+ * on signal handler functions.
+ *
+ * If the current process forks, fatal_signal_fork() may be called to clear the
+ * parent process's fatal signal hooks, so that 'hook_cb' is only called when
+ * the child terminates, not when the parent does. When fatal_signal_fork() is
+ * called, it calls the 'cancel_cb' function if it is nonnull, passing 'aux',
+ * to notify that the hook has been canceled. This allows the hook to free
+ * memory, etc. */