2:flock

From Linux Man Pages

      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 parameter operation is one of the follow-
      ing:
 
             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 opera-
      tion 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 a 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.
 
      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(2)  call  first appeared in 4.2BSD).  A version of flock(2), possibly implemented in terms of
      fcntl(2), appears on most Unices.

NOTES

      flock(2) 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(2) 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(2) and fcntl(2), and flock(2) does not detect deadlock.
 
      flock(2) places advisory locks only; given suitable permissions on a file, a process is free to ignore the use of
      flock(2) and perform I/O on the file.
 
      flock(2) and fcntl(2) locks have different semantics with respect to forked processes  and  dup(2).   On  systems
      that  implement  flock()  using  fcntl(), 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 behaviour, and occurs on many other implementations.)

RELATED

      close(2), dup(2), execve(2), fcntl(2), fork(2), open(2), lockf(3)
 
      There are also locks.txt and mandatory.txt in /usr/src/linux/Documentation.

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