ftrylockfile man page on CentOS

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

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() 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 *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.

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.

SEE ALSO
       unlocked_stdio(3)

				  2001-10-18			   LOCKFILE(3)
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