lockf

       #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
       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

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

ATTRIBUTES
       For  an  explanation  of  the  terms  used   in   this   section,   see
       attributes(7).

       +----------+---------------+---------+
       |Interface | Attribute     | Value   |
       +----------+---------------+---------+
       |lockf()   | Thread safety | MT-Safe |
       +----------+---------------+---------+
CONFORMING TO
       POSIX.1-2001, POSIX.1-2008, SVr4.

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 4.04 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                               2015-08-08                          LOCKF(3)
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