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LOCKF(3)                   Linux Programmer's Manual                  LOCKF(3)

       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)):

       lockf(): _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 descrip-
       tor 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

       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.

              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.

       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.)

       This  page  is part of release 3.22 of the Linux man-pages project.  A description of the project, and informa-
       tion about reporting bugs, can be found at

GNU                               2009-07-25                          LOCKF(3)