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Annotation of 43BSDTahoe/man/man2/flock.2, revision 1.1

1.1     ! root        1: .\" Copyright (c) 1983 Regents of the University of California.
        !             2: .\" All rights reserved.  The Berkeley software License Agreement
        !             3: .\" specifies the terms and conditions for redistribution.
        !             4: .\"
        !             5: .\"    @(#)flock.2     6.4 (Berkeley) 5/22/86
        !             6: .\"
        !             7: .TH FLOCK 2 "May 22, 1986"
        !             8: .UC 5
        !             9: .SH NAME
        !            10: flock \- apply or remove an advisory lock on an open file
        !            11: .SH SYNOPSIS
        !            12: .nf
        !            13: .ft B
        !            14: #include <sys/file.h>
        !            15: .PP
        !            16: .ft B
        !            17: .DT
        !            18: #define        LOCK_SH 1       /* shared lock */
        !            19: #define        LOCK_EX 2       /* exclusive lock */
        !            20: #define        LOCK_NB 4       /* don't block when locking */
        !            21: #define        LOCK_UN 8       /* unlock */
        !            22: .PP
        !            23: .ft B
        !            24: flock(fd, operation)
        !            25: int fd, operation;
        !            26: .fi
        !            27: .SH DESCRIPTION
        !            28: .I Flock
        !            29: applies or removes an
        !            30: .I advisory
        !            31: lock on the file associated with the file descriptor
        !            32: .IR fd .
        !            33: A lock is applied by specifying an
        !            34: .I operation
        !            35: parameter that is the inclusive or of
        !            36: LOCK_SH or LOCK_EX and, possibly, LOCK_NB.  To unlock
        !            37: an existing lock
        !            38: .I operation
        !            39: should be LOCK_UN.
        !            40: .PP
        !            41: Advisory locks allow cooperating processes to perform
        !            42: consistent operations on files, but do not guarantee
        !            43: consistency (i.e., processes may still access files
        !            44: without using advisory locks possibly resulting in
        !            45: inconsistencies).
        !            46: .PP
        !            47: The locking mechanism allows two types of locks:
        !            48: .I shared
        !            49: locks and
        !            50: .I exclusive
        !            51: locks.
        !            52: At any time multiple shared locks may be applied to a file,
        !            53: but at no time are multiple exclusive, or both shared and exclusive,
        !            54: locks allowed simultaneously on a file.  
        !            55: .PP
        !            56: A shared lock may be
        !            57: .I upgraded
        !            58: to an exclusive lock, and vice versa, simply by specifying
        !            59: the appropriate lock type; this results in the previous
        !            60: lock being released and the new lock applied (possibly
        !            61: after other processes have gained and released the lock).
        !            62: .PP
        !            63: Requesting a lock on an object that is already locked
        !            64: normally causes the caller to be blocked until the lock may be
        !            65: acquired.  If LOCK_NB is included in
        !            66: .IR operation ,
        !            67: then this will not happen; instead the call will fail and
        !            68: the error EWOULDBLOCK will be returned.
        !            69: .SH NOTES
        !            70: Locks are on files, not file descriptors.  That is, file descriptors
        !            71: duplicated through
        !            72: .IR dup (2)
        !            73: or
        !            74: .IR fork (2)
        !            75: do not result in multiple instances of a lock, but rather multiple
        !            76: references to a single lock.  If a process holding a lock on a file
        !            77: forks and the child explicitly unlocks the file, the parent will
        !            78: lose its lock.
        !            79: .PP
        !            80: Processes blocked awaiting a lock may be awakened by signals.
        !            81: .SH "RETURN VALUE
        !            82: Zero is returned if the operation was successful;
        !            83: on an error a \-1 is returned and an error code is left in
        !            84: the global location \fIerrno\fP.
        !            85: .SH "ERRORS
        !            86: The \fIflock\fP call fails if:
        !            87: .TP 20
        !            88: [EWOULDBLOCK]
        !            89: The file is locked and the LOCK_NB option was specified.
        !            90: .TP 20
        !            91: [EBADF]
        !            92: The argument \fIfd\fP is an invalid descriptor.
        !            93: .TP 20
        !            94: [EINVAL]
        !            95: The argument \fIfd\fP refers to an object other than a file.
        !            96: .SH "SEE ALSO"
        !            97: open(2), close(2), dup(2), execve(2), fork(2)

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