> For the complete documentation index, see [llms.txt](https://aceld.gitbook.io/libevent/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://aceld.gitbook.io/libevent/6_shi_jian/64_jian_cha_shi_jian_zhuang_tai.md).

# 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;
}
```


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