epoll的lt和et模式的实验
针对epoll api的两种触发模式，lt和et，仿照一些例子写了代码进行实验。
#include <sys/types.h> #include <sys/socket.h> #include <netinet/in.h> #include <arpa/inet.h> #include <sys/epoll.h> #include <pthread.h> #include <assert.h> #include <stdio.h> #include <unistd.h> #include <stdlib.h> #include <errno.h> #include <string.h> #include <fcntl.h> #define MAX_EVENT_NUMBER 1024 #define BUFFER_SIZE 10 int setnonblocking(int fd) { int old_option = fcntl(fd, F_GETFL); int new_option = old_option | O_NONBLOCK; fcntl(fd, F_SETFL, new_option); return old_option; } void addfd(int epollfd, int fd, bool enable_et) { epoll_event event; event.data.fd = fd; event.events = EPOLLIN; if (enable_et) { event.events |= EPOLLET; } epoll_ctl(epollfd, EPOLL_CTL_ADD, fd, &event); setnonblocking(fd); } void lt(epoll_event *events, int number, int epollfd, int listenfd) { char buf[BUFFER_SIZE]; for (int i=0; i<number; i++) { int sockfd = events[i].data.fd; if (sockfd == listenfd) { sockaddr_in client_address; socklen_t client_addrlen = sizeof(client_address); int connfd = accept(listenfd, (sockaddr*)&client_address, &client_addrlen); addfd(epollfd, connfd, false); } else if (events[i].events & EPOLLIN) { printf("lt event trigger once\n"); memset(buf, '\0', BUFFER_SIZE); int ret = recv(sockfd, buf, BUFFER_SIZE-1, 0); if (ret <= 0) { close(sockfd); continue; } printf("get %d bytes of content: %s\n", ret, buf); } else { printf("something else happened\n"); } } } void et(epoll_event *events, int number, int epollfd, int listenfd) { char buf[BUFFER_SIZE]; for (int i=0; i<number; i++) { int sockfd = events[i].data.fd; if (sockfd == listenfd) { sockaddr_in client_address; socklen_t client_addrlen = sizeof(client_address); int connfd = accept(listenfd, (sockaddr*)&client_address, &client_addrlen); addfd(epollfd, connfd, true); } else if(events[i].events & EPOLLIN) { // Need to read complete printf("et event trigger once\n"); while (true) { memset(buf, '\0', BUFFER_SIZE); int ret = recv(sockfd, buf, BUFFER_SIZE-1, 0); if (ret < 0) { // Below shows complete if ((errno == EAGAIN) || (errno == EWOULDBLOCK)) { printf("read later\n"); break; } printf("some error happens\n"); close(sockfd); break; } else if (ret == 0) { close(sockfd); break; } else { printf("get %d bytes of content: %s\n", ret, buf); } } } else { printf("something else happened\n"); } } } int main(int argc, char *argv[]) { if (argc <= 1) { printf("usage: %s port_number ip_address\n", basename(argv[0])); return 1; } int port = atoi(argv[1]); int ret = 0; sockaddr_in address; bzero(&address, sizeof(address)); address.sin_family = AF_INET; if (argc >= 3) { const char *ip =argv[2]; inet_pton(AF_INET, ip, &address.sin_addr); } else { address.sin_addr.s_addr = INADDR_ANY; } address.sin_port = htons(port); int listenfd = socket(PF_INET, SOCK_STREAM, 0); assert(listenfd >= 0); ret = bind(listenfd, (sockaddr*)&address, sizeof(address)); assert(ret != - 1); ret = listen(listenfd, 5); assert(ret != -1); epoll_event events[MAX_EVENT_NUMBER]; int epollfd = epoll_create(5); assert(epollfd != -1); addfd(epollfd, listenfd, true); while(true) { ret = epoll_wait(epollfd, events, MAX_EVENT_NUMBER, -1); if (ret < 0) { printf("epoll failure\n"); break; } lt(events, ret, epollfd, listenfd); // lt //et(events, ret, epollfd, listenfd); // et } close(listenfd); return 0; }
