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 following:

           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 nonblocking 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 descriptors  (cre-
       ated  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 operation 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  sig-
              nal(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  imple-
       ment 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 3.25 of the Linux man-pages project.  A description of the project,  and  information  about
       reporting bugs, can be found at http://www.kernel.org/doc/man-pages/.



Linux                                                      2009-07-25                                                   FLOCK(2)