C: socket connection timeout

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Set the socket non-blocking, and use select() (which takes a timeout parameter). If a non-blocking socket is trying to connect, then select() will indicate that the socket is writeable when the connect() finishes (either successfully or unsuccessfully). You then use getsockopt() to determine the outcome of the connect():

int main(int argc, char **argv) {
    u_short port;                /* user specified port number */
    char *addr;                  /* will be a pointer to the address */
    struct sockaddr_in address;  /* the libc network address data structure */
    short int sock = -1;         /* file descriptor for the network socket */
    fd_set fdset;
    struct timeval tv;

    if (argc != 3) {
        fprintf(stderr, "Usage %s <port_num> <address>\n", argv[0]);
        return EXIT_FAILURE;
    }

    port = atoi(argv[1]);
    addr = argv[2];

    address.sin_family = AF_INET;
    address.sin_addr.s_addr = inet_addr(addr); /* assign the address */
    address.sin_port = htons(port);            /* translate int2port num */

    sock = socket(AF_INET, SOCK_STREAM, 0);
    fcntl(sock, F_SETFL, O_NONBLOCK);

    connect(sock, (struct sockaddr *)&address, sizeof(address));

    FD_ZERO(&fdset);
    FD_SET(sock, &fdset);
    tv.tv_sec = 10;             /* 10 second timeout */
    tv.tv_usec = 0;

    if (select(sock + 1, NULL, &fdset, NULL, &tv) == 1)
    {
        int so_error;
        socklen_t len = sizeof so_error;

        getsockopt(sock, SOL_SOCKET, SO_ERROR, &so_error, &len);

        if (so_error == 0) {
            printf("%s:%d is open\n", addr, port);
        }
    }

    close(sock);
    return 0;
}

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