redhat man page for funlockfile

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 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 *file-
       handle, 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 nonzero 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

SEE ALSO
       unlocked_stdio(3)

								    2001-10-18							       LOCKFILE(3)