Name

flockfile, ftrylockfile, funlockfile — lock FILE for stdio

Synopsis

#include <stdio.h>
void flockfile( FILE *filehandle);
 
int ftrylockfile( FILE *filehandle);
 
void funlockfile( FILE *filehandle);
 
[Note] Note
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

DESCRIPTION

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] 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.

RETURN VALUE

The ftrylockfile() function returns zero for success (the lock was obtained), and nonzero for failure.

ERRORS

None.

CONFORMING TO

POSIX.1-2001.

AVAILABILITY

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.

SEE ALSO

unlocked_stdio(3)

COLOPHON

This page is part of release 3.24 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/.


  Copyright (C) 2001 Andries Brouwer <aebcwi.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.