網(wǎng)絡(luò)服務(wù)器通常使用一個獨(dú)立的進(jìn)程或線程來實(shí)現(xiàn)每個連接。由于高性能應(yīng)用程序需要同時處理大量的客戶端，這種方法就不太好用了，因?yàn)橘Y源占用和上下文切換時間等因素影響了同時處理大量客戶端的能力。另一種方法是在一個線程中使用非阻塞 I/O，以及一些就緒通知方法，即當(dāng)你可以在一個套接字上讀寫更多數(shù)據(jù)的時候告訴你。

本文介紹了 Linux 的 epoll(7) 機(jī)制，它是 Linux 最好的就緒通知機(jī)制。我們用 C 語言編寫了示例代碼，實(shí)現(xiàn)了一個完整的 TCP 服務(wù)器。 我假設(shè)您有一定 C 語言編程經(jīng)驗(yàn)，知道如何在 Linux 上編譯和運(yùn)行程序，并且可以閱讀手冊查看各種需要的 C 函數(shù)。

epoll 是在 Linux 2.6 中引入的，在其他類 UNIX 操作系統(tǒng)上不可用。它提供了一個類似于 select(2) 和 poll(2) 函數(shù)的功能：

• select(2) 一次可以監(jiān)測 FD_SETSIZE數(shù)量大小的描述符，FD_SETSIZE 通常是一個在 libc 編譯時指定的小數(shù)字。
• poll(2) 一次可以監(jiān)測的描述符數(shù)量并沒有限制，但撇開其它因素，我們每次都不得不檢查就緒通知，線性掃描所有通過描述符，這樣時間復(fù)雜度為 O(n)而且很慢。

epoll 沒有這些固定限制，也不執(zhí)行任何線性掃描。因此它可以更高效地執(zhí)行和處理大量事件。

一個 epoll 實(shí)例可由 epoll_create(2) 或 epoll_create1(2) （它們采用不同的參數(shù)）創(chuàng)建，它們的返回值是一個 epoll 實(shí)例。epoll_ctl(2) 用來添加或刪除監(jiān)聽 epoll 實(shí)例的描述符。epoll_wait(2) 用來等待被監(jiān)聽的描述符事件，一直阻塞到事件可用。更多信息請參見相關(guān)手冊。

當(dāng)描述符被添加到 epoll 實(shí)例時，有兩種模式：電平觸發(fā)和邊緣觸發(fā)（譯者注：借鑒電路里面的概念）。當(dāng)你使用電平觸發(fā)模式，并且數(shù)據(jù)可以被讀取，epoll_wait(2) 函數(shù)總是會返回就緒事件。如果你還沒有讀完數(shù)據(jù)，并且再次在 epoll 實(shí)例上調(diào)用?epoll_wait(2) 函數(shù)監(jiān)聽這個描述符，由于還有數(shù)據(jù)可讀，那么它會再次返回這個事件。在邊緣觸發(fā)模式下，你只會得到一次就緒通知。如果你沒有將數(shù)據(jù)全部讀走，并且再次在 epoll 實(shí)例上調(diào)用 epoll_wait(2) 函數(shù)監(jiān)聽這個描述符，它就會阻塞，因?yàn)榫途w事件已經(jīng)發(fā)送過了。

傳遞到 epoll_ctl(2) 的 epoll 事件結(jié)構(gòu)體如下。對每一個被監(jiān)聽的描述符，你可以關(guān)聯(lián)到一個整數(shù)或者一個用戶數(shù)據(jù)的指針。

C typedef union epoll_data {void *ptr;int fd;__uint32_t u32;__uint64_t u64; } epoll_data_t;struct epoll_event {__uint32_t events; /* Epoll events */epoll_data_t data; /* User data variable */ };

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typedef union epoll_data { ??void????????*ptr; ??int??????????fd; ??__uint32_t?? u32; ??__uint64_t?? u64; } epoll_data_t; struct epoll_event { ??__uint32_t?? events; /* Epoll events */ ??epoll_data_t data;?? /* User data variable */ };

現(xiàn)在我們開始寫代碼。我們將實(shí)現(xiàn)一個小的 TCP 服務(wù)器，將發(fā)送到這個套接字的所有數(shù)據(jù)打印到標(biāo)準(zhǔn)輸出上。首先編寫一個 create_and_bind() 函數(shù)，用來創(chuàng)建和綁定 TCP 套接字：

