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

       flockfile, ftrylockfile, funlockfile - lock FILE for stdio

       #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 ||

       The stdio functions are thread-safe.  This is achieved by assigning to each FILE object a lockcount and (if the
       lockcount  is non-zero) 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

       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  effi-
       ciency,  one  does  the  I/O  with  the  non-locking 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  non-blocking  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 non-zero for failure.



       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.


       This  page  is part of release 3.22 of the Linux man-pages project.  A description of the project, and informa-
       tion about reporting bugs, can be found at

                                  2008-08-29                      FLOCKFILE(3)