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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:

       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.


   Multithreading (see pthreads(7))
       The   flockfile(),  ftrylockfile(),  and	 funlockfile()	functions  are


       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


       This page is part of release 3.65 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/.

				  2013-07-23			  FLOCKFILE(3)

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