FLOCK(2)                 UNIX System V(March 27, 1992)                 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 advisory 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 exclusive, 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 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 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.


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FLOCK(2)                 UNIX System V(March 27, 1992)                 FLOCK(2)


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)









































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