.\" $OpenBSD: flockfile.3,v 1.1 2025/07/25 18:27:57 tedu Exp $
.\"
.\" Copyright (c) 2025 Ted Unangst <tedu@openbsd.org>
.\"
.\" Permission to use, copy, modify, and distribute this software for any
.\" purpose with or without fee is hereby granted, provided that the above
.\" copyright notice and this permission notice appear in all copies.
.\"
.\" THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL WARRANTIES
.\" WITH REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF
.\" MERCHANTABILITY AND FITNESS. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR
.\" ANY SPECIAL, DIRECT, INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES
.\" WHATSOEVER RESULTING FROM LOSS OF USE, DATA OR PROFITS, WHETHER IN AN
.\" ACTION OF CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF
.\" OR IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.
.\"
.Dd $Mdocdate: July 25 2025 $
.Dt FLOCKFILE 3
.Os
.Sh NAME
.Nm flockfile ,
.Nm ftrylockfile ,
.Nm funlockfile
.Nd locking for stdio streams
.Sh SYNOPSIS
.In stdio.h
.Ft void
.Fn flockfile "FILE *file"
.Ft int
.Fn ftrylockfile "FILE *file"
.Ft void
.Fn funlockfile "FILE *file"
.Sh DESCRIPTION
These functions provide application control over the locking of stdio streams.
.Fn flockfile
and
.Fn ftrylockfile
increment the lock count associated with
.Fa file
on behalf of the calling thread.
If another thread owns the lock,
.Fn flockfile
blocks until the lock becomes available, whereas
.Fn ftrylockfile
returns immediately and indicates failure.
.Pp
When called by a thread holding the lock,
.Fn funlockfile
decrements the lock count by one.
When the lock count reaches zero, the calling thread relinquishes
ownership of the lock.
.Pp
Functions such as
.Xr fread 3
and
.Xr fprintf 3
are internally thread safe by default, but additional locking may be used
to coordinate sequences of multiple calls.
.Sh RETURN VALUES
.Fn ftrylockfile
returns zero for success and non-zero for failure.
.Sh SEE ALSO
.Xr getc_unlocked 3 ,
.Xr pthreads 3 ,
.Xr putc_unlocked 3
.Sh STANDARDS
These functions conform to
.St -p1003.1-2024 .
.Sh HISTORY
These functions have been available since
.Ox 2.5 .
.Sh CAVEATS
Reading from one stream can flush a different buffered output stream,
leading to deadlocks.