FLOCKFILE(3)		   Linux Programmers Manual		 FLOCKFILE(3)



NAME
       flockfile, ftrylockfile, funlockfile - lock FILE for stdio

SYNOPSIS
       #include 

       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 || _XOPEN_SOURCE ||
       _BSD_SOURCE || _SVID_SOURCE

DESCRIPTION
       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 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 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 *filehan
       dle, 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.

RETURN VALUE
       The ftrylockfile() function returns zero  for  success  (the  lock  was
       obtained), and non-zero 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 3.05 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/.



				  2007-07-26			  FLOCKFILE(3)