epoll是Linux特有的I/O复用函数。它在现实和使用上与select，poll由很大的差异。首先，epoll使用一组函数来完成任务，而不是单个函数。其次，epoll把用户关心的文件描述符上的事件放在内核里的一个事件表中，从而无须像select和poll那样每次调用都要重复传入文件描述符集或事件集。但epoll需要使用一个额外的文件描述符，来唯一标识内核中的这个事件表，这个文件描述符使用使用如下epoll_create函数来创建:
#inlcude<sys/epoll.h>
int epoll_create(int size)
size参数提示内核事件表需要多大。该函数返回的文件描述符将用作其他所有epoll系统调用的第一个参数，以指定要访问的内核事件表。
#include<sys/epoll.h>
int epoll_ctl(int epfd, int op, int fd, struct epoll_event *event)
fd参数是要操作的文件描述符，op参数则指定操作类型。操作类型有：
EPOLL_CTL_ADD, 往事件表中注册fd上的事件。
EPOLL_CTL_MOD, 修改fd上的注册事件。
EPOLL_CTL_DEL, 删除fd上的注册事件。
event参数指定事件，他是epoll_event结构指针类型。epoll_event的定义如下：
struct epoll_event
{
__unit32_t events; //epoll事件
epoll_data_t data; //用户数据
}；
#include<sys/epoll.h>
int epoll_wait(int epfd, struct epoll_event* events, int maxevents, int timeout);
timeout参数与poll函数的timeout参数相同。
maxevents参数指定最多监听多少个事件，它必须大于0.
epoll_wait函数如果检测到事件，就将所有就绪的事件从内核事件表（由epfd参数指定）中复制到他的第二个参数events指向的数组中。这个数组值输出epoll_wait检测到的就绪事件。
epoll对文件描述符由两种模式为LT模式和ET模式。LT模式是默认的工作模式，在此模式下epoll相当与高效的poll。而当往内核事件表中注册一个文件描述符上的EPOLLET事件时，epoll将以ET模式来操作该文件描述符。
eg：
#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, int enable_et)
{
struct 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(struct 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 (listenfd == sockfd)
{
struct sockaddr_in client_address;
socklen_t client_addrlength = sizeof(client_address);
int connfd = accept(listenfd, (struct sockaddr *) &client_address, &client_addrlength);
addfd(epollfd, connfd, 0);
}
else if (events[i].events & EPOLLIN)
{
printf("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(struct 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)
{
struct sockaddr_in client_address;
socklen_t client_addrlength = sizeof(client_address);
int connfd = accept(listenfd, (struct sockaddr *) &client_address, &client_addrlength);
addfd(epollfd, connfd, 1);
}
else if(events[i].events & EPOLLIN)
{
printf("event trigger once\n");
while(1)
{
memset(buf, '\0', BUFFER_SIZE);
int ret = recv(sockfd, buf, BUFFER_SIZE-1, 0);
if (ret < 0)
{
if ((errno == EAGAIN) || (errno == EWOULDBLOCK))
{
printf("read later\n");
break;
}
close(sockfd);
break;
}
else if(ret == 0)
{
close(sockfd);
}
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 <= 2)
{
printf("usage: %s ip_address port_number\n", argv[0]);
return 1;
}
const char *ip = argv[1];
int port = atoi(argv[2]);
int ret = 0;
struct sockaddr_in address;
bzero(&address, sizeof(address));
address.sin_family = AF_INET;
inet_pton(AF_INET, ip, &address.sin_addr);
address.sin_port = htons(port);
int listenfd = socket(AF_INET, SOCK_STREAM, 0);
assert(ret != -1);
ret = bind(listenfd, (struct sockaddr *) &address, sizeof(address));
assert(ret != -1);
ret = listen(listenfd, 5);
assert(ret != -1);
struct epoll_event events[MAX_EVENT_NUMBER];
int epollfd = epoll_create(5);
assert(epollfd != -1);
addfd(epollfd, listenfd, 1);
while (1)
{
int ret = epoll_wait(epollfd, events, MAX_EVENT_NUMBER, -1);
if(ret < 0)
{
printf("epoll failure!\n");
break;
}
lt(events, ret, epollfd, listenfd);
// et(events, ret, epolfd, listenfd);
}
close(listenfd);
return 0;
}
2.eg:
#define MAX_EVENT_NUMBER 1024
#define BUFFER_SIZE 10
struct fds
{
int epollfd;
int sockfd;
};
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, int oneshot)
{
struct epoll_event event;
event.data.fd = fd;
event.events = EPOLLIN | EPOLLET;
if (oneshot)
{
event.events |= EPOLLONESHOT;
}
epoll_ctl(epollfd, EPOLL_CTL_ADD, fd, &event);
setnonblocking(fd);
}
void reset_oneshot(int epollfd, int fd)
{
struct epoll_event event;
event.data.fd = fd;
event.events = EPOLLIN | EPOLLET | EPOLLONESHOT;
epoll_ctl(epollfd, EPOLL_CTL_MOD, fd, &event);
}
void* worker(void *arg)
{
int sockfd = ((struct fds *)arg)->sockfd;
int epollfd = ((struct fds *)arg)->epollfd;
printf("start new thread to receive data on fd: %d\n", sockfd);
char buf[BUFFER_SIZE];
memset(buf, '\0', BUFFER_SIZE);
while(1)
{
int ret = recv(sockfd, buf, BUFFER_SIZE-1, 0);
if (ret == 0)
{
close(sockfd);
printf("foreiner closed the connection\n");
break;
}
else if (ret < 0)
{
if (errno == EAGAIN)
{
reset_oneshot(epollfd, sockfd);
printf("read later\n");
break;
}
}
else
{
printf("get content: %s\n", buf);
sleep(5);
}
}
printf("end thread recviving data on fd: %d\n", sockfd);
}
int main(int argc, char **argv)
{
if (argc <= 2)
{
printf("usage: %s ip_address port_number\n", argv[0]);
return -1;
}
const char *ip = argv[1];
int port = atoi(argv[2]);
int ret = 0;
struct sockaddr_in address;
bzero(&address, sizeof(address));
address.sin_family = AF_INET;
inet_pton(AF_INET, ip, &address.sin_addr);
address.sin_port = htons(port);
int listenfd = socket(AF_INET, SOCK_STREAM, 0);
assert( listenfd >= 0 );
ret = bind(listenfd, (struct sockaddr *) &address, sizeof(address));
assert( ret != -1 );
ret = listen(listenfd, 5);
assert( ret != -1 );
struct epoll_event events[MAX_EVENT_NUMBER];
int epollfd = epoll_create(5);
assert( epollfd != -1 );
addfd( epollfd, listenfd, 0 );
while (1)
{
int ret = epoll_wait(epollfd, events, MAX_EVENT_NUMBER, -1);
if (ret < 0)
{
printf("epoll failure\n");
break;
}
for(int i = 0; i < ret; i++)
{
int sockfd = events[i].data.fd;
if (sockfd == listenfd)
{
struct sockaddr_in client_address;
socklen_t client_addrlength = sizeof(client_address);
int connfd = accept(listenfd, (struct sockaddr *) &client_address, &client_addrlength);
addfd(epollfd, connfd, 1);
}
else if (events[i].events & EPOLLIN)
{
pthread_t thread;
struct fds fds_for_new_worker;
fds_for_new_worker.epollfd = epollfd;
fds_for_new_worker.sockfd = sockfd;
pthread_create(&thread, NULL, worker, (void *) &fds_for_new_worker);
}
else
{
printf("something else happened \n");
}
}
}
close(listenfd);
return 0;
}