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NAME | LIBRARY | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | ATTRIBUTES | STANDARDS | HISTORY | SEE ALSO | COLOPHON |
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flockfile(3) Library Functions Manual flockfile(3)
flockfile, ftrylockfile, funlockfile - lock FILE for stdio
Standard C library (libc, -lc)
#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:
/* Since glibc 2.24: */ _POSIX_C_SOURCE >= 199309L
|| /* glibc <= 2.23: */ _POSIX_C_SOURCE
|| /* glibc <= 2.19: */ _BSD_SOURCE || _SVID_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.
For an explanation of the terms used in this section, see
attributes(7).
┌─────────────────────────────────────┬───────────────┬─────────┐
│ Interface │ Attribute │ Value │
├─────────────────────────────────────┼───────────────┼─────────┤
│ flockfile(), ftrylockfile(), │ Thread safety │ MT-Safe │
│ funlockfile() │ │ │
└─────────────────────────────────────┴───────────────┴─────────┘
POSIX.1-2008.
POSIX.1-2001.
These functions are available when _POSIX_THREAD_SAFE_FUNCTIONS
is defined.
unlocked_stdio(3)
This page is part of the man-pages (Linux kernel and C library
user-space interface documentation) project. Information about
the project can be found at
⟨https://www.kernel.org/doc/man-pages/⟩. If you have a bug report
for this manual page, see
⟨https://git.kernel.org/pub/scm/docs/man-pages/man-pages.git/tree/CONTRIBUTING⟩.
This page was obtained from the tarball man-pages-6.9.1.tar.gz
fetched from
⟨https://mirrors.edge.kernel.org/pub/linux/docs/man-pages/⟩ on
2024-06-26. If you discover any rendering problems in this HTML
version of the page, or you believe there is a better or more up-
to-date source for the page, or you have corrections or
improvements to the information in this COLOPHON (which is not
part of the original manual page), send a mail to
man-pages@man7.org
Linux man-pages 6.9.1 2024-05-02 flockfile(3)
Pages that refer to this page: FILE(3type), stdio_ext(3), unlocked_stdio(3)
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HTML rendering created 2024-06-26 by Michael Kerrisk, author of The Linux Programming Interface. For details of in-depth Linux/UNIX system programming training courses that I teach, look here. Hosting by jambit GmbH. |
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