[转] 3个学习Socket编程的简单例子：TCP Server/Client, Select

    以前都是采用ACE的编写网络应用，最近由于工作需要，需要直接只用socket接口编写CS的代码，重新学习这方面的知识，给出自己所用到的3个简单例子，都是拷贝别人的程序。如果你能完全理解这3个例子，估计socket编程就已经基本入门了。

      建议：1) 多多查查所用到的网络接口; 2) 最好有一本书，如UNIX环境高级编程，UNIX网络编程，可查询；3) 可以直接使用书上的例子更好。

 http://blog.csdn.net/zhenjing/article/details/4770490

TCP Client代码：

#include   <sys/stat.h>   
#include   <sys/types.h>   
#include   <sys/socket.h>   
#include   <stdio.h>   
#include   <malloc.h>   
#include   <netdb.h>   
#include   <fcntl.h>
#include   <unistd.h>
#include   <netinet/in.h>
#include   <arpa/inet.h>
#define    RES_LENGTH  10240 //接受字符的最大长度
int     connect_socket(char * server,int serverPort);
int     send_msg(int sockfd,char * sendBuff);
char *  recv_msg(int sockfd);
int     close_socket(int sockfd);
int main(int argc, char ** argv)
{
    int   sockfd=0;
    char  sendMsg[30]="abc.org/r/n/r";
    char* res;
    int   port = 4242;
    char  ip[128] = {0};
    strncpy(ip, "127.0.0.1", 128);
    if(argc > 2)
    {
        strncpy(ip, argv[1], 128);
        port = atoi(argv[2]);
        printf("Input IP: %s, port : %d/n", ip, port);
    }
    else if(argc > 1)
    {    
        port = atoi(argv[1]);
        printf("Input port : %d/n", port);
    }
    sockfd=connect_socket(ip, port);
    
    send_msg(sockfd,sendMsg);
    /* res=recv_msg(sockfd); */
    
    printf("return from recv function/n");
    printf(res);
    free(res);
    close_socket(sockfd);
    return 0;
}
/************************************************************
 * 连接SOCKET服务器，如果出错返回-1，否则返回socket处理代码
 * server：服务器地址(域名或者IP),serverport：端口
 * ********************************************************/
int    connect_socket(char * server,int serverPort){
    int    sockfd=0;
    struct    sockaddr_in    addr;
    struct    hostent        * phost;
    //向系统注册，通知系统建立一个通信端口
    //AF_INET表示使用IPv4协议
    //SOCK_STREAM表示使用TCP协议
    if((sockfd=socket(AF_INET,SOCK_STREAM,0))<0){
        herror("Init socket error!");
        return -1;
    }
    bzero(&addr,sizeof(addr));
    addr.sin_family = AF_INET;
    addr.sin_port = htons(serverPort);
    addr.sin_addr.s_addr = inet_addr(server);//按IP初始化
    
    if(addr.sin_addr.s_addr == INADDR_NONE){//如果输入的是域名*******************
        phost = (struct hostent*)gethostbyname(server);
        if(phost==NULL){
            herror("Init socket s_addr error!");
            return -1;
        }
        addr.sin_addr.s_addr =((struct in_addr*)phost->h_addr)->s_addr;
    }
    if(connect(sockfd,(struct sockaddr*)&addr, sizeof(addr))<0)
    {
        perror("Connect server fail!");
        return -1; //0表示成功，-1表示失败
    }
    else
        return sockfd;
}
/**************************************************************
 * 发送消息，如果出错返回-1，否则返回发送的字符长度
 * sockfd：socket标识，sendBuff：发送的字符串
 * *********************************************************/
int send_msg(int sockfd,char * sendBuff)
{
    int sendSize=0;
    if((sendSize=send(sockfd,sendBuff,strlen(sendBuff),0))<=0){
        herror("Send msg error!");
        return -1;
    }else
        return sendSize;
}
/****************************************************************
 *接受消息，如果出错返回NULL，否则返回接受字符串的指针(动态分配，注意释放)
 *sockfd：socket标识
 * *********************************************************/
char* recv_msg(int sockfd){
    char * response;
    int  flag=0,recLenth=0;
    response=(char *)malloc(RES_LENGTH);
    memset(response,0,RES_LENGTH);
    
    for(flag=0;;)
    {
        printf("======recv data:/n");
        if(( recLenth=recv(sockfd,response+flag,RES_LENGTH-flag,0))==-1 )
        {
            free(response);
            printf("Return value : %d/n", recLenth);
            perror("Recv msg error : ");
            return NULL;
        }
        else if(recLenth==0)
            break;
        else
        {
            printf("%d char recieved data : %s./n", recLenth, response+flag);
            flag+=recLenth;
            recLenth=0;
        }
    }
    printf("Return value : %d/n", recLenth);
    response[flag]='/0';
    return response;
}
/**************************************************
 *关闭连接
 * **********************************************/
int close_socket(int sockfd)
{
    close(sockfd);
    return 0;
}

