在不同程序中运行时，选择总是返回0

Question

#define MY_TIMEOUT_S    5   
    int mySock;
    if ((mySock = socket(AF_INET, SOCK_DGRAM, 0)) == -1) {
        perror("Cannot create UDP socket");
        return -1;
    }

    struct sockaddr_in myAddr;
    myAddr.sin_family       = AF_INET;
    myAddr.sin_port         = 0;
    myAddr.sin_addr.s_addr  = INADDR_ANY;
    memset(&(myAddr.sin_zero), '\0', 8);
    if (bind(mySock, (struct sockaddr*)&myAddr, 
            sizeof(struct sockaddr)) == -1) {
        perror("Cannot bind UDP socket to desired port");
        close(mySock);
        return -1;
    }

    // here send to remote target host on udp socket and port 

    char buf[MAX_UDP_PACKET_SIZE];
    struct sockaddr_in theirAddr;
    int addrLen = sizeof(struct sockaddr);

    fd_set readSet, curReadSet;
    FD_ZERO(&readSet);
    FD_SET(mySock, &readSet);
    int maxFD = mySock;
    while (true) {
        struct timeval timeOutTV = {MY_TIMEOUT_S, 0};

        memcpy(&curReadSet, &readSet, sizeof(fd_set));

        int selectRet = select(maxFD+1, &curReadSet, NULL, NULL, &timeOutTV);
        cout<<"select value =" <<selectRet<<endl;

        if (selectRet == -1) {
            if (errno == EINTR) {
                continue; // Interrupted by signal, retry
            }
            close(mySock);
            return -1;

        } else if (selectRet == 0) {
            close(mySock);
            return -1;
        }
        if (FD_ISSET(mySock, &curReadSet)) {
            int numBytes = recvfrom(mySock, buf, MAX_UDP_PACKET_SIZE, 0,
                    (struct sockaddr*)&theirAddr, (socklen_t*)&addrLen);
            if (numBytes == -1) {
                close(mySock);
            }   

        }
    }

目标16061根3R FIFO 0,8 0T0 153467管道
目标16061根4U IPv4 153470 0T0 UDP*:3290
目标16061根5U sock 0,6 0T0 153471无法识别协议
目标16061根6U IPv4 153472 0T0 TCP*:3290(LISTEN)
目标16061根7U raw 0t0 153473 00000000:0001->00000000:0000 ST=07

2）这是我在一个单独的exe中运行的上面的代码，其中只有这是在main中运行的代码，因为fd=3工作正常

sudo LSOF-A-P 16112

3）这是一个web服务器，我在这里集成了上面的代码，它独立工作，但不是现在

这是我的web服务器运行的输出

webServ 18431根3U unix 0xC09F9B00 0T0 157975套接字
webServ 18431根4U IPv4 157976 0T0 UDP*:8080
webServ 18431根5U IPv4 157977 0T0 TCP*:8080(LISTEN)

Answer 1

做那个：

int maxFD = mySock + 1;

select(2)的手册页说明：

int select(int nfds, fd_set *readfds, fd_set *writefds,
           fd_set *exceptfds, struct timeval *timeout);

nfds是三个集合中编号最高的文件描述符，加上1。