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  posi‐
       tions.  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 mix‐
       ing 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 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.

       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 prohib‐
              ited 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 cmd.

       ENOLCK Too many segment locks open, lock table is full.

ATTRIBUTES
   Multithreading (see pthreads(7))
       The lockf() function is thread-safe.

CONFORMING TO
       SVr4, POSIX.1-2001.

SEE ALSO
       fcntl(2), flock(2)

       locks.txt and mandatory-locking.txt  in  the  Linux  kernel  source  directory  Documenta‐
       tion/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 3.74 of the Linux man-pages project.  A  description  of  the
       project,  information  about  reporting  bugs, and the latest version of this page, can be
       found at http://www.kernel.org/doc/man-pages/.



GNU                                         2014-06-13                                   LOCKF(3)