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
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 non-blocking version of
flockfile(). It does nothing in case some other thread owns *
filehandle, and it obtains ownership and increments the lockcount