C static int create_and_bind (char *port) {struct addrinfo hints;struct addrinfo *result, *rp;int s, sfd;memset (&hints, 0, sizeof (struct addrinfo));hints.ai_family = AF_UNSPEC; /* Return IPv4 and IPv6 choices */hints.ai_socktype = SOCK_STREAM; /* We want a TCP socket */hints.ai_flags = AI_PASSIVE; /* All interfaces */s = getaddrinfo (NULL, port, &hints, &result);if (s != 0){fprintf (stderr, "getaddrinfo: %sn", gai_strerror (s));return -1;}for (rp = result; rp != NULL; rp = rp->ai_next){sfd = socket (rp->ai_family, rp->ai_socktype, rp->ai_protocol);if (sfd == -1)continue;s = bind (sfd, rp->ai_addr, rp->ai_addrlen);if (s == 0){/* We managed to bind successfully! */break;}close (sfd);}if (rp == NULL){fprintf (stderr, "Could not bindn");return -1;}freeaddrinfo (result);return sfd; }

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static int create_and_bind (char *port) { ??struct addrinfo hints; ??struct addrinfo *result, *rp; ??int s, sfd; ??memset (&hints, 0, sizeof (struct addrinfo)); ??hints.ai_family = AF_UNSPEC;???? /* Return IPv4 and IPv6 choices */ ??hints.ai_socktype = SOCK_STREAM; /* We want a TCP socket */ ??hints.ai_flags = AI_PASSIVE;???? /* All interfaces */ ??s = getaddrinfo (NULL, port, &hints, &result); ??if (s != 0) ????{ ??????fprintf (stderr, "getaddrinfo: %sn", gai_strerror (s)); ??????return -1; ????} ??for (rp = result; rp != NULL; rp = rp->ai_next) ????{ ??????sfd = socket (rp->ai_family, rp->ai_socktype, rp->ai_protocol); ??????if (sfd == -1) ????????continue; ??????s = bind (sfd, rp->ai_addr, rp->ai_addrlen); ??????if (s == 0) ????????{ ??????????/* We managed to bind successfully! */ ??????????break; ????????} ??????close (sfd); ????} ??if (rp == NULL) ????{ ??????fprintf (stderr, "Could not bindn"); ??????return -1; ????} ??freeaddrinfo (result); ??return sfd; }

create_and_bind() 包含一個標(biāo)準(zhǔn)代碼塊，用一種可移植的方式來獲得 IPv4 和 IPv6 套接字。它接受一個 port 字符串參數(shù)，可由 argv[1] 傳遞。getaddrinfo(3) 函數(shù)返回一堆 addrinfo?結(jié)構(gòu)體到 result 變量中，它們與傳入的 hints參數(shù)是兼容的。addrinfo結(jié)構(gòu)體像這樣：

C struct addrinfo {int ai_flags;int ai_family;int ai_socktype;int ai_protocol;size_t ai_addrlen;struct sockaddr *ai_addr;char *ai_canonname;struct addrinfo *ai_next; };

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struct addrinfo { ??int??????????????ai_flags; ??int??????????????ai_family; ??int??????????????ai_socktype; ??int??????????????ai_protocol; ??size_t?????????? ai_addrlen; ??struct sockaddr *ai_addr; ??char????????????*ai_canonname; ??struct addrinfo *ai_next; };

我們依次遍歷這些結(jié)構(gòu)體并用它們創(chuàng)建套接字，直到可以創(chuàng)建并綁定一個套接字。如果成功了，create_and_bind() 返回這個套接字描述符。如果失敗則返回 -1。

下面我們編寫一個函數(shù)，用于將套接字設(shè)置為非阻塞狀態(tài)。make_socket_non_blocking() 為傳入的 sfd?參數(shù)設(shè)置 O_NONBLOCK 標(biāo)志：

C static int make_socket_non_blocking (int sfd) {int flags, s;flags = fcntl (sfd, F_GETFL, 0);if (flags == -1){perror ("fcntl");return -1;}flags |= O_NONBLOCK;s = fcntl (sfd, F_SETFL, flags);if (s == -1){perror ("fcntl");return -1;}return 0; }

