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 ||

       The stdio functions are thread-safe.  This is achieved by assigning  to
       each  FILE object a lockcount and (if the lockcount is nonzero) an own-
       ing 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 inter-
       rupted 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 *filehan-
       dle, 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.


       For   an   explanation   of   the  terms  used  in  this  section,  see

       |Interface                     | Attribute     | Value   |
       |flockfile(),  ftrylockfile(), | Thread safety | MT-Safe |
       |funlockfile()                 |               |         |
       POSIX.1-2001, POSIX.1-2008.

       These  functions  are  available  when  _POSIX_THREAD_SAFE_FUNCTIONS is


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                                  2015-08-08                      FLOCKFILE(3)
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