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

       flockfile, ftrylockfile, funlockfile - lock FILE for stdio

       #include <stdio.h>

       void flockfile(FILE *filehandle);
       int ftrylockfile(FILE *filehandle);
       void funlockfile(FILE *filehandle);

       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  non-locking
       versions  of  the  stdio  functions:  with  getc_unlocked() and putc_unlocked() instead of
       getc() and putc().

       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 lock-
       count otherwise.

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



					    2001-10-18				      LOCKFILE(3)
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