Section (3) flockfile
flockfile, ftrylockfile, funlockfile — lock FILE for stdio
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.
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).
waits for *
to be no longer locked by a different thread, then makes the
current thread owner of *
filehandle, and increments the
decrements the lock count.
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.
returns zero for success (the lock was obtained), and nonzero
For an explanation of the terms used in this section, see attributes(7).
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