👁️ watch模块 - libevent源码深度解析系列（十六）
802 字
4 分钟
👁️ watch模块 - libevent源码深度解析系列（十六）
👁️ watch模块
一、概述
libevent的watch模块提供了在事件循环的不同阶段执行回调的能力。这对于监控、调试或在特定时机执行操作非常有用。
二、watch回调类型
| 回调类型 | 触发时机 |
|---|---|
| prepare | 在调用I/O复用机制（如epoll_wait）之前 |
| check | 在I/O复用机制返回之后，处理活跃事件之前 |
| fork | 在fork子进程之后 |
三、API
3.1 添加watch回调
/* 添加prepare回调 * 在每次调用I/O复用机制之前执行 */int event_base_add_virtual_event_(struct event_base *base, struct event *ev);
/* 添加check回调 * 在I/O复用机制返回之后执行 */int event_base_add_virtual_event_(struct event_base *base, struct event *ev);3.2 结构体
struct evwatch { struct event_base *base; struct evwatch_callback callback; void *arg; TAILQ_ENTRY(evwatch) next;};
typedef void (*evwatch_callback)(struct evwatch *, const struct evwatch_callbacks *, void *);四、使用示例
4.1 监控事件循环
#include <stdio.h>#include <stdlib.h>#include <unistd.h>#include <event2/event.h>
static int loop_count = 0;static struct timeval last_time;
static void prepare_cb(struct evwatch *watcher, const struct evwatch_callbacks *cbs, void *arg) { struct timeval now; evutil_gettimeofday(&now, NULL);
printf("[PREPARE] Loop iteration %d\n", loop_count);
// 计算距离上次循环的时间 if (last_time.tv_sec != 0) { long diff = (now.tv_sec - last_time.tv_sec) * 1000000 + (now.tv_usec - last_time.tv_usec); printf("[PREPARE] Time since last loop: %ld us\n", diff); }
last_time = now;}
static void check_cb(struct evwatch *watcher, const struct evwatch_callbacks *cbs, void *arg) { struct event_base *base = arg;
int active_count = event_base_get_num_events(base, EVENT_BASE_COUNT_ACTIVE); int total_count = event_base_get_num_events(base, EVENT_BASE_COUNT_ADDED);
printf("[CHECK] Active events: %d, Total events: %d\n", active_count, total_count);
loop_count++;}
static void timer_cb(evutil_socket_t fd, short events, void *arg) { printf("Timer fired!\n");}
int main() { struct event_base *base = event_base_new();
// 添加watch回调 struct evwatch *prepare_watcher = evwatch_prepare_new(base, prepare_cb, NULL); struct evwatch *check_watcher = evwatch_check_new(base, check_cb, base);
// 创建定时器 struct event *timer = event_new(base, -1, EV_PERSIST, timer_cb, NULL); struct timeval tv = {1, 0}; event_add(timer, &tv);
// 5秒后退出 struct timeval exit_tv = {5, 0}; event_base_loopexit(base, &exit_tv);
event_base_dispatch(base);
evwatch_free(prepare_watcher); evwatch_free(check_watcher); event_free(timer); event_base_free(base);
printf("Total loop iterations: %d\n", loop_count); return 0;}4.2 性能监控
#include <stdio.h>#include <stdlib.h>#include <unistd.h>#include <event2/event.h>
struct perf_stats { long total_loops; long total_events; long max_latency; struct timeval last_prepare;};
static void prepare_cb(struct evwatch *watcher, const struct evwatch_callbacks *cbs, void *arg) { struct perf_stats *stats = arg; evutil_gettimeofday(&stats->last_prepare, NULL);}
static void check_cb(struct evwatch *watcher, const struct evwatch_callbacks *cbs, void *arg) { struct perf_stats *stats = arg; struct event_base *base = evwatch_get_base(watcher);
struct timeval now; evutil_gettimeofday(&now, NULL);
// 计算I/O复用耗时 long latency = (now.tv_sec - stats->last_prepare.tv_sec) * 1000000 + (now.tv_usec - stats->last_prepare.tv_usec);
if (latency > stats->max_latency) { stats->max_latency = latency; }
stats->total_loops++; stats->total_events += event_base_get_num_events(base, EVENT_BASE_COUNT_ACTIVE);}
int main() { struct event_base *base = event_base_new();
struct perf_stats stats = {0};
struct evwatch *prepare = evwatch_prepare_new(base, prepare_cb, &stats); struct evwatch *check = evwatch_check_new(base, check_cb, &stats);
// 创建一些事件 struct event *timer = event_new(base, -1, EV_PERSIST, [](evutil_socket_t fd, short events, void *arg) {}, NULL); struct timeval tv = {0, 100000}; // 100ms event_add(timer, &tv);
// 运行10秒 struct timeval exit_tv = {10, 0}; event_base_loopexit(base, &exit_tv);
event_base_dispatch(base);
printf("Performance Statistics:\n"); printf(" Total loops: %ld\n", stats.total_loops); printf(" Total events processed: %ld\n", stats.total_events); printf(" Max I/O latency: %ld us\n", stats.max_latency); printf(" Avg events per loop: %.2f\n", (double)stats.total_events / stats.total_loops);
evwatch_free(prepare); evwatch_free(check); event_free(timer); event_base_free(base);
return 0;}五、实际应用场景
| 场景 | 说明 |
|---|---|
| 性能监控 | 统计事件循环次数、延迟等指标 |
| 调试 | 在关键点打印调试信息 |
| 资源管理 | 在循环前后检查资源状态 |
| 心跳检测 | 检测事件循环是否正常运行 |
| 动态调整 | 根据负载动态调整参数 |
六、注意事项
| 注意事项 | 说明 |
|---|---|
| 回调效率 | watch回调在每次循环都会执行，应尽量高效 |
| 避免阻塞 | 回调中不应执行阻塞操作 |
| 资源释放 | 使用evwatch_free释放watcher |
| 线程安全 | 多线程环境下需要加锁保护 |
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👁️ watch模块 - libevent源码深度解析系列（十六）
https://xutopia.top/blog/libevent-watch-module/