Linux为多路复用IO提供了较多的接口，有select(),pselect(),poll()的方式，继承自BSD和System V 两大派系。
select模型比较简单，“轮询”检测fd_set的状态，然后再采取相应的措施。
信号驱动模型有必要仔细研究一下，一般有如下步骤：
- 设置安装函数，信号是驱动信号是SIGIO（最好使用sigaction的方式，方便设置flag为SA_RESTART，因为client中读取终端的syscall可能会被中断，有必要重启。当然，使用signal()的方式然后再对errno进行判断是否为ETNTR，自行重启也是一种方法。但是signal()的可移植性问题，我强烈不建议使用）
- 设置fd的属主。F_SETOWN,要接受信号的进程，fcntl().
- 设置fd的异步标志。小细节，用'|'添加，要把之前的状态保留，也就是先F_GETFL再F_SETFL。（注意：在打开文件open()时设置标识O_ASYNC无实质效果）
On Linux, specifying the O_ASYNC
flag when calling open() has no effect. To enable signal-driven I/O, we must
instead set this flag using the fcntl() F_SETFL operation (Section 5.3).————《the linux programming interface》 4.3.1 - sigaction()安装
具体进下文client例程。
写了一个聊天程序的demo，把这两种技术都使用了。服务端采取多路复用的IO方式，代替多进（线）程的模型，客服端采取的是信号驱动，如下：
容易产生bug的地方都写注释里边了。
serv.c
#include <sys/select.h>
#include <sys/socket.h>
#include <netinet/in.h>
#include <arpa/inet.h>
#include <unistd.h>
#include <fcntl.h>
#include <stdlib.h>
#include <string.h>
#include <stdio.h>
#include <signal.h> void endServ(int sig)
{
printf("Server ended!\n");
exit(-);
} int main()
{
// signal
struct sigaction act;
sigemptyset(&act.sa_mask);
act.sa_handler = endServ;
act.sa_flags = ;
sigaction(SIGINT,&act,);
printf("This server is started,enter CTRL+C to end. \n"); int servfd = socket(AF_INET,SOCK_STREAM,);
if(servfd == -) {
printf("something wrong with socket():%m\n");
exit(-);
} struct sockaddr_in addr;
addr.sin_family = AF_INET;
addr.sin_port = htons();
inet_pton(AF_INET,"127.0.0.1",&addr.sin_addr); if (bind(servfd,(struct sockaddr*)&addr,sizeof(addr)) == -) {
printf("Some thing wrong with bind():%m\n");
exit(-);
}
printf("Bind success!\n"); listen(servfd,); int numbers=;//how many clients has accepted
fd_set fs;
FD_ZERO(&fs);
int client_fd[];
int i;
for(i=;i<;++i)
{
client_fd[i] = -;
} int maxfd=;
char buf[];
for(;;)
{
maxfd =;
FD_ZERO(&fs);
FD_SET(servfd,&fs);
maxfd = maxfd>servfd?maxfd:servfd;
for(i=;i<numbers;++i)
{
if(client_fd[i] != -) {
FD_SET(client_fd[i],&fs);
maxfd = maxfd>client_fd[i]?maxfd:client_fd[i];
}
} int res = select(maxfd+,&fs,,,);
if(res == -) {
printf("Something wrong with select():%m\n");
exit(-);
} if(FD_ISSET(servfd,&fs) && numbers < ) {
printf("New client!\n");
client_fd[numbers] = accept(servfd,,);
numbers++;
} for(i=;i<numbers;++i)
{
bzero(buf,sizeof(buf));
//judge if client_fd[i] equal -1 is necessary
//if a client quited,next time the program will
//have a segment default
//also it should be in the front.
if(client_fd[i] != - && FD_ISSET(client_fd[i],&fs))
{
res = recv(client_fd[i],buf,sizeof(buf),);
if(res == ) {
printf("A client quit\n");
close(client_fd[i]);
FD_CLR(client_fd[i],&fs);
client_fd[i] = -;
}
else if(res == -) {
printf("Something wrong with net.\n");
exit(-);
}
else {
int j;
for(j=;j<numbers;++j)
{
if(client_fd[j] != -)
send(client_fd[j],buf,sizeof(buf),);
}
}
}
}
}
}
client：
#include <signal.h>
#include <unistd.h>
#include <sys/socket.h>
#include <fcntl.h>
#include <arpa/inet.h>
#include <netinet/in.h>
#include <stdio.h>
#include <string.h>
#include <stdlib.h>
#include <errno.h> static int fd; void show(int sig)
{
char buf[] = {};
int n = read(fd,buf,sizeof(buf));
buf[n] = ;
write(,"MSG:",strlen("MSG:"));
write(,buf,strlen(buf));
} int main()
{
struct sigaction act;
sigemptyset(&act.sa_mask);
act.sa_handler = show;
//This is necessary,in last loop read() counld be interrupt;
act.sa_flags = SA_RESTART;
sigaction(SIGIO,&act,); fd = socket(AF_INET,SOCK_STREAM,);
if(fd == -) {
printf("Cannot get socketfd!:%m\n");
exit(-);
} struct sockaddr_in addr;
addr.sin_family = AF_INET;
addr.sin_port = htons();
inet_pton(AF_INET,"127.0.0.1",&addr.sin_addr); if(connect(fd,(struct sockaddr*)&addr,sizeof(addr)) != -)
printf("connect success!\n");
else
exit(-); int flag = fcntl(fd,F_GETFL);
flag |= O_ASYNC;
fcntl(fd,F_SETFL,flag);
fcntl(fd,F_SETOWN,getpid()); char buffer[]={};
for(;;)
{
int n = read(,buffer,sizeof(buffer));
if(n==-)
break;
send(fd,buffer,n,);
} write(,"Closed.",strlen("Closed."));
}