這兩天公司代碼中用到了epoll。然后在跟同事閑扯的過程中發現了Linux中有eventfd。兩者雖然名字看起來差不多，但是相關性倒是不多。
為了弄明白這兩個東西到底在內核上是怎么實現的，這兩天將內核這兩個部分的相關代碼看了下，也終于明白了這兩個東西的實現機制。
后續幾篇博客我盡量將這兩個東西的工作原理闡述清楚，但是自己的語言表達能力比較差，也只能是盡量了。

今天這篇博客首先是介紹兩者的使用方式，恰巧兩者能夠在一個程序中搞定，我就寫了下面的小程序來展示兩者的功能。

#include <stdio.h>

#include <unistd.h>

#include <sys/time.h>

#include <stdint.h>

#include <pthread.h>

#include <sys/eventfd.h>

#include <sys/epoll.h>

int efd = -1;

void *read_thread(void *dummy)

{

????int ret = 0;

????uint64_t count = 0;

????int ep_fd = -1;

????struct epoll_event events[10];

????if (efd < 0)

????{

????????printf("efd not inited.\n");

????????goto fail;

????}

????ep_fd = epoll_create(1024);

????if (ep_fd < 0)

????{

????????perror("epoll_create fail: ");

????????goto fail;

????}

????{

????????struct epoll_event read_event;

????????read_event.events = EPOLLHUP | EPOLLERR | EPOLLIN;

????????read_event.data.fd = efd;

????????ret = epoll_ctl(ep_fd, EPOLL_CTL_ADD, efd, &read_event);

????????if (ret < 0)

????????{

????????????perror("epoll ctl failed:");

????????????goto fail;

????????}

????}

????while (1)

????{

????????ret = epoll_wait(ep_fd, &events[0], 10, 5000);

????????if (ret > 0)

????????{

????????????int i = 0;

????????????for (; i < ret; i++)

????????????{

????????????????if (events[i].events & EPOLLHUP)

????????????????{

????????????????????printf("epoll eventfd has epoll hup.\n");

????????????????????goto fail;

????????????????}

????????????????else if (events[i].events & EPOLLERR)

????????????????{

????????????????????printf("epoll eventfd has epoll error.\n");

????????????????????goto fail;

????????????????}

????????????????else if (events[i].events & EPOLLIN)

????????????????{

????????????????????int event_fd = events[i].data.fd;

????????????????????ret = read(event_fd, &count, sizeof(count));

????????????????????if (ret < 0)

????????????????????{

????????????????????????perror("read fail:");

????????????????????????goto fail;

????????????????????}

????????????????????else

????????????????????{

????????????????????????struct timeval tv;

????????????????????????gettimeofday(&tv, NULL);

????????????????????????printf("success read from efd, read %d bytes(%llu) at %lds %ldus\n",

???????????????????????????????ret, count, tv.tv_sec, tv.tv_usec);

????????????????????}

????????????????}

????????????}

????????}

????????else if (ret == 0)

????????{

????????????/* time out */

????????????printf("epoll wait timed out.\n");

????????????break;

????????}

????????else

????????{

????????????perror("epoll wait error:");

????????????goto fail;

????????}

????}

fail:

????if (ep_fd >= 0)

????{

????????close(ep_fd);

????????ep_fd = -1;

????}

????return NULL;

}

int main(int argc, char *argv[])

{

????pthread_t pid = 0;

????uint64_t count = 0;

????int ret = 0;

????int i = 0;

????efd = eventfd(0, 0);

????if (efd < 0)

????{

????????perror("eventfd failed.");

????????goto fail;

????}

????ret = pthread_create(&pid, NULL, read_thread, NULL);

????if (ret < 0)

????{

????????perror("pthread create:");

????????goto fail;

????}

????for (i = 0; i < 5; i++)

????{

????????count = 4;

????????ret = write(efd, &count, sizeof(count));

????????if (ret < 0)

????????{

????????????perror("write event fd fail:");

????????????goto fail;

????????}

????????else

????????{

????????????struct timeval tv;

????????????gettimeofday(&tv, NULL);

????????????printf("success write to efd, write %d bytes(%llu) at %lds %ldus\n",

???????????????????ret, count, tv.tv_sec, tv.tv_usec);

????????}

????????sleep(1);

????}

fail:

????if (0 != pid)

????{

????????pthread_join(pid, NULL);

????????pid = 0;

????}

????if (efd >= 0)

????{

????????close(efd);

????????efd = -1;

????}

????return ret;

}

gcc main.c -Werror -Wall -lpthread

最后執行效果為

success write to efd, write 8 bytes(4) at 1328805612s 21939us

success read from efd, read 8 bytes(4) at 1328805612s 21997us

success write to efd, write 8 bytes(4) at 1328805613s 22247us

success read from efd, read 8 bytes(4) at 1328805613s 22287us

success write to efd, write 8 bytes(4) at 1328805614s 22462us

success read from efd, read 8 bytes(4) at 1328805614s 22503us

success write to efd, write 8 bytes(4) at 1328805615s 22688us

success read from efd, read 8 bytes(4) at 1328805615s 22726us

success write to efd, write 8 bytes(4) at 1328805616s 22973us

success read from efd, read 8 bytes(4) at 1328805616s 23007us

epoll wait timed out.

eventfd具體與pipe有點像，用來完成兩個線程之間事件觸發，但是同事說現在已經支持到進程級別，現在我還沒有驗證過。能夠用來作為線程之間簡單通訊，類似于pthread_cond_t。
epoll則是linux提供的一種多路復用技術，完成與select，poll等一樣的功能，完成對多個文件描述符進行等待。本文上述代碼僅僅用到了一個文件描述符。
epoll比select的優勢網絡隨處可見，這就不多說了。

應用程序就已經寫到這了，對于內核里面具體是怎么搞的，將會在以后的博客中盡量解釋清楚。

總結

以上是生活随笔為你收集整理的eventfd以及epoll原理分析的全部內容，希望文章能夠幫你解決所遇到的問題。

如果覺得生活随笔網站內容還不錯，歡迎將生活随笔推薦給好友。