高性能网络编程 - select系统调用

IO复用使得程序可以同一时候监听多个文件描写叙述符，比方client须要同一时候处理用户输入和网络连接，server端须要同一时候处理监听套接字和连接套接字，select系统调用可以使得我们监听自己感兴趣描写叙述符。可读，可写，异常等事件。select能处理的异常仅仅有带外数据。能同一时候处理描写叙述符的数量受限于FD_SETSIZE的大小（一般1024）。以下这个程序展示了它的一般使用方法。

#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <string.h>
#include <libgen.h> //basename
#include <arpa/inet.h>
#include <sys/types.h>
#include <sys/socket.h>
#include <assert.h>
#include <errno.h>

int main(int argc, char *argv[]){
int i, maxi;
char buf[1024];
fd_set read_fds, all_read_fds;
fd_set exception_fds, all_exception_fds;
int nready, connectfd[FD_SETSIZE];
int connfd,sockfd,maxfd;

struct sockaddr_in client_address;
socklen_t client_addrlen = sizeof(client_address);

if(argc <= 2){
printf("usage: %s ip_address port_number\n", basename(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);

maxfd = listenfd;
maxi = -1;
for(i= 0; i < FD_SETSIZE; i++)
connectfd[i] = -1;
FD_ZERO(&all_read_fds);
FD_ZERO(&all_exception_fds);
FD_SET(listenfd, &all_read_fds);
//do not care about the OOB of listen socket

while(1){
read_fds = all_read_fds;
exception_fds = all_exception_fds;
memset(buf, 0, sizeof(buf));
nready = select(maxfd + 1, &read_fds, NULL, &exception_fds, NULL);
//new client connection coming
if(FD_ISSET(listenfd, &read_fds)){
connfd = accept(listenfd, (struct sockaddr *)&client_address, &client_addrlen);
if(connfd < 0){
perror("accept failed\n");
exit(-1);
}

for(i=0; i < FD_SETSIZE; i++)
if(connectfd[i] < 0){
connectfd[i] = connfd;
break;
}
if( i == FD_SETSIZE){
printf("too many clients..\n");
exit(-1);
}
// add new descriptor to the read set and exception set
FD_SET(connfd, &all_read_fds);
FD_SET(connfd, &all_exception_fds);
if(connfd > maxfd)
maxfd = connfd;
if(i > maxi)
maxi = i;// update the max index in connectfd[]
//we complete the listen socktet processing
if(--nready <= 0)
continue;
}
for(i = 0; i <= maxi; i++){
//check all the connected clients for data
if((sockfd = connectfd[i]) < 0)
continue;
if(FD_ISSET(sockfd, &read_fds)){
ret = recv(sockfd, buf, sizeof(buf) - 1, 0);
if(ret <= 0)
break;//connection closed
// process logic
printf("get %d bytes of normal data:%s\n", ret, buf);
//we complete the listen socktet processing
if(--nready <= 0)
continue;
}
for(i = 0; i <= maxi; i++){
//check all the connected clients for data
if((sockfd = connectfd[i]) < 0)
continue;
if(FD_ISSET(sockfd, &read_fds)){
ret = recv(sockfd, buf, sizeof(buf) - 1, 0);
if(ret <= 0)
break;//connection closed
// process logic
printf("get %d bytes of normal data:%s\n", ret, buf);
}else if(FD_ISSET(sockfd, &exception_fds)){
//handle exceptions
//get out of band data
ret = recv(connfd, buf, sizeof(buf) - 1, MSG_OOB);
if(ret <= 0)
break;
printf("get %d bytes of OOB data: %s", ret, buf);

}
FD_CLR(sockfd, &all_read_fds);
FD_CLR(sockfd, &all_exception_fds);
//after we deal with this client
close(sockfd);
if(--nready <= 0)
break;
}
}

close(listenfd);
return 0;
}

參考：《高性能server编程》和《UNP》