关于select函数

关于select函数：
其函数原型为：
int select(int n, fd_set *readfds, fd_set *writefds, fd_set *exceptfds, struct tim *timeout);
此函数的功能是由内核检测在timeout时间内，是否有readfds，writefds，exceptfds三个句柄集(file descriptors)里的某个句柄（file descriptor)的状态符合寻求，即readfds句柄集里有句柄可读或writefds句柄集里有可写或exceptfds句柄集里有例外发生，任何一个有变化函数就立即返回，返回值为timeout发生状态变化的句柄个数。
n是所有readfds，writefds，exceptfds三个句柄集(file descriptors)里编号最大值加1。比如：要检测两个socket句柄fd1和fd2在timeout时间内是否分别可读和可写就可以这样：
先把两个句柄集(file descriptors)清零：

       FD_ZERO (&readfds);
        FD_ZERO (&writefds);
然后把fd1加入读检测集：
        FD_SET (fd1, &readfds);
然后把fd2加入写检测集：
        FD_SET (fd2, &writefds);
再给timeout设置值，timeout是这样的一个结构：
              struct tim {
                  long    tv_sec;        
                  long    tv_usec;       
              };
你可以这样赋值：
        timeout.tv_sec=1;
        timeout.tv_uec=0;
表示检测在1秒钟内是否有句柄状态发生变化。
如果有句柄发生变化，就可以用FD_ISSET检测各个句柄，比如：
                FD_ISSET (fd1, &readfds);//检测是否fd1变成可读的了
                FD_ISSET (fd2, &writefds);//检测是否fd2变成可写的了
示意程序代码如下：

    fd1 = socket();//创建一个socket
    fd2 = socket();//创建一个socket
    while(1) {
        FD_ZERO (&readfds);
        FD_ZERO (&writefds);
        FD_SET (fd1, &readfds);
        FD_SET (fd2, &writefds);
        timeout.tv_sec=1;
        timeout.tv_uec=0;
        ret = select(fd1>fd2?(fd1+1):(fd2+1), &readfds, &writefds, NULL, &timeout);
        if(ret < 0) {printf("系统错误，select出错，错误代码：%d, 错误信息：%s", errno, strerror(errno));}
        else if(ret == 0) {printf("select超时返回，没有任何句柄状态发生变化！");}
        //有句柄状态发生了变化
        if(FD_ISSET(fd1, &readfds)) {
            fd1有数据可读;
            fd1里的数据被读出来;
        }
        if(FD_ISSET(fd2, &writefds)) {
            fd2可写;
            fd2里发送数据给对方;
        }
    }

经常用到的几个自定义函数：
1、开启监听的函数

int
OpenSCPServer(int port, int total, int sendbuflen, int recvbuflen, int blockORnot, int reuseORnot)    {

