linux非阻塞socket教程
http://blog.csdn.net/favormm/article/details/5296621
本文并非解释什么是非阻塞socket，也不是介绍socket API的用法， 取而代替的是让你感受实际工作中的代码编写。虽然很简陋，但你可以通过man手册与其它资源非富你的代码。请注意本教程所说的主题，如果细说，内容可以达到一本书内容，你会发现本教程很有用。
本教程内容如下：
1. 改变一个阻塞的socket为非阻塞模式。
2. select模型
3. FD宏
4. 读写函数
5. 写一个非阻塞socket代码片
6. 整个代码
7.下一步
如果你在此有许多问题，那么恭喜你，在初级阶段，任何人都没有剥夺你发现问题的权利。关于你所发现的问题，请不要犹豫email我。
你可以自由发表本教程到任何WWW或FTP网部，但无论如何也要保持原教程的原型。这样我将会非常感谢你。
1.改变一个阻塞的socket为非阻塞模式
简单的几行代码就可以创建一个socket 并连接，看起来如此简单。（你可以自己加入错误处理）
- s = socket(AF_INET, SOCK_STREAM, 0);
- memset(&sin, 0, sizeof(struct sockaddr_in));
- sin.sin_family = AF_INET;
- sin.sin_port = htons(port);
- sin.sin_addr.s_addr = inet_addr(hstname);
- if(sin.sin_addr.s_addr == INADDR_NONE) {
- connect(s, (struct sockaddr *)&sin, sizeof(sin))
s = socket(AF_INET, SOCK_STREAM, 0); memset(&sin, 0, sizeof(struct sockaddr_in)); sin.sin_family = AF_INET; sin.sin_port = htons(port); sin.sin_addr.s_addr = inet_addr(hstname); if(sin.sin_addr.s_addr == INADDR_NONE) { connect(s, (struct sockaddr *)&sin, sizeof(sin))
有很多种方法可以设置socket为非阻塞模式， 我在unix下常用的方法如下：
- int x;
- x=fcntl(s,F_GETFL,0);
- fcntl(s,F_SETFL,x | O_NONBLOCK);
int x; x=fcntl(s,F_GETFL,0); fcntl(s,F_SETFL,x | O_NONBLOCK);
到现在为此， 这个socket已为非阻塞模式了，我们可以把焦点放在如何用它了。 但是在接着写代码之前，我们要看看我们将要用到的命令。
2.选择模型
select这个方法用来检测一个socket是否有数据到来或是是否有准备好的数据要发送。声明如下:
- select(s, &read_flags, &write_flags, &exec_flags, timer);
select(s, &read_flags, &write_flags, &exec_flags, timer);
s socket的句柄
read_flags 读描述字集合。检查socket上是否有数据可读。
write_flags 写描述字集合。检查socket上是否已有数据可发送。
exec_flags 错误描述字集合。（本教程这儿不介绍）
timer 等待某个条件为真时超时时间。
在本教程中，我们将如下用：
- select(s, &read_flags, &write_flags, NULL, timer);
select(s, &read_flags, &write_flags, NULL, timer);
exec_flags参数设为null，因为在我们的程序中我们不需要关心exec事件。（解释它，超出了本教程的范围）
3.FD宏
在select方法中的描述字集合是用来检测socket上发生的读取事件的，它用法如下：
- FD_ZERO(s, &write_flags) sets all associated flags
- in the socket to 0
- FD_SET(s, &write_flags) used to set a socket for checking
- FD_CLR (s, &write_flags) used to clear a socket from being checked
- FD_ISSET(s, &write_flags) used to query as to if the socket is ready
- for reading or writing.
FD_ZERO(s, &write_flags) sets all associated flags in the socket to 0 FD_SET(s, &write_flags) used to set a socket for checking FD_CLR (s, &write_flags) used to clear a socket from being checked FD_ISSET(s, &write_flags) used to query as to if the socket is ready for reading or writing.
如何用，我们在后面的代码中展示。
4.读取方法
你应知道如何用下面的两个方法，但是在非阻塞模式下，它们的用法有一点点不同。
- write(s,buffer,sizeof(buffer)) send the text in "buffer"
- read(s,buffer,sizeof(buffer)) read available data into "buffer"
write(s,buffer,sizeof(buffer)) send the text in "buffer" read(s,buffer,sizeof(buffer)) read available data into "buffer"
当一个socket用非阻塞模式时，当调用这两个方法的时候，它们将立即返回，比如，如果没有数据的时候，它们将不会阻塞等待数据，而是返回一个错误。从现在开始，我们就要看代码了。
5.写一个非阻塞socket代码片
首先声明要用到的变量：
- fd_set read_flags,write_flags; // the flag sets to be used
- struct timeval waitd; // the max wait time for an event
- char buffer[8196]; // input holding buffer
- int stat; // holds return value for select();
fd_set read_flags,write_flags; // the flag sets to be used struct timeval waitd; // the max wait time for an event char buffer[8196]; // input holding buffer int stat; // holds return value for select();
我们程序运行的大部份时间都花费在不断调用select(它将花费我们大部份CPU时间)，至到有数据准备好读或取。此时，timer就体现了它的意义。它决定select将等待多久。下面就是用法，请仔细分析：
- // Insert Code to create a socket
- while(1) // put program in an infinite loop of reading and writing data
- {
- waitd.tv_sec = 1; // Make select wait up to 1 second for data
- waitd.tv_usec = 0; // and 0 milliseconds.
