man3/flockfile.3

.\" Copyright (C) 2001 Andries Brouwer <aeb@cwi.nl>.
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.TH FLOCKFILE 3  2017-07-13 "" "Linux Programmer's Manual"
.SH NAME
flockfile, ftrylockfile, funlockfile \- lock FILE for stdio
.SH SYNOPSIS
.nf
.B #include <stdio.h>
.PP
.BI "void flockfile(FILE *" filehandle );
.BI "int ftrylockfile(FILE *" filehandle );
.BI "void funlockfile(FILE *" filehandle );
.fi
.PP
.in -4n
Feature Test Macro Requirements for glibc (see
.BR feature_test_macros (7)):
.in
.ad l
.PP
All functions shown above:
.RS 4
/* Since glibc 2.24: */ _POSIX_C_SOURCE\ >=\ 199309L
    || /* Glibc versions <= 2.23: */ _POSIX_C_SOURCE
    || /* Glibc versions <= 2.19: */ _BSD_SOURCE || _SVID_SOURCE
.RE
.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 nonzero)
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.
.PP
(Note: this locking has nothing to do with the file locking done
by functions like
.BR flock (2)
and
.BR lockf (3).)
.PP
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.
.PP
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 nonlocking versions of the stdio functions: with
.BR getc_unlocked (3)
and
.BR putc_unlocked (3)
instead of
.BR getc (3)
and
.BR putc (3).
.PP
The
.BR flockfile ()
function waits for
.I *filehandle
to be
no longer locked by a different thread, then makes the
current thread owner of
.IR *filehandle ,
and increments
the lockcount.
.PP
The
.BR funlockfile ()
function decrements the lock count.
.PP
The
.BR ftrylockfile ()
function is a nonblocking version
of
.BR flockfile ().
It does nothing in case some other thread
owns
.IR *filehandle ,
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 nonzero for failure.
.SH ERRORS
None.
.SH ATTRIBUTES
For an explanation of the terms used in this section, see
.BR attributes (7).
.TS
allbox;
lbw29 lb lb
l l l.
Interface	Attribute	Value
T{
.BR flockfile (),
.BR ftrylockfile (),
.BR funlockfile ()
T}	Thread safety	MT-Safe
.TE
.SH CONFORMING TO
POSIX.1-2001, POSIX.1-2008.
.SH AVAILABILITY
These functions are available when
.B _POSIX_THREAD_SAFE_FUNCTIONS
is defined.
.SH SEE ALSO
.BR unlocked_stdio (3)