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static int make_socket_non_blocking (int sfd) { ??int flags, s; ??flags = fcntl (sfd, F_GETFL, 0); ??if (flags == -1) ????{ ??????perror ("fcntl"); ??????return -1; ????} ??flags |= O_NONBLOCK; ??s = fcntl (sfd, F_SETFL, flags); ??if (s == -1) ????{ ??????perror ("fcntl"); ??????return -1; ????} ??return 0; }

現(xiàn)在說說?main() 函數(shù)吧，它里面包含了這個程序的事件循環(huán)。這是主要代碼:

#define MAXEVENTS 64int main (int argc, char *argv[]) {int sfd, s;int efd;struct epoll_event event;struct epoll_event *events;if (argc != 2){fprintf (stderr, "Usage: %s [port]n", argv[0]);exit (EXIT_FAILURE);}sfd = create_and_bind (argv[1]);if (sfd == -1)abort ();s = make_socket_non_blocking (sfd);if (s == -1)abort ();s = listen (sfd, SOMAXCONN);if (s == -1){perror ("listen");abort ();}efd = epoll_create1 (0);if (efd == -1){perror ("epoll_create");abort ();}event.data.fd = sfd;event.events = EPOLLIN | EPOLLET;s = epoll_ctl (efd, EPOLL_CTL_ADD, sfd, &event);if (s == -1){perror ("epoll_ctl");abort ();}/* Buffer where events are returned */events = calloc (MAXEVENTS, sizeof event);/* The event loop */while (1){int n, i;n = epoll_wait (efd, events, MAXEVENTS, -1);for (i = 0; i < n; i++){if ((events[i].events & EPOLLERR) ||(events[i].events & EPOLLHUP) ||(!(events[i].events & EPOLLIN))){/* An error has occured on this fd, or the socket is notready for reading (why were we notified then?) */fprintf (stderr, "epoll errorn");close (events[i].data.fd);continue;}else if (sfd == events[i].data.fd){/* We have a notification on the listening socket, whichmeans one or more incoming connections. */while (1){struct sockaddr in_addr;socklen_t in_len;int infd;char hbuf[NI_MAXHOST], sbuf[NI_MAXSERV];in_len = sizeof in_addr;infd = accept (sfd, &in_addr, &in_len);if (infd == -1){if ((errno == EAGAIN) ||(errno == EWOULDBLOCK)){/* We have processed all incomingconnections. */break;}else{perror ("accept");break;}}s = getnameinfo (&in_addr, in_len,hbuf, sizeof hbuf,sbuf, sizeof sbuf,NI_NUMERICHOST | NI_NUMERICSERV);if (s == 0){printf("Accepted connection on descriptor %d ""(host=%s, port=%s)n", infd, hbuf, sbuf);}/* Make the incoming socket non-blocking and add it to thelist of fds to monitor. */s = make_socket_non_blocking (infd);if (s == -1)abort ();event.data.fd = infd;event.events = EPOLLIN | EPOLLET;s = epoll_ctl (efd, EPOLL_CTL_ADD, infd, &event);if (s == -1){perror ("epoll_ctl");abort ();}}continue;}else{/* We have data on the fd waiting to be read. Read anddisplay it. We must read whatever data is availablecompletely, as we are running in edge-triggered modeand won't get a notification again for the samedata. */int done = 0;while (1){ssize_t count;char buf[512];count = read (events[i].data.fd, buf, sizeof buf);if (count == -1){/* If errno == EAGAIN, that means we have read alldata. So go back to the main loop. */if (errno != EAGAIN){perror ("read");done = 1;}break;}else if (count == 0){/* End of file. The remote has closed theconnection. */done = 1;break;}/* Write the buffer to standard output */s = write (1, buf, count);if (s == -1){perror ("write");abort ();}}if (done){printf ("Closed connection on descriptor %dn",events[i].data.fd);/* Closing the descriptor will make epoll remove itfrom the set of descriptors which are monitored. */close (events[i].data.fd);}}}}free (events);close (sfd);return EXIT_SUCCESS; }

