The reentrant version of the fgetc function is locked against
    multiple threads calling it simultaneously. This incurs overhead
    to ensure integrity of the stream. The unlocked version of this
    call, fgetc_unlocked can be used to avoid the overhead. The
    fgetc_unlocked function is functionally identical to the fgetc
    function, except that fgetc_unlocked 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 fgetc_unlocked is used.

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

    See also getc_unlocked, flockfile, ftrylockfile, and funlockfile.