Question

Any ideas why when the server writes a socket while the client is waiting on select, select never finishes?

I am using c to communicate between sockets. My client connects to my server fine.

socket_desc=socket(AF_INET,SOCK_STREAM,0);//create the socket descriptor
client->address.sin_addr.s_addr = inet_addr(ipAddress);
client->address.sin_family = AF_INET;
client->address.sin_port = htons(port);

bind(socket_desc,&address,sizeof(address));
connect(socket_desc, &address, sizeof(address));

When I use recv to block and listen for data, everything works fine:

int bytesRead = 1;
while(bytesRead){
    int bufsize=1024;        
    char *buffer=malloc(bufsize);
    bytesRead = recv(socket_desc, buffer, bufsize, 0);
    printf("CLIENT RECV: %s", buffer);
}

If I try to use select, it doesn’t seem to read any data. If I add STDIN to the fd_set, I can force it to read from the socket, but select doesn’t seem to get triggered from the socket_desc reading in data…?

int running = 1;
while(running){
    /* wait for something to happen on the socket */
    struct timeval selTimeout;
    selTimeout.tv_sec = 2;       /* timeout (secs.) */
    selTimeout.tv_usec = 0;            /* 0 microseconds */
    fd_set readSet;
    FD_ZERO(&readSet);
    FD_SET(STDIN_FILENO, &readSet);//stdin manually trigger reading
    FD_SET(socket_desc, &readSet);//tcp socket

    int numReady = select(3, &readSet, NULL, NULL, &selTimeout);
            //IT ONLY GETS PAST SELECT ON RETURN FROM THE KEYBOARD
    if(numReady > 0){
        char buffer[100] = {'\0'};
        int bytesRead = read(socket_desc, &buffer, sizeof(buffer));
        printf("bytesRead %i : %s", bytesRead, buffer);
        if(bytesRead == 0){
            running = FALSE;
            printf("Shutdowning client.\n");

        }
    }

Answer 1

The first parameter to select should be the maximum socket id plus 1. So in your case, it should be

socket_desc+1

Can you try with that and see if it works?

The reason it only gets when you press a key on the keyboard is because stdin is 0, which would be within 0 – (3 – 1) range, which is what is checked. If you set the first parameter to socket_desc+1, then 0 – (socket_desc) range should be checked for ready sockets