flockfile(3) — Linux manual page


flockfile(3)            Library Functions Manual            flockfile(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 (see

       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 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.

RETURN VALUE         top

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

ERRORS         top


ATTRIBUTES         top

       For an explanation of the terms used in this section, see
       │ Interface                           Attribute     Value   │
       │ flockfile(), ftrylockfile(),        │ Thread safety │ MT-Safe │
       │ funlockfile()                       │               │         │

STANDARDS         top


HISTORY         top


       These functions are available when _POSIX_THREAD_SAFE_FUNCTIONS
       is defined.

SEE ALSO         top


Linux man-pages (unreleased)     (date)                     flockfile(3)

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