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)):

           _XOPEN_SOURCE >= 500
               || /* Glibc since 2.19: */ _DEFAULT_SOURCE
               || /* Glibc versions <= 2.19: */ _BSD_SOURCE || _SVID_SOURCE

       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

       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 F_LOCK and this lock operation would cause a

       EINTR  While waiting to acquire a lock, the call was interrupted by
              delivery of a signal caught by a handler; see signal(7).

       EINVAL An invalid operation was specified in cmd.

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

       For an explanation of the terms used in this section, see

       │Interface Attribute     Value   │
       │lockf()   │ Thread safety │ MT-Safe │
       POSIX.1-2001, POSIX.1-2008, SVr4.

       fcntl(2), flock(2)

       locks.txt and mandatory-locking.txt in the Linux 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 5.07 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

GNU                               2019-03-06                          LOCKF(3)