# 6.4 检查事件状态 有时候需要了解事件是否已经添加,检查事件代表什么。 ```cpp int event_pending(const struct event *ev, short what, struct timeval *tv_out); #define event_get_signal(ev) /* ... */ evutil_socket_t event_get_fd(const struct event *ev); struct event_base *event_get_base(const struct event *ev); short event_get_events(const struct event *ev); event_callback_fn event_get_callback(const struct event *ev); void *event_get_callback_arg(const struct event *ev); int event_get_priority(const struct event *ev); void event_get_assignment(const struct event *event, struct event_base **base_out, evutil_socket_t *fd_out, short *events_out, event_callback_fn *callback_out, void **arg_out); ``` event_pending()函数确定给定的事件是否是未决的或者激活的。如果是,而且 what 参 数设置了 EV_READ、EV_WRITE、EV_SIGNAL 或者 EV_TIMEOUT 等标志,则函数会返回事件当前为之未决或者激活的所有标志 。如果提供了 tv_out 参数,并且 what 参数中设置了 EV_TIMEOUT 标志,而事件当前正因超时事件而未决或者激活,则 tv_out 会返回事件 的超时值。 event_get_fd()和 event_get_signal()返回为事件配置的文件描述符或者信号值。 event_get_base()返回为事件配置的 event_base。event_get_events()返回事件的标志(EV_READ、EV_WRITE 等)。event_get_callback()和 event_get_callback_arg() 返回事件的回调函数及其参数指针。 event_get_assignment()复制所有为事件分配的字段到提供的指针中。任何为 NULL 的参数会被忽略。 ###实例 ```cpp #include <event2/event.h> #include <stdio.h> /* Change the callback and callback_arg of 'ev', which must not be * pending. */ int replace_callback(struct event *ev, event_callback_fn new_callback, void *new_callback_arg) { struct event_base *base; evutil_socket_t fd; short events; int pending; pending = event_pending(ev, EV_READ|EV_WRITE|EV_SIGNAL|EV_TIMEOUT, NULL); if (pending) { /* We want to catch this here so that we do not re-assign a * pending event. That would be very very bad. */ fprintf(stderr, "Error! replace_callback called on a pending event!\n"); return -1; } event_get_assignment(ev, &base, &fd, &events, NULL /* ignore old callback */ , NULL /* ignore old callback argument */); event_assign(ev, base, fd, events, new_callback, new_callback_arg); return 0; } ```