1.\" $NetBSD: flockfile.3,v 1.3 2003/04/16 13:34:44 wiz Exp $ 2.\" 3.\" Copyright (c) 2003 The NetBSD Foundation, Inc. 4.\" All rights reserved. 5.\" 6.\" This code is derived from software contributed to The NetBSD Foundation 7.\" by Klaus Klein. 8.\" 9.\" Redistribution and use in source and binary forms, with or without 10.\" modification, are permitted provided that the following conditions 11.\" are met: 12.\" 1. Redistributions of source code must retain the above copyright 13.\" notice, this list of conditions and the following disclaimer. 14.\" 2. Redistributions in binary form must reproduce the above copyright 15.\" notice, this list of conditions and the following disclaimer in the 16.\" documentation and/or other materials provided with the distribution. 17.\" 3. All advertising materials mentioning features or use of this software 18.\" must display the following acknowledgement: 19.\" This product includes software developed by the NetBSD 20.\" Foundation, Inc. and its contributors. 21.\" 4. Neither the name of The NetBSD Foundation nor the names of its 22.\" contributors may be used to endorse or promote products derived 23.\" from this software without specific prior written permission. 24.\" 25.\" THIS SOFTWARE IS PROVIDED BY THE NETBSD FOUNDATION, INC. AND CONTRIBUTORS 26.\" ``AS IS'' AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED 27.\" TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR 28.\" PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE FOUNDATION OR CONTRIBUTORS 29.\" BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR 30.\" CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF 31.\" SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS 32.\" INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN 33.\" CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) 34.\" ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE 35.\" POSSIBILITY OF SUCH DAMAGE. 36.\" 37.Dd January 28, 2003 38.Dt FLOCKFILE 3 39.Os 40.Sh NAME 41.Nm flockfile , 42.Nm ftrylockfile , 43.Nm funlockfile 44.Nd stdio stream locking functions 45.Sh LIBRARY 46.Lb libc 47.Sh SYNOPSIS 48.In stdio.h 49.Ft void 50.Fn flockfile "FILE *file" 51.Ft int 52.Fn ftrylockfile "FILE *file" 53.Ft void 54.Fn funlockfile "FILE *file" 55.Sh DESCRIPTION 56The 57.Fn flockfile , 58.Fn ftrylockfile , 59and 60.Fn funlockfile 61functions provide applications with explicit control of locking of 62stdio stream objects. 63They can be used by a thread to execute a sequence of I/O operations 64as a unit, without interference from another thread. 65.Pp 66Locks on stdio streams are recursive, and a lock count is maintained. 67stdio streams are created unlocked, with a lock count of zero. 68After successful acquisition of the lock, its count is incremented 69to one, indicating locked state of the stdio stream. 70Each subsequent relock operation performed by the owner thread 71increments the lock count by one, and each subsequent unlock 72operation performed by the owner thread decrements the lock count by one, 73allowing matching lock and unlock operations to be nested. 74After its lock count is decremented to zero, the stdio stream returns 75to unlocked state, and ownership of the stdio stream is relinquished. 76.Pp 77The 78.Fn flockfile 79function acquires the ownership of 80.Fa file 81for the calling thread. 82If 83.Fa file 84is already owned by another thread, the calling thread is suspended 85until the acquisition is possible (i.e., 86.Fa file 87is relinquished again and the calling thread is scheduled to acquire it). 88.Pp 89The 90.Fn ftrylockfile 91function acquires the ownership of 92.Fa file 93for the calling thread only if 94.Fa file 95is available. 96.Pp 97The 98.Fn funlockfile 99function relinquishes the ownership of 100.Fa file 101previously granted to the calling thread. 102Only the current owner of 103.Fa file 104may 105.Fn funlockfile 106it. 107.Sh RETURN VALUES 108If successful, the 109.Fn ftrylockfile 110function returns 0. 111Otherwise, it returns non-zero to indicate that the lock cannot be acquired. 112.Sh SEE ALSO 113.Xr getc_unlocked 3 , 114.Xr getchar_unlocked 3 , 115.Xr putc_unlocked 3 , 116.Xr putchar_unlocked 3 117.Sh STANDARDS 118The 119.Fn flockfile , 120.Fn ftrylockfile 121and 122.Fn funlockfile 123functions conform to 124.St -p1003.1-2001 . 125.Sh BUGS 126The design of these interfaces does not allow for addressing the 127problem of priority inversion. 128