#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 */

     int
     flock(int fd, int 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 pa-
     rameter that is one of LOCK_SH or LOCK_EX with the optional addition of
     LOCK_NB. To unlock an existing lock operation should be LOCK_UN.

     Advisory locks allow cooperating processes to perform consistent opera-
     tions on files, but do not guarantee consistency (i.e., processes may
     still access files without using advisory locks possibly resulting in in-
     consistencies).

     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 exclusive, or both shared and exclu-
     sive, locks allowed simultaneously on a file.

     A shared lock may be upgraded to an exclusive lock, and vice versa, sim-
     ply by specifying the appropriate lock type; this results in the previous
     lock being released and the new lock applied (possibly after other pro-
     cesses 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 in-
     cluded 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 du-
     plicated through dup(2) or fork(2) do not result in multiple instances of
     a lock, but rather multiple references 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 VALUES

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

ERRORS

     The flock() call fails if:

     [EWOULDBLOCK]
     The flock() function call appeared in 4.2BSD.

4.2 Berkeley Distribution      December 11, 1993                             2