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flockfile(3) — Linux manual page

NAME \|LIBRARY \|SYNOPSIS \|DESCRIPTION \|RETURN VALUE \|ERRORS \|ATTRIBUTES \|STANDARDS \|HISTORY \|SEE ALSO \|COLOPHON

flockfile(3)             Library Functions Manualflockfile(3)

NAME top

       flockfile, ftrylockfile, funlockfile - lock FILE for stdio

LIBRARY top

       Standard C library (libc,-lc)

SYNOPSIS top

#include <stdio.h>void flockfile(FILE *filehandle);int ftrylockfile(FILE *filehandle);void funlockfile(FILE *filehandle);   Feature Test Macro Requirements for glibc (seefeature_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

DESCRIPTION top

       The stdio functions are thread-safe.  This is achieved by       assigning to eachFILE object a lockcount and (if the lockcount is       nonzero) an owning thread.  For each library call, these functions       wait until theFILE 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 likeflock(2) andlockf(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 theFILE 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: withgetc_unlocked(3)       andputc_unlocked(3) instead ofgetc(3) andputc(3).       Theflockfile() 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.       Thefunlockfile() function decrements the lock count.       Theftrylockfile() function is a nonblocking version offlockfile().  It does nothing in case some other thread owns*filehandle, and it obtains ownership and increments the lockcount       otherwise.

RETURN VALUE top

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

ERRORS top

       None.

ATTRIBUTES top

       For an explanation of the terms used in this section, seeattributes(7).       ┌──────────────────────────────────────┬───────────────┬─────────┐       │Interface│Attribute│Value│       ├──────────────────────────────────────┼───────────────┼─────────┤       │flockfile(),ftrylockfile(),         │ Thread safety │ MT-Safe │       │funlockfile()                        │               │         │       └──────────────────────────────────────┴───────────────┴─────────┘

STANDARDS top

       POSIX.1-2008.

HISTORY top

       POSIX.1-2001.       These functions are available when_POSIX_THREAD_SAFE_FUNCTIONSis       defined.

COLOPHON top

       This page is part of theman-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-6.15.tar.gz       fetched from       ⟨https://mirrors.edge.kernel.org/pub/linux/docs/man-pages/⟩ on       2025-08-11.  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 isnot       part of the original manual page), send a mail to       man-pages@man7.orgLinux man-pages 6.15            2025-05-17flockfile(3)

Pages that refer to this page:FILE(3type), stdio_ext(3), unlocked_stdio(3)

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