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=; 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-, );
if (ret <= ) {
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=; 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-, );
if (ret < ) {
// Below shows complete
if ((errno == EAGAIN) || (errno == EWOULDBLOCK)) {
printf("read later\n");
break;
}
printf("some error happens\n");
close(sockfd);
break;
}
else if (ret == ) {
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 <= ) {
printf("usage: %s port_number ip_address\n", basename(argv[]));
return ;
} int port = atoi(argv[]);
int ret = ;
sockaddr_in address;
bzero(&address, sizeof(address));
address.sin_family = AF_INET;
if (argc >= ) {
const char *ip =argv[];
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, );
assert(listenfd >= ); ret = bind(listenfd, (sockaddr*)&address, sizeof(address));
assert(ret != - ); ret = listen(listenfd, );
assert(ret != -); epoll_event events[MAX_EVENT_NUMBER];
int epollfd = epoll_create();
assert(epollfd != -);
addfd(epollfd, listenfd, true); while(true) {
ret = epoll_wait(epollfd, events, MAX_EVENT_NUMBER, -);
if (ret < ) {
printf("epoll failure\n");
break;
}
lt(events, ret, epollfd, listenfd); // lt
//et(events, ret, epollfd, listenfd); // et
} close(listenfd);
return ; }
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
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
lt event trigger once
get bytes of content:
lt event trigger once
get bytes of content:
lt event trigger once
get bytes of content:
lt event trigger once
get bytes of content: lt event trigger once
get bytes of content:
lt event trigger once
get bytes of content:
lt event trigger once
get bytes of content:
lt event trigger once
get bytes of content:
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
Trying 127.0.0.1...
Connected to 127.0.0.1.
Escape character is '^]'.
服务器显示：
$./epoll_test
et event trigger once
get bytes of content:
get bytes of content:
get bytes of content:
get bytes of content: read later
et event trigger once
get bytes of content:
get bytes of content:
get bytes of content:
get bytes of content: 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-, );
if (ret < ) {
// 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 == ) {
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
Trying 127.0.0.1...
Connected to 127.0.0.1.
Escape character is '^]'.
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
et event trigger once
get bytes of content:
如果客户端，再输入一行：
$telnet 127.0.0.1
Trying 127.0.0.1...
Connected to 127.0.0.1.
Escape character is '^]'.
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
et event trigger once
get bytes of content:
et event trigger once
get bytes of content:
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 == ) {
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
Trying 127.0.0.1...
Connected to 127.0.0.1.
Escape character is '^]'.
服务器端：
$./epoll_test
et event trigger once
get bytes of content:
get bytes of content:
get bytes of content:
get 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。:)