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FLOCKFILE(3) Linux Programmer's Manual FLOCKFILE(3)
flockfile, ftrylockfile, funlockfile - lock FILE for stdio
#include <stdio.h>
void flockfile(FILE *filehandle);
int ftrylockfile(FILE *filehandle);
void funlockfile(FILE *filehandle);
Feature Test Macro Requirements for glibc (see feature_test_macros(7)):
All functions shown above:
_POSIX_C_SOURCE >= 1 || _XOPEN_SOURCE || _BSD_SOURCE || _SVID_SOURCE ||
_POSIX_SOURCE
The stdio functions are thread-safe. This is achieved by assigning to each
FILE object a lockcount and (if the lockcount is nonzero) an owning thread.
For each library call, these functions wait until the FILE object is no longer
locked by a different thread, then lock it, do the requested I/O, and unlock
the object again.
(Note: this locking has nothing to do with the file locking done by functions
like flock(2) and lockf(3).)
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.
To this end, a thread can explicitly lock the 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
getc_unlocked(3) and putc_unlocked(3) instead of getc(3) and putc(3).
The flockfile() function waits for *filehandle to be no longer locked by a
different thread, then makes the current thread owner of *filehandle, and
increments the lockcount.
The funlockfile() function decrements the lock count.
The ftrylockfile() function is a nonblocking version of flockfile(). It does
nothing in case some other thread owns *filehandle, and it obtains ownership
and increments the lockcount otherwise.
The ftrylockfile() function returns zero for success (the lock was obtained),
and nonzero for failure.
None.
POSIX.1-2001.
These functions are available when _POSIX_THREAD_SAFE_FUNCTIONS is defined.
They are in libc since libc 5.1.1 and in glibc since glibc 2.0.
unlocked_stdio(3)
This page is part of release 3.32 of the Linux man-pages project. A
description of the project, and information about reporting bugs, can be found
at http://www.kernel.org/doc/man-pages/.
2008-08-29 FLOCKFILE(3)
HTML rendering created 2010-12-03 by Michael Kerrisk, author of The Linux Programming Interface