lockf(3) — Linux manual page


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

NAME         top

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

SYNOPSIS         top

       #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

DESCRIPTION         top

       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 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

RETURN VALUE         top

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

ERRORS         top

              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.

ATTRIBUTES         top

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

       │Interface Attribute     Value   │
       │lockf()   │ Thread safety │ MT-Safe │

CONFORMING TO         top

       POSIX.1-2001, POSIX.1-2008, SVr4.

SEE ALSO         top

       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)

COLOPHON         top

       This page is part of release 5.08 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)

Pages that refer to this page: fcntl(2)fcntl64(2)flock(2)flockfile(3)ftrylockfile(3)funlockfile(3)lslocks(8)