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#define MAXEVENTS 64 int main (int argc, char *argv[]) { ??int sfd, s; ??int efd; ??struct epoll_event event; ??struct epoll_event *events; ??if (argc != 2) ????{ ??????fprintf (stderr, "Usage: %s [port]n", argv[0]); ??????exit (EXIT_FAILURE); ????} ??sfd = create_and_bind (argv[1]); ??if (sfd == -1) ????abort (); ??s = make_socket_non_blocking (sfd); ??if (s == -1) ????abort (); ??s = listen (sfd, SOMAXCONN); ??if (s == -1) ????{ ??????perror ("listen"); ??????abort (); ????} ??efd = epoll_create1 (0); ??if (efd == -1) ????{ ??????perror ("epoll_create"); ??????abort (); ????} ??event.data.fd = sfd; ??event.events = EPOLLIN | EPOLLET; ??s = epoll_ctl (efd, EPOLL_CTL_ADD, sfd, &event); ??if (s == -1) ????{ ??????perror ("epoll_ctl"); ??????abort (); ????} ??/* Buffer where events are returned */ ??events = calloc (MAXEVENTS, sizeof event); ??/* The event loop */ ??while (1) ????{ ??????int n, i; ??????n = epoll_wait (efd, events, MAXEVENTS, -1); ??????for (i = 0; i < n; i++) ????{ ??????if ((events[i].events & EPOLLERR) || ??????????????(events[i].events & EPOLLHUP) || ??????????????(!(events[i].events & EPOLLIN))) ????????{ ??????????????/* An error has occured on this fd, or the socket is not ???????????????? ready for reading (why were we notified then?) */ ??????????fprintf (stderr, "epoll errorn"); ??????????close (events[i].data.fd); ??????????continue; ????????} ??????else if (sfd == events[i].data.fd) ????????{ ??????????????/* We have a notification on the listening socket, which ???????????????? means one or more incoming connections. */ ??????????????while (1) ????????????????{ ??????????????????struct sockaddr in_addr; ??????????????????socklen_t in_len; ??????????????????int infd; ??????????????????char hbuf[NI_MAXHOST], sbuf[NI_MAXSERV]; ??????????????????in_len = sizeof in_addr; ??????????????????infd = accept (sfd, &in_addr, &in_len); ??????????????????if (infd == -1) ????????????????????{ ??????????????????????if ((errno == EAGAIN) || ??????????????????????????(errno == EWOULDBLOCK)) ????????????????????????{ ??????????????????????????/* We have processed all incoming ???????????????????????????? connections. */ ??????????????????????????break; ????????????????????????} ??????????????????????else ????????????????????????{ ??????????????????????????perror ("accept"); ??????????????????????????break; ????????????????????????} ????????????????????} ??????????????????s = getnameinfo (&in_addr, in_len, ?????????????????????????????????? hbuf, sizeof hbuf, ?????????????????????????????????? sbuf, sizeof sbuf, ?????????????????????????????????? NI_NUMERICHOST | NI_NUMERICSERV); ??????????????????if (s == 0) ????????????????????{ ??????????????????????printf("Accepted connection on descriptor %d " ???????????????????????????? "(host=%s, port=%s)n", infd, hbuf, sbuf); ????????????????????} ??????????????????/* Make the incoming socket non-blocking and add it to the ???????????????????? list of fds to monitor. */ ??????????????????s = make_socket_non_blocking (infd); ??????????????????if (s == -1) ????????????????????abort (); ??????????????????event.data.fd = infd; ??????????????????event.events = EPOLLIN | EPOLLET; ??????????????????s = epoll_ctl (efd, EPOLL_CTL_ADD, infd, &event); ??????????????????if (s == -1) ????????????????????{ ??????????????????????perror ("epoll_ctl"); ??????????????????????abort (); ????????????????????} ????????????????} ??????????????continue; ????????????} ??????????else ????????????{ ??????????????/* We have data on the fd waiting to be read. Read and ???????????????? display it. We must read whatever data is available ???????????????? completely, as we are running in edge-triggered mode ???????????????? and won't get a notification again for the same ???????????????? data. */ ??????????????int done = 0; ??????????????while (1) ????????????????{ ??????????????????ssize_t count; ??????????????????char buf[512]; ??????????????????count = read (events[i].data.fd, buf, sizeof buf); ??????????????????if (count == -1) ????????????????????{ ??????????????????????/* If errno == EAGAIN, that means we have read all ???????????????????????? data. So go back to the main loop. */ ??????????????????????if (errno != EAGAIN) ????????????????????????{ ??????????????????????????perror ("read"); ??????????????????????????done = 1; ????????????????????????} ??????????????????????break; ????????????????????} ??????????????????else if (count == 0) ????????????????????{ ??????????????????????/* End of file. The remote has closed the ???????????????????????? connection. */ ??????????????????????done = 1; ??????????????????????break; ????????????????????} ??????????????????/* Write the buffer to standard output */ ??????????????????s = write (1, buf, count); ??????????????????if (s == -1) ????????????????????{ ??????????????????????perror ("write"); ??????????????????????abort (); ????????????????????} ????????????????} ??????????????if (done) ????????????????{ ??????????????????printf ("Closed connection on descriptor %dn", ??????????????????????????events[i].data.fd); ??????????????????/* Closing the descriptor will make epoll remove it ???????????????????? from the set of descriptors which are monitored. */ ??????????????????close (events[i].data.fd); ????????????????} ????????????} ????????} ????} ??free (events); ??close (sfd); ??return EXIT_SUCCESS; }