TCP SERVER

#include <unistd.h> /* fork, close */
#include <stdlib.h> /* exit */
#include <string.h> /* strlen */
#include <stdio.h> /* perror, fdopen, fgets */
#include <sys/socket.h>
#include <sys/wait.h> /* waitpid */
#include <netdb.h> /* getaddrinfo */
#define die(msg) do { perror(msg); exit(EXIT_FAILURE); } while (0)
#define PORT "4242"
#define NUM_CHILDREN 3
#define MAXLEN 1024
int readline(int fd, char *buf, int maxlen); // forward declaration
int recvdata(int fd, char *buf, int maxlen); // forward declaration
int main(int argc, char** argv)
{
    int i, n, sockfd, clientfd;
    int yes = 1;    // used in setsockopt(2)
    struct addrinfo *ai;
    struct sockaddr_in *client;
    socklen_t client_t;
    pid_t cpid;     // child pid
    char line[MAXLEN];
    char cpid_s[32];
    char welcome[32];
    /* Create a socket and get its file descriptor -- socket(2) */
    sockfd = socket(AF_INET, SOCK_STREAM, 0);
    if (sockfd == -1) {
        die("Couldn't create a socket");
    }
    /* Prevents those dreaded "Address already in use" errors */
    if (setsockopt(sockfd, SOL_SOCKET, SO_REUSEADDR, (const void *)&yes, sizeof(int)) == -1) {
        die("Couldn't setsockopt");
    }
    /* Fill the address info struct (host + port) -- getaddrinfo(3) */
    if (getaddrinfo(NULL, PORT, NULL, &ai) != 0) {   // get localhost 
        die("Couldn't get address");
    }
    /* Assign address to this socket's fd */
    if (bind(sockfd, ai->ai_addr, ai->ai_addrlen) != 0) {  // only bind on localhost ip
        die("Couldn't bind socket to address");
    }
    /* Free the memory used by our address info struct */
    freeaddrinfo(ai);
    /* Mark this socket as able to accept incoming connections */
    /* printf("Process %d Listening/n", getpid()); */
    if (listen(sockfd, 10) == -1) {
        die("Couldn't make socket listen");
    }
    printf("One new connection is coming!/n");
    /* Fork you some child processes. */
    for (i = 0; i < NUM_CHILDREN; i++) {
        cpid = fork();
        if (cpid == -1) {
            die("Couldn't fork");
        }
        if (cpid == 0) { // We're in the child ...
            for (;;) { // Run forever ...
                /* Necessary initialization for accept(2) */
                client_t = sizeof client;
                /* Blocks! */
                printf("Waiting new connection!/n");
                clientfd = accept(sockfd, (struct sockaddr *)&client, &client_t);
                if (clientfd == -1) {
                    die("Couldn't accept a connection");
                }
                /* Send a welcome message/prompt */
                bzero(cpid_s, 32);
                bzero(welcome, 32);
                sprintf(cpid_s, "%d", getpid());
                sprintf(welcome, "Child %s echo> ", cpid_s);
                send(clientfd, welcome, strlen(welcome), 0);
                /* Read a line from the client socket ... */
                /* n = readline(clientfd, line, MAXLEN);
                if (n == -1) {
                    die("Couldn't read line from connection");
                }                                                 */
            
                n = recvdata(clientfd, line, MAXLEN);
                printf("recieve data: %s", line);
                /* ... and echo it back */
                send(clientfd, line, n, 0);
                /* Clean up the client socket */
                close(clientfd);
                printf("Close client socket./n");
            }
        }
    }
    /* Sit back and wait for all child processes to exit */
    while (waitpid(-1, NULL, 0) > 0);
    /* Close up our socket */
    close(sockfd);
    printf("Close server socket./n");
    return 0;
}
  
/**
 * Simple utility function that reads a line from a file descriptor fd,
 * up to maxlen bytes -- ripped from Unix Network Programming, Stevens.
 */
int readline(int fd, char *buf, int maxlen)
{
    int n, rc;
    char c;
    for (n = 1; n < maxlen; n++) {
        if ((rc = read(fd, &c, 1)) == 1) {
            *buf++ = c;
            if (c == '/n')
                break;
        } else if (rc == 0) {
            if (n == 1)
                return 0; // EOF, no data read
            else
                break; // EOF, read some data
        } else
            return -1; // error
    }
    *buf = '/0'; // null-terminate
    
    return n;
}
int recvdata(int fd, char *buf, int maxlen)
{
    return recv(fd, buf, maxlen, 0);
}

