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BSD 2.11 - man page for flock (bsd section 2)

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FLOCK(2)										 FLOCK(2)

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

SYNOPSIS
       #include <sys/file.h>

       #define	 LOCK_SH   1	/* shared lock */
       #define	 LOCK_EX   2	/* exclusive lock */
       #define	 LOCK_NB   4	/* don't block when locking */
       #define	 LOCK_UN   8	/* unlock */

       flock(fd, operation)
       int fd, operation;

DESCRIPTION
       Flock  applies or removes an advisory lock on the file associated with the file descriptor
       fd.  A lock is applied by specifying an operation parameter that is the	inclusive  or  of
       LOCK_SH or LOCK_EX and, possibly, LOCK_NB.  To unlock an existing lock operation should be
       LOCK_UN.

       Advisory locks allow cooperating processes to perform consistent operations on files,  but
       do  not	guarantee consistency (i.e., processes may still access files without using advi-
       sory locks possibly resulting in inconsistencies).

       The locking mechanism allows two types of locks: shared locks and exclusive locks.  At any
       time  multiple  shared  locks may be applied to a file, but at no time are multiple exclu-
       sive, or both shared and exclusive, locks allowed simultaneously on a file.

       A shared lock may be upgraded to an exclusive lock, and vice versa, simply  by  specifying
       the  appropriate  lock  type; this results in the previous lock being released and the new
       lock applied (possibly after other processes have gained and released the lock).

       Requesting a lock on an object that is already locked normally causes  the  caller  to  be
       blocked	until  the  lock may be acquired.  If LOCK_NB is included in operation, then this
       will not happen; instead the call will fail and the error EWOULDBLOCK will be returned.

NOTES
       Locks are on files, not file descriptors.  That is, file  descriptors  duplicated  through
       dup(2)  or fork(2) do not result in multiple instances of a lock, but rather multiple ref-
       erences to a single lock.  If a process holding a lock on  a  file  forks  and  the  child
       explicitly unlocks the file, the parent will lose its lock.

       Processes blocked awaiting a lock may be awakened by signals.

RETURN VALUE
       Zero  is  returned  if  the  operation was successful; on an error a -1 is returned and an
       error code is left in the global location errno.

ERRORS
       The flock call fails if:

       [EWOULDBLOCK]	   The file is locked and the LOCK_NB option was specified.

       [EBADF]		   The argument fd is an invalid descriptor.

       [EINVAL] 	   The argument fd refers to an object other than a file.

SEE ALSO
       open(2), close(2), dup(2), execve(2), fork(2)

4.2 Berkeley Distribution		   May 22, 1986 				 FLOCK(2)
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