1. 测试代码： 

//https://www.jianshu.com/p/d7ebac8dc9f8
#include <stdio.h>
#include <unistd.h>
#include <stdint.h>
#include <pthread.h>
#include <sys/eventfd.h>
#include <sys/epoll.h>int event_fd = -1;void *read_thread(void *dummy)
{uint64_t inc = 1;int ret = 0;int i = 0;for (; i < 2; i++) {ret = write(event_fd, &inc, sizeof(uint64_t));if (ret < 0) {perror("child thread write event_fd fail.");} else {printf("child thread completed write %llu (0x%llx) to event_fd\n", (unsigned long long) inc, (unsigned long long) inc);}sleep(4);}
}int main(int argc, char *argv[])
{int ret = 0;pthread_t pid = 0;event_fd = eventfd(0, EFD_NONBLOCK | EFD_CLOEXEC);if (event_fd < 0) {perror("event_fd create fail.");}ret = pthread_create(&pid, NULL, read_thread, NULL);if (ret < 0) {perror("pthread create fail.");}uint64_t counter;int epoll_fd = -1;struct epoll_event events[16];if (event_fd < 0){printf("event_fd not inited.\n");}epoll_fd = epoll_create(8);if (epoll_fd < 0){perror("epoll_create fail:");}struct epoll_event read_event;read_event.events = EPOLLIN;read_event.data.fd = event_fd;ret = epoll_ctl(epoll_fd, EPOLL_CTL_ADD, event_fd, &read_event);if (ret < 0) {perror("epoll_ctl failed:");}while (1) {printf("main thread epoll is waiting......\n");ret = epoll_wait(epoll_fd, events, 16, 2000);printf("main thread epoll_wait return ret : %d\n", ret);if (ret > 0) {int i = 0;for (; i < ret; i++) {int fd = events[i].data.fd;if (fd == event_fd) {uint32_t epollEvents = events[i].events;if (epollEvents & EPOLLIN) {ret = read(event_fd, &counter, sizeof(uint64_t));if (ret < 0) {printf("main thread read fail\n");} else {printf("main thread read %llu (0x%llx) from event_fd\n", (unsigned long long) counter, (unsigned long long) counter);}} else {printf("main thread unexpected epoll events on event_fd\n");}}}} else if (ret == 0) {printf("main thread epoll_wait timed out. continue epoll\n");} else {perror("main thread epoll_wait error.");}}
}

输出结果：

wufan@Frank-Linux:~/Linux/test$ ./epoll_eventfd 
main thread epoll is waiting...... // main 线程阻塞在读端
child thread completed write 1 (0x1) to event_fd // 第一次写入后阻塞 4 秒
main thread epoll_wait return ret : 1 // 第一次写完后，立即唤醒 main 线程去进行读操作
main thread read 1 (0x1) from event_fd // main 线程读到了数据
main thread epoll is waiting...... // main 线程阻塞又在读端，超时时间为 2 秒
main thread epoll_wait return ret : 0 // main 线程阻塞等待时间到，返回
main thread epoll_wait timed out. continue epoll
main thread epoll is waiting...... // main 线程阻塞又在读端，超时时间为 2 秒
child thread completed write 1 (0x1) to event_fd // // 第二次写入后阻塞 4 秒
main thread epoll_wait return ret : 1 // 第二次写完后，立即唤醒 main 线程去进行读操作
main thread read 1 (0x1) from event_fd // main 线程读到了数据
main thread epoll is waiting...... // main 线程阻塞又在读端，超时时间为 2 秒
main thread epoll_wait return ret : 0 // main 线程阻塞等待时间到，返回
main thread epoll_wait timed out. continue epoll
main thread epoll is waiting...... // main 线程阻塞又在读端，超时时间为 2 秒
只要没有写入数据，就会在这个死循环中阻塞 -> 超时 -> 阻塞...