3:flockfile

From Linux Man Pages

      flockfile, ftrylockfile, funlockfile - lock FILE for stdio
      

Contents 1 SYNOPSIS 2 DESCRIPTION 3 RETURN VALUE 4 ERRORS 5 AVAILABILITY 6 CONFORMING TO 7 RELATED 8 CATEGORY

SYNOPSIS

      #include <stdio.h>
 
      void flockfile(FILE *filehandle);
      int ftrylockfile(FILE *filehandle);
      void funlockfile(FILE *filehandle);

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 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  cur-
      rent 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 lockcount otherwise.

RETURN VALUE

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

ERRORS

      None.

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.

CONFORMING TO

      POSIX.1-2001.

RELATED

      unlocked_stdio(3)

CATEGORY