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FLOCK(2)                   Linux Programmer's Manual                  FLOCK(2)



NAME
       flock - apply or remove an advisory lock on an open file

SYNOPSIS
       #include <sys/file.h>

       int flock(int fd, int operation);

DESCRIPTION
       Apply  or  remove an advisory lock on the open file specified by fd.  The argument operation is one of the fol-
       lowing:

           LOCK_SH  Place a shared lock.  More than one process may hold a shared lock for a given  file  at  a  given
                    time.

           LOCK_EX  Place  an exclusive lock.  Only one process may hold an exclusive lock for a given file at a given
                    time.

           LOCK_UN  Remove an existing lock held by this process.

       A call to flock() may block if an incompatible lock is  held  by  another  process.   To  make  a  non-blocking
       request, include LOCK_NB (by ORing) with any of the above operations.

       A single file may not simultaneously have both shared and exclusive locks.

       Locks created by flock() are associated with an open file table entry.  This means that duplicate file descrip-
       tors (created by, for example, fork(2) or dup(2)) refer to the same lock, and this  lock  may  be  modified  or
       released using any of these descriptors.  Furthermore, the lock is released either by an explicit LOCK_UN oper-
       ation on any of these duplicate descriptors, or when all such descriptors have been closed.

       If a process uses open(2) (or similar) to obtain more than one descriptor for the same file, these  descriptors
       are  treated  independently by flock().  An attempt to lock the file using one of these file descriptors may be
       denied by a lock that the calling process has already placed via another descriptor.

       A process may only hold one type of lock (shared or exclusive) on a  file.   Subsequent  flock()  calls  on  an
       already locked file will convert an existing lock to the new lock mode.

       Locks created by flock() are preserved across an execve(2).

       A shared or exclusive lock can be placed on a file regardless of the mode in which the file was opened.

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

ERRORS
       EBADF  fd is not an open file descriptor.

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

       EINVAL operation is invalid.

       ENOLCK The kernel ran out of memory for allocating lock records.

       EWOULDBLOCK
              The file is locked and the LOCK_NB flag was selected.

CONFORMING TO
       4.4BSD (the flock() call first appeared in 4.2BSD).  A version of flock(), possibly  implemented  in  terms  of
       fcntl(2), appears on most Unix systems.

NOTES
       flock()  does  not  lock  files  over NFS.  Use fcntl(2) instead: that does work over NFS, given a sufficiently
       recent version of Linux and a server which supports locking.

       Since kernel 2.0, flock() is implemented as a system call in its own right rather than being  emulated  in  the
       GNU C library as a call to fcntl(2).  This yields true BSD semantics: there is no interaction between the types
       of lock placed by flock() and fcntl(2), and flock() does not detect deadlock.

       flock() places advisory locks only; given suitable permissions on a file, a process is free to ignore  the  use
       of flock() and perform I/O on the file.

       flock()  and  fcntl(2)  locks have different semantics with respect to forked processes and dup(2).  On systems
       that implement flock() using fcntl(2), the semantics of flock() will be different from those described in  this
       manual page.

       Converting  a  lock  (shared  to exclusive, or vice versa) is not guaranteed to be atomic: the existing lock is
       first removed, and then a new lock is established.  Between these two steps, a pending lock request by  another
       process  may  be granted, with the result that the conversion either blocks, or fails if LOCK_NB was specified.
       (This is the original BSD behavior, and occurs on many other implementations.)

SEE ALSO
       close(2), dup(2), execve(2), fcntl(2), fork(2), open(2), lockf(3)

       See also Documentation/filesystem/locks.txt in the kernel source (Documentation/locks.txt in older kernels).

COLOPHON
       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 http://www.kernel.org/doc/man-pages/.



Linux                             2009-07-25                          FLOCK(2)