  but  note  that
       POSIX.1-2001 leaves the relationship between lockf() and fcntl(2) locks
       unspecified.  A portable application should probably avoid mixing calls
       to these interfaces.
       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; or cmd is F_LOCK  or  F_TLOCK
              and fd is not a writable file descriptor.
       EDEADLK
              The  command  was  F_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)
       locks.txt and mandatory-locking.txt in the Linux kernel  source  direc-
       tory  Documentation/filesystems  (on  older  kernels,  these  files are
       directly under the Documentation directory,  and  mandatory-locking.txt
       is called mandatory.txt)
COLOPHON
       This  page  is  part of release 3.53 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                               2012-07-07                          LOCKF(3)