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flockfile(3)


NAME

   flockfile, ftrylockfile, funlockfile - lock FILE for stdio

SYNOPSIS

   #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
           || /* Glibc versions <= 2.19: */ _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  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 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
   *filehandle, 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.

RETURN VALUE

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

ERRORS

   None.

ATTRIBUTES

   For  an  explanation  of  the  terms  used   in   this   section,   see
   attributes(7).

   
   Interface                      Attribute      Value   
   
   flockfile(),  ftrylockfile(),  Thread safety  MT-Safe 
   funlockfile()

CONFORMING TO

   POSIX.1-2001, POSIX.1-2008.

AVAILABILITY

   These functions  are  available  when  _POSIX_THREAD_SAFE_FUNCTIONS  is
   defined.

SEE ALSO

   unlocked_stdio(3)

COLOPHON

   This  page  is  part of release 4.09 of the Linux man-pages project.  A
   description of the project, information about reporting bugs,  and  the
   latest     version     of     this    page,    can    be    found    at
   https://www.kernel.org/doc/man-pages/.

                              2016-03-15                      FLOCKFILE(3)



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