    int    sockfd = 0, ret = 0, opt = 0, flags=1;
    struct sockaddr_in    laddr;
    ret = sockfd = socket(PF_INET, SOCK_STREAM, 0);
    if(ret < 0)    {
        sprintf(errorMessage, "OpenTCPServer socket() error! return:%d, errno=%d, errortext:'%s' %s", ret, errno, strerror(errno), GetCurrentTime(0, 0));
        return -1;
    }
    ret = setsockopt(sockfd, SOL_SOCKET, SO_REUSEADDR, &reuseORnot, sizeof(int));
    if(ret < 0)    {
        sprintf(errorMessage, "OpenTCPServer setsockopt() reuse error! return:%d, errno=%d, errortext:'%s' %s", ret, errno, strerror(errno), GetCurrentTime(0, 0));
        return -2;
    }
    ret = setsockopt(sockfd, SOL_SOCKET, SO_RCVBUF, &recvbuflen, sizeof(int));
    if ( ret < 0)    {
        sprintf(errorMessage, "OpenTCPServer setsockopt() recvbuf error! return:%d, errno=%d, errortext:'%s' %s", ret, errno, strerror(errno), GetCurrentTime(0, 0));
        return -3;
    }
    ret = setsockopt(sockfd, SOL_SOCKET, SO_SNDBUF, &sendbuflen, sizeof(int));
    if (ret < 0)    {
        sprintf(errorMessage, "OpenTCPServer setsockopt() sendbuf error! return:%d, errno=%d, errortext:'%s' %s", ret, errno, strerror(errno), GetCurrentTime(0, 0));
        return -4;
    }
    ioctl(sockfd,FIONBIO,&blockORnot);
    laddr.sin_family = PF_INET;
    laddr.sin_port = htons(port);
    laddr.sin_addr.s_addr = INADDR_ANY;
    bzero(&(laddr.sin_zero), 8);
    ret = bind(sockfd, (struct sockaddr *)&laddr, sizeof(struct sockaddr));
    if(ret < 0)    {
        sprintf(errorMessage, "OpenTCPServer bind() error! return:%d, errno=%d, errortext:'%s' %s", ret, errno, strerror(errno), GetCurrentTime(0, 0));
        close(sockfd);
        return -5;
    }
    ret = listen(sockfd, total);
    if(ret < 0)    {
        sprintf(errorMessage, "OpenTCPServer listen() error! return:%d, errno=%d, errortext:'%s' %s", ret, errno, strerror(errno), GetCurrentTime(0, 0));
        close(sockfd);
        return -6;
    }
    sprintf(errorMessage, "OpenTCPServer opened on port.%d(%d) OK, socket(%d), buf(%d:%d)! %s", port, total, sockfd, sendbuflen, recvbuflen, GetCurrentTime(0, 0));
    return sockfd;
}

2、连接服务器的函数

int
ConnectSCPServer(char * serverip, int serverport, int blockORnot)    {

    int    serversock = 0, ret = 0;
    unsigned long    addr;
    struct sockaddr_in    sin;
    struct hostent *he;
    if((he=gethostbyname(serverip))== 0) {
        sprintf(errorMessage, "ConnectSCPServer IP address '%s' error! return:-1 %s", serverip, GetCurrentTime(0, 0));
        return -1;
    }
    serversock = socket(PF_INET, SOCK_STREAM, 0);
    if(serversock == -1)    {
        sprintf(errorMessage, "ConnectSCPServer socket() error! return:-2, errno=%d, errortext:'%s' %s", errno, strerror(errno), GetCurrentTime(0, 0));
        return -2;
    }
    ioctl(serversock, FIONBIO, &blockORnot); //block or not
    memset((char*)&sin, 0, sizeof(struct sockaddr_in));
    sin.sin_family = PF_INET;
    sin.sin_port = htons(serverport);
    sin.sin_addr = *((struct in_addr *)he->h_addr);
    ret = connect(serversock, (struct sockaddr *)&sin, sizeof(sin));
    if(ret == -1)    {
        sprintf(errorMessage, "ConnectSCPServer connect() error! return:-3, errno=%d, errortext:'%s' %s", errno, strerror(errno), GetCurrentTime(0, 0));
        close(serversock);
        return -3;
    }
    return serversock;
}