Makefile文件：
epoll_test : epoll_test.cpp
g++ -o epoll_test epoll_test.cpp -lpthread
以上程序有个问题，就是在端口被占用时候，因为bind失败，会assert失败然后core dump. 在重复测试时候，可以换个端口。
首先，注释掉et，使用lt:
lt(events, ret, epollfd, listenfd); // lt //et(events, ret, epollfd, listenfd); // et
运行 ./epoll_test 12888 并在另一个窗口用telnet输入超过10个（BUFFERSIZE）字符：
$telnet 127.0.0.1 12888 Trying 127.0.0.1... Connected to 127.0.0.1. Escape character is '^]'. 12345678901234567890123456
1234567890123456789012345678
在服务器端得到：
$./epoll_test 12888 lt event trigger once get 9 bytes of content: 123456789 lt event trigger once get 9 bytes of content: 012345678 lt event trigger once get 9 bytes of content: 90123456 lt event trigger once get 1 bytes of content: lt event trigger once get 9 bytes of content: 123456789 lt event trigger once get 9 bytes of content: 012345678 lt event trigger once get 9 bytes of content: 901234567 lt event trigger once get 3 bytes of content: 8
可以看出因为buffersize的限制，服务器端进行了多次读取，event也触发了多次。
换成et模式：
//lt(events, ret, epollfd, listenfd); // lt et(events, ret, epollfd, listenfd); // et
运行服务器后，telnet客户端输入：
$telnet 127.0.0.1 12889 Trying 127.0.0.1... Connected to 127.0.0.1. Escape character is '^]'. 12345678901234567890123456 1234567890123456789012345678
服务器显示：
$./epoll_test 12889 et event trigger once get 9 bytes of content: 123456789 get 9 bytes of content: 012345678 get 9 bytes of content: 90123456 get 1 bytes of content: read later et event trigger once get 9 bytes of content: 123456789 get 9 bytes of content: 012345678 get 9 bytes of content: 901234567 get 3 bytes of content: 8 read later
可以看出，每次客户端的字符串，只触发了一次。
其实，上面的例子还不够严谨，因为服务器一次已经把字符都读完了。那么如果没读完，会继续出发吗。
如下修改服务器代码：
//修改 //while (true) { if (true) { memset(buf, '\0', BUFFER_SIZE); int ret = recv(sockfd, buf, BUFFER_SIZE-1, 0); if (ret < 0) { // Below shows complete if ((errno == EAGAIN) || (errno == EWOULDBLOCK)) { printf("read later\n"); break; } printf("some error happens\n"); close(sockfd); //修改 //break; } else if (ret == 0) { close(sockfd); //修改 //break; } else { printf("get %d bytes of content: %s\n", ret, buf); } }
运行服务器之后，telnet输入长字符串：
$telnet 127.0.0.1 12888 Trying 127.0.0.1... Connected to 127.0.0.1. Escape character is '^]'. 12345678901234567890123456
服务器端只显示了BUFFERSIZE长度的一行，没有读入的数据也没有进行event触发：
$./epoll_test 12888 et event trigger once get 9 bytes of content: 123456789
如果客户端，再输入一行：
$telnet 127.0.0.1 12888 Trying 127.0.0.1... Connected to 127.0.0.1. Escape character is '^]'. 12345678901234567890123456 1234567890123456789012345678
服务器端也仅仅把之前没读入的上一次客户端发来的数据中，再读入BUFFERSIZE长度：
$./epoll_test 12888 et event trigger once get 9 bytes of content: 123456789 et event trigger once get 9 bytes of content: 012345678
另外，对上面的服务器端程序，增加了et模式下对recv函数的返回ret=0的打印：
else if (ret == 0) { printf("get 0 data\n"); close(sockfd); break; }
发现在et模式下，没有走到ret==0的分支：
$telnet 127.0.0.1 12889 Trying 127.0.0.1... Connected to 127.0.0.1. Escape character is '^]'. 12345678901234567890123456
服务器端：
$./epoll_test 12889 et event trigger once get 9 bytes of content: 123456789 get 9 bytes of content: 012345678 get 9 bytes of content: 90123456 get 1 bytes of content: read later
走的是如下的判断结束的分支：
if ((errno == EAGAIN) || (errno == EWOULDBLOCK)) { printf("read later\n"); break; }
以上。:)
