void silc_mutex_lock(SilcMutex mutex);

    Locks the mutex. If the mutex is locked by another thread the
    current thread will block until the other thread has issued
    silc_mutex_unlock for the mutex.  If `mutex' is NULL this function
    has no effect.

    The caller must not call silc_mutex_lock for mutex that has been
    already locked in the current thread.  In this case deadlock will
    occur.