lockf(3) — Linux manual page


lockf(3)                Library Functions Manual                lockf(3)

NAME         top

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

LIBRARY         top

       Standard C library (libc, -lc)

SYNOPSIS         top

       #include <unistd.h>

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

   Feature Test Macro Requirements for glibc (see

           _XOPEN_SOURCE >= 500
               || /* glibc >= 2.19: */ _DEFAULT_SOURCE
               || /* glibc <= 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

       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 lock.

RETURN VALUE         top

       On success, zero is returned.  On error, -1 is returned, and
       errno is set to indicate the error.

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 deadlock.

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

       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 │

STANDARDS         top


HISTORY         top

       POSIX.1-2001, 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)

Linux man-pages (unreleased)     (date)                         lockf(3)

Pages that refer to this page: fcntl(2)flock(2)flockfile(3)off_t(3type)lslocks(8)