The reentrant version of the putc macro is locked against
    multiple threads calling it simultaneously. This incurs overhead
    to ensure integrity of the stream. The unlocked version of this
    call, putc_unlocked can be used to avoid the overhead. The putc_
    unlocked macro is functionally identical to the putc macro,
    except that it is not required to be implemented in a thread-
    safe manner. The putc_unlocked macro can be safely used only
    within a scope that is protected by the flockfile and funlockfile
    functions used as a pair. The caller must ensure that the stream
    is locked before putc_unlocked is used.

    Since putc_unlocked is a macro, a file pointer argument with side
    effects might be evaluated incorrectly. In such a case, use the
    fputc_unlocked function instead.

    Compiling with the __UNIX_PUTC macro defined enables an
    optimization that uses a faster, inlined version of this
    function.

    See also flockfile, ftrylockfile, and funlockfile.