lockf(3) - Linux manual page (original) (raw)

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** _op_**, off_t** _size_**);**

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

   **lockf**():
       _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 _op_, and the section consists of byte positions
   _pos_.._pos_+_size_-1 if _size_ is positive, and _pos_-_size_.._pos_-1 if _size_
   is negative, where _pos_ is the current file position, and if _size_
   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)](../man2/fcntl.2.html) locking.
   Many other systems implement **lockf**() in this way, but note that
   POSIX.1 leaves the relationship between **lockf**() and [fcntl(2)](../man2/fcntl.2.html) 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.

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

   **F_ULOCK**
          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](../man3/errno.3.html)_ 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](../man3/errno.3.html)_
   is set to indicate the error.

ERRORS top

   **EACCES** or **EAGAIN**
          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 _op_ is **F_LOCK** or
          **F_TLOCK** and _fd_ is not a writable file descriptor.

   **EDEADLK**
          _op_ 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 [signal(7)](../man7/signal.7.html).

   **EINVAL** An invalid operation was specified in _op_.

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

ATTRIBUTES top

   For an explanation of the terms used in this section, see
   [attributes(7)](../man7/attributes.7.html).
   ┌──────────────────────────────────────┬───────────────┬─────────┐
   │ **Interface** │ **Attribute** │ **Value** │
   ├──────────────────────────────────────┼───────────────┼─────────┤
   │ **lockf**()                              │ Thread safety │ MT-Safe │
   └──────────────────────────────────────┴───────────────┴─────────┘

STANDARDS top

   POSIX.1-2008.

HISTORY top

   POSIX.1-2001, SVr4.

SEE ALSO top

   [fcntl(2)](../man2/fcntl.2.html), [flock(2)](../man2/flock.2.html)

   _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_)

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Linux man-pages 6.10 2024-11-17 lockf(3)

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