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FLOCK(2)                  FreeBSD System Calls Manual                 FLOCK(2)

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

SYNOPSIS
     #include <sys/file.h>
     #define   LOCK_SH        0x01      /* shared file lock */
     #define   LOCK_EX        0x02      /* exclusive file lock */
     #define   LOCK_NB        0x04      /* don't block when locking */
     #define   LOCK_UN        0x08      /* unlock file */

     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
     parameter 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
     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.

IMPLEMENTATION NOTES
     In the non-threaded library flock() is implemented as the flock syscall.

     In the threaded library, the flock syscall is assembled to
     _thread_sys_flock() and flock() is implemented as a function which locks
     fd for read and write, then calls _thread_sys_flock().  Before returning,
     flock() unlocks fd.

RETURN VALUES
     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.

     [EOPNOTSUPP]   The argument fd refers to an object that does not support
                    file locking.

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

HISTORY
     The flock() function call appeared in 4.2BSD.

BSD 4.2                        December 11, 1993                       BSD 4.2

NAME | SYNOPSIS | DESCRIPTION | NOTES | IMPLEMENTATION NOTES | RETURN VALUES | ERRORS | SEE ALSO | HISTORY

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