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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 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 description (see open(2)).  This
       means that duplicate file descriptors (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 descrip‐
       tors.  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 hold only 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 imple‐
       mented in terms of fcntl(2), appears on most UNIX systems.

NOTES
       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).   With  this  implementation,
       there  is  no  interaction  between  the types of lock placed by flock() and fcntl(2), and
       flock() does not detect deadlock.  (Note, however, that on some systems, such as the  mod‐
       ern BSDs, flock() and fcntl(2) locks do interact with one another.)

       In  Linux  kernels  up to 2.6.11, flock() does not lock files over NFS (i.e., the scope of
       locks was limited to the local system).  Instead, one could use fcntl(2) byte-range  lock‐
       ing,  which  does work over NFS, given a sufficiently recent version of Linux and a server
       which supports locking.  Since Linux 2.6.12, NFS clients support flock() locks by  emulat‐
       ing  them  as  byte-range  locks on the entire file.  This means that fcntl(2) and flock()
       locks do interact with one another over NFS.  Since Linux 2.6.37, the  kernel  supports  a
       compatibility  mode  that allows flock() locks (and also fcntl(2) byte region locks) to be
       treated as local; see the discussion of the local_lock option in nfs(5).

       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
       flock(1), close(2), dup(2), execve(2), fcntl(2), fork(2), open(2), lockf(3)

       Documentation/filesystems/locks.txt   in   the   Linux   kernel  source  tree  (Documenta‐
       tion/locks.txt in older kernels)

COLOPHON
       This page is part of release 3.74 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 http://www.kernel.org/doc/man-pages/.



Linux                                       2014-09-21                                   FLOCK(2)


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