FLOCKFILE(3)                                        Linux Programmer's Manual                                       FLOCKFILE(3)



NAME
       flockfile, ftrylockfile, funlockfile - lock FILE for stdio

SYNOPSIS
       #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

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: 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 3.25 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)