main() 首先調(diào)用 create_and_bind() 新建套接字。然后把套接字設(shè)置非阻塞模式，再調(diào)用listen(2)。接下來它創(chuàng)建一個 epoll 實(shí)例 efd，添加監(jiān)聽套接字 sfd ，用電平觸發(fā)模式來監(jiān)聽輸入事件。

外層的 while 循環(huán)是主要事件循環(huán)。它調(diào)用epoll_wait(2)，線程保持阻塞以等待事件到來。當(dāng)事件就緒，epoll_wait(2) 用 events 參數(shù)返回事件，這個參數(shù)是一群 epoll_event 結(jié)構(gòu)體。

當(dāng)我們添加新的監(jiān)聽輸入連接以及刪除終止的現(xiàn)有連接時，efd 這個 epoll 實(shí)例在事件循環(huán)中不斷更新。

當(dāng)事件是可用的，它們可以有三種類型：

• 錯誤：當(dāng)一個錯誤連接出現(xiàn)，或事件不是一個可以讀取數(shù)據(jù)的通知，我們只要簡單地關(guān)閉相關(guān)的描述符。關(guān)閉描述符會自動地移除 efd 這個 epoll 實(shí)例的監(jiān)聽列表。
• 新連接：當(dāng)監(jiān)聽描述符 sfd 是可讀狀態(tài)，這表明一個或多個連接已經(jīng)到達(dá)。當(dāng)有一個新連接， accept(2) 接受這個連接，打印一條相應(yīng)的消息，把這個到來的套接字設(shè)置為非阻塞狀態(tài)，并將其添加到 efd 這個?epoll 實(shí)例的監(jiān)聽列表。
• 客戶端數(shù)據(jù)：當(dāng)任何一個客戶端描述符的數(shù)據(jù)可讀時，我們在內(nèi)部 while 循環(huán)中用 read(2) 以 512 字節(jié)大小讀取數(shù)據(jù)。這是因?yàn)楫?dāng)前我們必須讀走所有可讀的數(shù)據(jù)，當(dāng)監(jiān)聽描述符是邊緣觸發(fā)模式下，我們不會再得到事件。被讀取的數(shù)據(jù)使用 write(2) 被寫入標(biāo)準(zhǔn)輸出(fd=1)。如果 read(2) 返回 0，這表示 EOF 并且我們可以關(guān)閉這個客戶端的連接。如果返回 -1，errno 被設(shè)置為 EAGAIN，這表示這個事件的所有數(shù)據(jù)被讀走，我們可以返回主循環(huán)。

就是這樣。它在一個循環(huán)中運(yùn)行，在監(jiān)聽列表中添加和刪除描述符。

下載 epoll-example.c 代碼。

更新1：水平和邊緣觸發(fā)的定義被顛倒錯誤了（雖然代碼是正確的）。這是被Reddit用戶 bodski 發(fā)現(xiàn)的。文章現(xiàn)在正確了。我應(yīng)該在發(fā)布前校對的。對不起，并感謝謝指出錯誤。:)

更新2：代碼被修改成連接將被阻塞時才執(zhí)行accept(2)，所以如果多個連接到達(dá)，我們?nèi)拷邮堋＿@是Reddit用戶 pitchford 提出。謝謝你的評論。 :)

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