3、发送数据函数Send

int
Send(int sock, char * buf, size_t size, int flag, int timeout)    {

    int i = 0, ret = 0, intretry = 0;
    struct tim tival;
    fd_set writefds;
    int maxfds = 0;
    tival.tv_sec = timeout;
    tival.tv_usec = 0;
    FD_ZERO(&writefds);
    if(sock > 0) {
        FD_SET(sock, &writefds);
        maxfds=((sock > maxfds)?sock:maxfds);
    }
    else    {
        sprintf(errorMessage, "Send socket:%d error! return:-2 %s", sock, GetCurrentTime(0, 0));
        return -2;
    }
    ret = select(maxfds + 1, NULL, &writefds, NULL, &tival);
    if(ret <= 0) {
        if(ret < 0)    sprintf(errorMessage, "Send socket:%d select() error! return:%d, errno=%d, errortext:'%s' %s", sock, ret, errno, strerror(errno), GetCurrentTime(0, 0));
        else sprintf(errorMessage, "Send socket:%d select timeout(%d)! %s", sock, timeout, GetCurrentTime(0, 0));
        close(sock);
        return -3;
    }
    if(!(FD_ISSET(sock, &writefds)))    {
        sprintf(errorMessage, "Send socket:%d not in writefds! %s", sock, GetCurrentTime(0, 0));
        close(sock);
        return -4;
    }
    while(i < size)    {
        ret = send(sock, buf + i, size - i, flag);
        if(ret <= 0)    {
            sprintf(errorMessage, "Send socket:%d send() error! return:%d, errno=%d, errortext:'%s' %s", sock, ret, errno, strerror(errno), GetCurrentTime(0, 0));
            if (EINTR == errno)
              if(intretry < 10) {intretry++;continue;}
              else sprintf(errorMessage, "Send socket:%d send() error!EINTR 10 times! %s", sock, GetCurrentTime(0, 0));
            close(sock);
            return -1;
        }
        else i += ret;
    }
    sprintf(errorMessage, "Send socket:%d send() OK! %d/%d bytes sent! %s", sock, i, size, GetCurrentTime(0, 0));
    return i;
}

4、接收数据函数Recv

int
Recv(int sock, char * buf, size_t size, int flag, int timeout)    {

    int i = 0, ret = 0, intretry = 0;
    struct tim tival;
    fd_set readfds;
    int maxfds = 0;
    tival.tv_sec = timeout;
    tival.tv_usec = 0;
    FD_ZERO(&readfds);
    if(sock > 0) {
        FD_SET(sock, &readfds);
        maxfds=((sock > maxfds)?sock:maxfds);
    }
    else    {
        sprintf(errorMessage, "Recv socket:%d error! return:-2 %s", sock, GetCurrentTime(0, 0));
        return -2;
    }
    ret = select(maxfds + 1, &readfds, NULL, NULL, &tival);
    if(ret <= 0) {
        if(ret < 0)    sprintf(errorMessage, "Recv socket:%d select() error! return:%d, errno=%d, errortext:'%s' %s", sock, ret, errno, strerror(errno), GetCurrentTime(0, 0));
        else sprintf(errorMessage, "Recv socket:%d select timeout(%d)! %s", sock, timeout, GetCurrentTime(0, 0));
        close(sock);
        return -3;
    }
    if(!(FD_ISSET(sock, &readfds)))    {
        sprintf(errorMessage, "Recv socket:%d not in readfds! %s", sock, GetCurrentTime(0, 0));
        close(sock);
        return -4;
    }
    while(i < size)    {
        ret = recv(sock, buf + i, size - i, flag);
        if(ret <= 0){
            sprintf(errorMessage, "Recv socket:%d recv() error! return:%d, errno=%d, errortext:'%s' %s", sock, ret, errno, strerror(errno), GetCurrentTime(0, 0));
            if(errno == EINTR)   
                if(intretry < 10) {intretry++;continue;}
                else sprintf(errorMessage, "Recv socket:%d recv() error! EINTR 10 times! %s", sock, GetCurrentTime(0, 0));
            close(sock);
            return -1;
        }
        else i += ret;
    }
    sprintf(errorMessage, "Recv socket:%d recv() OK! %d/%d bytes received! %s", sock, i, size, GetCurrentTime(0, 0));
    return i;
}
最后需要说明的是：我这里讲到的源程序并不能实际地作为一个产品程序来用，实际情况下可能会有其它许多工作要做，比如可能要建立共享队列来存放 socket里读到的消息，也可能把发送消息先进行排队然后再调用Send函数。还有，如果不是全数字，在发送前一定要htonl转换为网络字节序，同理接收到后一定要先ntohl由网络字节序转换为主机字节序，否则对方发送过来的0x00000001在你这里可能是0x00010000，因为高低位顺序不同。