- FD_ZERO(&read_flags); // Zero the flags ready for using
- FD_ZERO(&write_flags);
- // Set the sockets read flag, so when select is called it examines
- // the read status of available data.
- FD_SET(thefd, &read_flags);
- // If there is data in the output buffer to be sent then we
- // need to also set the write flag to check the write status
- // of the socket
- if(strlen(outbuff)!=0) FD_SET(thefd, &write_flags);
- // Now call select
- stat=select(s+1, &read_flags,&write_flags,(fd_set*)0,&waitv);
- if(stat < 0) { // If select breaks then pause for 5 seconds
- sleep(5); // then continue
- continue;
- }
- // Now select will have modified the flag sets to tell us
- // what actions can be performed
- // Check if data is available to read
- if (FD_ISSET(thefd, &read_flags)) {
- FD_CLR(thefd, &read_flags);
- // here is where you use the read().
- // If read returns an error then the socket
- // must be dead so you must close it.
- }
- //Check if the socket is prepared to accept data
- if (FD_ISSET(thefd, &write_flags)) {
- FD_CLR(thefd, &write_flags);
- // this means the socket is ready for you to use write()
- }
- // Now here you can put in any of the precedures that you want
- // to happen every 1 second or so.
- // now the loop repeats over again
// Insert Code to create a socket while(1) // put program in an infinite loop of reading and writing data { waitd.tv_sec = 1; // Make select wait up to 1 second for data waitd.tv_usec = 0; // and 0 milliseconds. FD_ZERO(&read_flags); // Zero the flags ready for using FD_ZERO(&write_flags); // Set the sockets read flag, so when select is called it examines // the read status of available data. FD_SET(thefd, &read_flags); // If there is data in the output buffer to be sent then we // need to also set the write flag to check the write status // of the socket if(strlen(outbuff)!=0) FD_SET(thefd, &write_flags); // Now call select stat=select(s+1, &read_flags,&write_flags,(fd_set*)0,&waitv); if(stat < 0) { // If select breaks then pause for 5 seconds sleep(5); // then continue continue; } // Now select will have modified the flag sets to tell us // what actions can be performed // Check if data is available to read if (FD_ISSET(thefd, &read_flags)) { FD_CLR(thefd, &read_flags); // here is where you use the read(). // If read returns an error then the socket // must be dead so you must close it. } //Check if the socket is prepared to accept data if (FD_ISSET(thefd, &write_flags)) { FD_CLR(thefd, &write_flags); // this means the socket is ready for you to use write() } // Now here you can put in any of the precedures that you want // to happen every 1 second or so. // now the loop repeats over again
补充：请确保只有在你有数据发送的情况下才设置write_flag这个描述字集合，因为socket一量创建总是可写的。也就是说，如果你设置了这个参数，select将不会等待，而是马上返回并一直循环，它将抢占CPU99%的利用率，这是不允许的。
6.整个代码
最后利用我们所学，写一个简单的客户端。当然用非阻塞模式写一个客户端有点大采小用，这儿我们只是为了展示用法。更多示例请看第7节内容。
- #include <sys/types.h>
- #include <sys/socket.h>
- #include <sys/time.h>
- #include <netinet/in.h>
- #include <netdb.h>
- #include <stdio.h>
- #include <string.h>
- #include <unistd.h>
- #include <stdlib.h>
- #include <fcntl.h>
- // this routine simply converts the address into an
- // internet ip
- unsigned long name_resolve(char *host_name)
- {
- struct in_addr addr;
- struct hostent *host_ent;
- if((addr.s_addr=inet_addr(host_name))==(unsigned)-1) {
- host_ent=gethostbyname(host_name);
- if(host_ent==NULL) return(-1);
- memcpy(host_ent->h_addr, (char *)&addr.s_addr, host_ent->h_length);
- }
- return (addr.s_addr);
- }
- // The connect routine including the command to set
- // the socket non-blocking.
