mirror of https://github.com/mkerrisk/man-pages
126 lines
3.6 KiB
Groff
126 lines
3.6 KiB
Groff
.\" Copyright (C) 2001 Andries Brouwer <aeb@cwi.nl>.
|
|
.\"
|
|
.\" Permission is granted to make and distribute verbatim copies of this
|
|
.\" manual provided the copyright notice and this permission notice are
|
|
.\" preserved on all copies.
|
|
.\"
|
|
.\" Permission is granted to copy and distribute modified versions of this
|
|
.\" manual under the conditions for verbatim copying, provided that the
|
|
.\" entire resulting derived work is distributed under the terms of a
|
|
.\" permission notice identical to this one.
|
|
.\"
|
|
.\" Since the Linux kernel and libraries are constantly changing, this
|
|
.\" manual page may be incorrect or out-of-date. The author(s) assume no
|
|
.\" responsibility for errors or omissions, or for damages resulting from
|
|
.\" the use of the information contained herein. The author(s) may not
|
|
.\" have taken the same level of care in the production of this manual,
|
|
.\" which is licensed free of charge, as they might when working
|
|
.\" professionally.
|
|
.\"
|
|
.\" Formatted or processed versions of this manual, if unaccompanied by
|
|
.\" the source, must acknowledge the copyright and authors of this work.
|
|
.\"
|
|
.TH FLOCKFILE 3 2007-07-26 "" "Linux Programmer's Manual"
|
|
.SH NAME
|
|
flockfile, ftrylockfile, funlockfile \- lock FILE for stdio
|
|
.SH SYNOPSIS
|
|
.nf
|
|
.B #include <stdio.h>
|
|
.sp
|
|
.BI "void flockfile(FILE *" filehandle );
|
|
.br
|
|
.BI "int ftrylockfile(FILE *" filehandle );
|
|
.br
|
|
.BI "void funlockfile(FILE *" filehandle );
|
|
.fi
|
|
.sp
|
|
.in -4n
|
|
Feature Test Macro Requirements for glibc (see
|
|
.BR feature_test_macros (7)):
|
|
.in
|
|
.ad l
|
|
.sp
|
|
All functions shown above:
|
|
_POSIX_C_SOURCE || _XOPEN_SOURCE || _BSD_SOURCE || _SVID_SOURCE
|
|
.ad b
|
|
.SH DESCRIPTION
|
|
The stdio functions are thread-safe.
|
|
This is achieved by assigning
|
|
to each
|
|
.I FILE
|
|
object a lockcount and (if the lockcount is non-zero)
|
|
an owning thread.
|
|
For each library call, these functions wait until the
|
|
.I FILE
|
|
object
|
|
is no longer locked by a different thread, then lock it, do the
|
|
requested I/O, and unlock the object again.
|
|
.LP
|
|
(Note: this locking has nothing to do with the file locking done
|
|
by functions like
|
|
.BR flock (2)
|
|
and
|
|
.BR lockf (3).)
|
|
.LP
|
|
All this is invisible to the C-programmer, but there may be two
|
|
reasons to wish for more detailed control.
|
|
On the one hand, maybe
|
|
a series of I/O actions by one thread belongs together, and should
|
|
not be interrupted by the I/O of some other thread.
|
|
On the other hand, maybe the locking overhead should be avoided
|
|
for greater efficiency.
|
|
.LP
|
|
To this end, a thread can explicitly lock the
|
|
.I FILE
|
|
object,
|
|
then do its series of I/O actions, then unlock.
|
|
This prevents
|
|
other threads from coming in between.
|
|
If the reason for doing
|
|
this was to achieve greater efficiency, one does the I/O with
|
|
the non-locking versions of the stdio functions: with
|
|
.BR getc_unlocked (3)
|
|
and
|
|
.BR putc_unlocked (3)
|
|
instead of
|
|
.BR getc (3)
|
|
and
|
|
.BR putc (3).
|
|
.LP
|
|
The
|
|
.BR flockfile ()
|
|
function waits for \fI*filehandle\fP to be
|
|
no longer locked by a different thread, then makes the
|
|
current thread owner of \fI*filehandle\fP, and increments
|
|
the lockcount.
|
|
.LP
|
|
The
|
|
.BR funlockfile ()
|
|
function decrements the lock count.
|
|
.LP
|
|
The
|
|
.BR ftrylockfile ()
|
|
function is a non-blocking version
|
|
of
|
|
.BR flockfile ().
|
|
It does nothing in case some other thread
|
|
owns \fI*filehandle\fP, and it obtains ownership and increments
|
|
the lockcount otherwise.
|
|
.SH "RETURN VALUE"
|
|
The
|
|
.BR ftrylockfile ()
|
|
function returns zero for success
|
|
(the lock was obtained), and non-zero for failure.
|
|
.SH ERRORS
|
|
None.
|
|
.SH "CONFORMING TO"
|
|
POSIX.1-2001.
|
|
.SH AVAILABILITY
|
|
These functions are available when
|
|
.B _POSIX_THREAD_SAFE_FUNCTIONS
|
|
is defined.
|
|
They are in libc since libc 5.1.1 and in glibc
|
|
since glibc 2.0.
|
|
.SH "SEE ALSO"
|
|
.BR unlocked_stdio (3)
|