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


       #include <unistd.h>

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

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

           _BSD_SOURCE || _SVID_SOURCE || _XOPEN_SOURCE >= 500 ||


       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
              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.

              Same  as  F_LOCK  but the call never blocks and returns an error
              instead if the file is already locked.

              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.


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


              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.

              The  command  was  T_LOCK  and this lock operation would cause a

       EINVAL An invalid operation was specified in fd.

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


       SVr4, POSIX.1-2001.


       fcntl(2), flock(2)
       There are also locks.txt and mandatory-locking.txt in the kernel source
       directory  Documentation/filesystems.   (On  older kernels, these files
       are  directly  under  the  Documentation/  directory,  and   mandatory-
       locking.txt is called mandatory.txt.)


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