- int doconnect(char *address, int port)
- {
- int x,s;
- struct sockaddr_in sin;
- s=socket(AF_INET, SOCK_STREAM, 0);
- x=fcntl(s,F_GETFL,0); // Get socket flags
- fcntl(s,F_SETFL,x | O_NONBLOCK); // Add non-blocking flag
- memset(&sin, 0, sizeof(struct sockaddr_in));
- sin.sin_family=AF_INET;
- sin.sin_port=htons(port);
- sin.sin_addr.s_addr=name_resolve(address);
- if(sin.sin_addr.s_addr==NULL) return(-1);
- printf("ip: %s/n",inet_ntoa(sin.sin_addr));
- x=connect(s, (struct sockaddr *)&sin, sizeof(sin));
- if(x<0) return(-1);
- return(s);
- }
- int main (void)
- {
- fd_set read_flags,write_flags; // you know what these are
- struct timeval waitd;
- int thefd; // The socket
- char outbuff[512]; // Buffer to hold outgoing data
- char inbuff[512]; // Buffer to read incoming data into
- int err; // holds return values
- memset(&outbuff,0,sizeof(outbuff)); // memset used for portability
- thefd=doconnect("203.1.1.1",79); // Connect to the finger port
- if(thefd==-1) {
- printf("Could not connect to finger server/n");
- exit(0);
- }
- strcat(outbuff,"jarjam/n"); //Add the string jarjam to the output
- //buffer
- while(1) {
- waitd.tv_sec = 1; // Make select wait up to 1 second for data
- waitd.tv_usec = 0; // and 0 milliseconds.
- FD_ZERO(&read_flags); // Zero the flags ready for using
- FD_ZERO(&write_flags);
- FD_SET(thefd, &read_flags);
- if(strlen(outbuff)!=0) FD_SET(thefd, &write_flags);
- err=select(thefd+1, &read_flags,&write_flags,
- (fd_set*)0,&waitd);
- if(err < 0) continue;
- if(FD_ISSET(thefd, &read_flags)) { //Socket ready for reading
- FD_CLR(thefd, &read_flags);
- memset(&inbuff,0,sizeof(inbuff));
- if (read(thefd, inbuff, sizeof(inbuff)-1) <= 0) {
- close(thefd);
- break;
- }
- else printf("%s",inbuff);
- }
- if(FD_ISSET(thefd, &write_flags)) { //Socket ready for writing
- FD_CLR(thefd, &write_flags);
- write(thefd,outbuff,strlen(outbuff));
- memset(&outbuff,0,sizeof(outbuff));
- }
- // now the loop repeats over again
- }
- }
#include <sys/types.h> #include <sys/socket.h> #include <sys/time.h> #include <netinet/in.h> #include <netdb.h> #include <stdio.h> #include <string.h> #include <unistd.h> #include <stdlib.h> #include <fcntl.h> // this routine simply converts the address into an // internet ip unsigned long name_resolve(char *host_name) { struct in_addr addr; struct hostent *host_ent; if((addr.s_addr=inet_addr(host_name))==(unsigned)-1) { host_ent=gethostbyname(host_name); if(host_ent==NULL) return(-1); memcpy(host_ent->h_addr, (char *)&addr.s_addr, host_ent->h_length); } return (addr.s_addr); } // The connect routine including the command to set // the socket non-blocking. int doconnect(char *address, int port) { int x,s; struct sockaddr_in sin; s=socket(AF_INET, SOCK_STREAM, 0); x=fcntl(s,F_GETFL,0); // Get socket flags fcntl(s,F_SETFL,x | O_NONBLOCK); // Add non-blocking flag memset(&sin, 0, sizeof(struct sockaddr_in)); sin.sin_family=AF_INET; sin.sin_port=htons(port); sin.sin_addr.s_addr=name_resolve(address); if(sin.sin_addr.s_addr==NULL) return(-1); printf("ip: %s/n",inet_ntoa(sin.sin_addr)); x=connect(s, (struct sockaddr *)&sin, sizeof(sin)); if(x<0) return(-1); return(s); } int main (void) { fd_set read_flags,write_flags; // you know what these are struct timeval waitd; int thefd; // The socket char outbuff[512]; // Buffer to hold outgoing data char inbuff[512]; // Buffer to read incoming data into int err; // holds return values memset(&outbuff,0,sizeof(outbuff)); // memset used for portability thefd=doconnect("203.1.1.1",79); // Connect to the finger port if(thefd==-1) { printf("Could not connect to finger server/n"); exit(0); } strcat(outbuff,"jarjam/n"); //Add the string jarjam to the output //buffer while(1) { waitd.tv_sec = 1; // Make select wait up to 1 second for data waitd.tv_usec = 0; // and 0 milliseconds. FD_ZERO(&read_flags); // Zero the flags ready for using FD_ZERO(&write_flags); FD_SET(thefd, &read_flags); if(strlen(outbuff)!=0) FD_SET(thefd, &write_flags); err=select(thefd+1, &read_flags,&write_flags, (fd_set*)0,&waitd); if(err < 0) continue; if(FD_ISSET(thefd, &read_flags)) { //Socket ready for reading FD_CLR(thefd, &read_flags); memset(&inbuff,0,sizeof(inbuff)); if (read(thefd, inbuff, sizeof(inbuff)-1) <= 0) { close(thefd); break; } else printf("%s",inbuff); } if(FD_ISSET(thefd, &write_flags)) { //Socket ready for writing FD_CLR(thefd, &write_flags); write(thefd,outbuff,strlen(outbuff)); memset(&outbuff,0,sizeof(outbuff)); } // now the loop repeats over again } }
7.下一步
其它更多的示例代码从此教程中分离，以zip文件的方式给出。为了更好的理解所学， 你最好参考一些结构更复杂，技术更强的代码：
http://users.cybernex.net.au/jj/sock.zip