采用Select的TCP Server:

#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <errno.h>
#include <string.h>
#include <sys/types.h>
#include <sys/socket.h>
#include <netinet/in.h>
#include <arpa/inet.h>
#define MYPORT 1234    // the port users will be connecting to
#define BACKLOG 5     // how many pending connections queue will hold
#define BUF_SIZE 1024
int fd_A[BACKLOG];    // accepted connection fd
int conn_amount;      // current connection amount
void showclient()
{
    int i;
    printf("client amount: %d/n", conn_amount);
    for (i = 0; i < BACKLOG; i++) {
        printf("[%d]:%d  ", i, fd_A[i]);
    }
    printf("/n/n");
}
int main(void)
{
    int sock_fd, new_fd;             // listen on sock_fd, new connection on new_fd
    struct sockaddr_in server_addr;  // server address information
    struct sockaddr_in client_addr;  // connector's address information
    socklen_t sin_size;
    int yes = 1;
    char buf[BUF_SIZE];
    int ret;
    int i;
    if ((sock_fd = socket(AF_INET, SOCK_STREAM, 0)) == -1) {
        perror("socket");
        exit(1);
    }
    if (setsockopt(sock_fd, SOL_SOCKET, SO_REUSEADDR, &yes, sizeof(int)) == -1) {
        perror("setsockopt");
        exit(1);
    }
    server_addr.sin_family = AF_INET;         // host byte order
    server_addr.sin_port = htons(MYPORT);     // short, network byte order
    server_addr.sin_addr.s_addr = INADDR_ANY; // automatically fill with my IP
    memset(server_addr.sin_zero, '/0', sizeof(server_addr.sin_zero));
    if (bind(sock_fd, (struct sockaddr *)&server_addr, sizeof(server_addr)) == -1) {
        perror("bind");
        exit(1);
    }
    if (listen(sock_fd, BACKLOG) == -1) {
        perror("listen");
        exit(1);
    }
    printf("listen port %d/n", MYPORT);
    fd_set fdsr;
    int maxsock;
    struct timeval tv;
    conn_amount = 0;
    sin_size = sizeof(client_addr);
    maxsock = sock_fd;
    while (1) 
    {
        // initialize file descriptor set
        FD_ZERO(&fdsr);
        FD_SET(sock_fd, &fdsr);  // add fd
        // timeout setting
        tv.tv_sec = 30;
        tv.tv_usec = 0;
        // add active connection to fd set
        for (i = 0; i < BACKLOG; i++) {
            if (fd_A[i] != 0) {
                FD_SET(fd_A[i], &fdsr);
            }
        }
        ret = select(maxsock + 1, &fdsr, NULL, NULL, &tv);
        if (ret < 0) {          // error
            perror("select");
            break;
        } else if (ret == 0) {  // time out
            printf("timeout/n");
            continue;
        }
        // check every fd in the set
        for (i = 0; i < conn_amount; i++) 
        {
            if (FD_ISSET(fd_A[i], &fdsr)) // check which fd is ready
            {
                ret = recv(fd_A[i], buf, sizeof(buf), 0);
                if (ret <= 0) 
                {        // client close
                    printf("ret : %d and client[%d] close/n", ret, i);
                    close(fd_A[i]);
                    FD_CLR(fd_A[i], &fdsr);  // delete fd 
                    fd_A[i] = 0;
                    conn_amount--;
                }
                else 
                {        // receive data
                    if (ret < BUF_SIZE)
                        memset(&buf[ret], '/0', 1); // add NULL('/0')
                    printf("client[%d] send:%s/n", i, buf);
                }
            }
        }
        // check whether a new connection comes
        if (FD_ISSET(sock_fd, &fdsr))  // accept new connection 
        {
            new_fd = accept(sock_fd, (struct sockaddr *)&client_addr, &sin_size);
            if (new_fd <= 0) 
            {
                perror("accept");
                continue;
            }
            // add to fd queue
            if (conn_amount < BACKLOG) 
            {
                fd_A[conn_amount++] = new_fd;
                printf("new connection client[%d] %s:%d/n", conn_amount,
                        inet_ntoa(client_addr.sin_addr), ntohs(client_addr.sin_port));
                if (new_fd > maxsock)  // update the maxsock fd for select function
                    maxsock = new_fd;
            }
            else 
            {
                printf("max connections arrive, exit/n");
                send(new_fd, "bye", 4, 0);
                close(new_fd);
                break;   
            }
        }
        showclient();
    }
    // close other connections
    for (i = 0; i < BACKLOG; i++) 
    {
        if (fd_A[i] != 0) 
        {
            close(fd_A[i]);
        }
    }
    exit(0);
}