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.\" Copyright (C) 2001 Andries Brouwer <aeb@cwi.nl>.
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.\"
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.\" Permission is granted to make and distribute verbatim copies of this
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.\" manual provided the copyright notice and this permission notice are
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.\" preserved on all copies.
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.\"
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.\" Permission is granted to copy and distribute modified versions of this
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.\" manual under the conditions for verbatim copying, provided that the
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.\" entire resulting derived work is distributed under the terms of a
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.\" permission notice identical to this one.
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.\"
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.\" Since the Linux kernel and libraries are constantly changing, this
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.\" manual page may be incorrect or out-of-date. The author(s) assume no
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.\" responsibility for errors or omissions, or for damages resulting from
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.\" the use of the information contained herein. The author(s) may not
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.\" have taken the same level of care in the production of this manual,
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.\" which is licensed free of charge, as they might when working
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.\" professionally.
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.\"
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.\" Formatted or processed versions of this manual, if unaccompanied by
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.\" the source, must acknowledge the copyright and authors of this work.
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.\"
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.TH LOCKFILE 3 2001-10-18 "" "Linux Programmer's Manual"
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.SH NAME
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flockfile, ftrylockfile, funlockfile \- lock FILE for stdio
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.SH SYNOPSIS
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.nf
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.B #include <stdio.h>
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.sp
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.BI "void flockfile(FILE *" filehandle );
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.br
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.BI "int ftrylockfile(FILE *" filehandle );
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.br
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.BI "void funlockfile(FILE *" filehandle );
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.fi
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.SH DESCRIPTION
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The stdio functions are thread-safe. This is achieved by assigning
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to each FILE object a lockcount and (if the lockcount is non-zero)
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an owning thread.
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For each library call, these functions wait until the FILE object
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is no longer locked by a different thread, then lock it, do the
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requested I/O, and unlock the object again.
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.LP
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(Note: this locking has nothing to do with the file locking done
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by functions like
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.BR flock (2)
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and
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.BR lockf (3).)
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.LP
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All this is invisible to the C-programmer, but there may be two
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reasons to wish for more detailed control. On the one hand, maybe
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a series of I/O actions by one thread belongs together, and should
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not be interrupted by the I/O of some other thread.
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On the other hand, maybe the locking overhead should be avoided
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for greater efficiency.
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.LP
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To this end, a thread can explicitly lock the FILE object,
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then do its series of I/O actions, then unlock. This prevents
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other threads from coming in between. If the reason for doing
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this was to achieve greater efficiency, one does the I/O with
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the non-locking versions of the stdio functions: with
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\fIgetc_unlocked\fP() and \fIputc_unlocked\fP() instead of
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\fIgetc\fP() and \fIputc\fP().
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.LP
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The \fBflockfile\fP() function waits for *\fIfilehandle\fP to be
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no longer locked by a different thread, then makes the
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current thread owner of *\fIfilehandle\fP, and increments
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the lockcount.
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.LP
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The \fBfunlockfile\fP() function decrements the lock count.
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.LP
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The \fBftrylockfile\fP() function is a non-blocking version
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of \fBflockfile\fP(). It does nothing in case some other thread
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owns *\fIfilehandle\fP, and it obtains ownership and increments
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the lockcount otherwise.
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.SH "RETURN VALUE"
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The \fBftrylockfile\fP() function returns zero for success
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(the lock was obtained), and non-zero for failure.
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.SH ERRORS
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None.
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.SH AVAILABILITY
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These functions are available when _POSIX_THREAD_SAFE_FUNCTIONS
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is defined. They are in libc since libc 5.1.1 and in glibc
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since glibc 2.0.
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.SH "CONFORMING TO"
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POSIX.1-2001.
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.SH "SEE ALSO"
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.BR unlocked_stdio (3)
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