因为et模式需要循环读取，但是在读取过程中，如果有新的事件到达，很可能触发了其他线程来处理这个socket，那就乱了。
EPOLL_ONESHOT就是用来避免这种情况。注意在一个线程处理完一个socket的数据，也就是触发EAGAIN errno时候，就应该重置EPOLL_ONESHOT的flag，这时候，新到的事件，就可以重新进入触发流程了。
注：EPOLL_ONESHOT的原理其实是，每次触发事件之后，就将事件注册从fd上清除了，也就不会再被追踪到；下次需要用epoll_ctl的EPOLL_CTL_MOD来手动加上才行。
服务器代码如下：
#include <stdio.h> #include <stdlib.h> #include <unistd.h> #include <errno.h> #include <string.h> #include <fcntl.h> #include <assert.h> #include <sys/types.h> #include <sys/socket.h> #include <netinet/in.h> #include <arpa/inet.h> #include <sys/epoll.h> #include <pthread.h> #define MAX_EVENT_NUMBER 1024 #define BUFFER_SIZE 1024 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, bool oneshot) { 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) { 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 = ((fds*)arg)->sockfd; int epollfd = ((fds*)arg)->epollfd; pthread_t pid = pthread_self(); printf("start new thread %u to recv data on fd: %d ", pid, sockfd); char buf[BUFFER_SIZE]; memset(buf, ' ', BUFFER_SIZE); while(1) { int ret = recv(sockfd, buf, BUFFER_SIZE-1, 0); if (ret == 0) { close(sockfd); printf("foreiner closed the connection "); break; } else if (ret < 0) { if (errno == EAGAIN) { reset_oneshot(epollfd, sockfd); printf("EAGAIN read later "); break; } } else { buf[ret] = ' '; printf("thread %u get content: %s ", pid, buf); printf("thread %u about to sleep ", pid); sleep(5); printf("thread %u back from sleep ", pid); } } //printf("end thread %u receiving data on fd: %d ", pid, sockfd); } int main(int argc, char *argv[]) { if (argc <= 1) { printf("usage: %s port_number [ip_address] ", 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, false); while(1) { int ret = epoll_wait(epollfd, events, MAX_EVENT_NUMBER, -1); if (ret < 0) { printf("epoll failure "); break; } for (int i=0; i<ret; i++) { int sockfd = events[i].data.fd; if (sockfd == listenfd) { sockaddr_in client_address; socklen_t client_addrlength = sizeof(client_address); int connfd = accept(listenfd, (sockaddr*)&client_address, &client_addrlength); addfd(epollfd, connfd, true); printf("new connection is added to epollfd "); } else if (events[i].events & EPOLLIN) { pthread_t thread; fds fds_for_new_worker; fds_for_new_worker.epollfd = epollfd; fds_for_new_worker.sockfd = sockfd; // new thread pthread_create(&thread, NULL, worker, (void*)&fds_for_new_worker); } else { printf("something else happened "); } } } close(listenfd); return 0; }
以下是使用telnet客户端发送的文本，匀速敲入代码：
$telnet 127.0.0.1 12346 Trying 127.0.0.1... Connected to 127.0.0.1. Escape character is '^]'. hi1 hi2 hi3 hi4 hi5 hi6 hi7 hi8 Connection closed by foreign host.
以下是服务器的运行和输出：
$./epoll_oneshot 12346 new connection is added to epollfd start new thread 1734051584 to recv data on fd: 5 thread 1734051584 get content: hi1 thread 1734051584 about to sleep thread 1734051584 back from sleep thread 1734051584 get content: hi2 hi3 thread 1734051584 about to sleep thread 1734051584 back from sleep thread 1734051584 get content: hi4 hi5 hi6 thread 1734051584 about to sleep thread 1734051584 back from sleep thread 1734051584 get content: hi7 thread 1734051584 about to sleep thread 1734051584 back from sleep EAGAIN read later start new thread 1723561728 to recv data on fd: 5 thread 1723561728 get content: hi8 thread 1723561728 about to sleep thread 1723561728 back from sleep EAGAIN read later ^C
最后用Ctrl+C来结束服务器。
可以看出，在hi7文本和hi8文本之间，服务器收到了EAGAIN，表示读取告一段落。而之后的线程id也换成了新线程id。在hi7之前，因为每次服务器sleep结束之后，都还有没有读完的数据，所以线程id始终没有变，始终是同一个线程处理数据。
另外，要注意的是，EPOLL_ONESHOT既可以在et下也可以在lt下设置。效果是一样的，都是同一个fd上面的相同事件只会触发一次。上面是et的例子，对于lt，如果设置了EPOLL_ONESHOT，也是需要把数据读完，然后重置event。而不能像原始lt编程方式那样依赖于事件通知来读取数据了。