.\" Copyright (c) 2003 Tim J. Robbins
.\" All rights reserved.
.\"
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.\" modification, are permitted provided that the following conditions
.\" are met:
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.\" $FreeBSD: src/lib/libc/stdio/flockfile.3,v 1.3 2003/01/13 01:29:14 tjr Exp $
.\"
.Dd March 22, 2009
.Dt FLOCKFILE 3
.Os
.Sh NAME
.Nm flockfile ,
.Nm ftrylockfile ,
.Nm funlockfile
.Nd "stdio locking functions"
.Sh LIBRARY
.Lb libc
.Sh SYNOPSIS
.In stdio.h
.Ft void
.Fn flockfile "FILE *stream"
.Ft int
.Fn ftrylockfile "FILE *stream"
.Ft void
.Fn funlockfile "FILE *stream"
.Sh DESCRIPTION
These functions provide explicit application-level locking of stdio streams.
They can be used to avoid output from multiple threads being interspersed,
input being dispersed among multiple readers, and to avoid the overhead
of locking the stream for each operation.
.Pp
The
.Fn flockfile
function acquires an exclusive lock on the specified stream.
If another thread has already locked the stream,
.Fn flockfile
will block until the lock is released.
.Pp
The
.Fn ftrylockfile
function is a non-blocking version of
.Fn flockfile ;
if the lock cannot be acquired immediately,
.Fn ftrylockfile
returns non-zero instead of blocking.
.Pp
The
.Fn funlockfile
function releases the lock on a stream acquired by an earlier call to
.Fn flockfile
or
.Fn ftrylockfile .
.Pp
These functions behave as if there is a lock count associated
with each stream.
Each time
.Fn flockfile
is called on the stream, the count is incremented,
and each time
.Fn funlockfile
is called on the stream, the count is decremented.
The lock is only actually released when the count reaches zero.
.Sh RETURN VALUES
The
.Fn flockfile
and
.Fn funlockfile
functions return no value.
.Pp
The
.Fn ftrylockfile
function
returns zero if the stream was successfully locked,
non-zero otherwise.
.Sh SEE ALSO
.Xr getc_unlocked 3 ,
.Xr putc_unlocked 3
.Sh STANDARDS
The
.Fn flockfile ,
.Fn ftrylockfile
and
.Fn funlockfile
functions conform to
.St -p1003.1-2001 .