Error and unexpected behavior with TCP connection

Question

hi..

I am wanting my TCP server and client to "talk" to each other depending on what the other says. But when I run, it seems to go through one iteration, but then both the server and client seem to just "hang" there. Once I kill it on the client side, I end up getting a broken pipe error.

Can anyone interpret what I am doing wrong here?

SERVER code snippet:

while(feof(file_ptr) == 0)
{
    fscanf(file_ptr, "%s", supplies);
    printf("%s\n", supplies);

    //SEND supplies to all clients (currently just one client)

    n = write(newsockfd, supplies, strlen(supplies));

    if (n < 0)
    {
        error("ERROR writing to socket");
    }


    //Receive a request from client that they can use supply

    bzero(buffer,2);
    n = read(newsockfd,buffer,2);

    if (n < 0)
    {
        error("ERROR reading from socket");
    }

    printf("Who is using supply? %s\n", buffer);


    if(strcmp(buffer, "A"))
    {
        aRounds++;  
    }


    while(done != 1)
    {
        //WAIT loop until message CO is received from client
        bzero(buffer,2);
        n = read(newsockfd,buffer,2);

        if (n < 0)
        {
            error("ERROR reading from socket");
        }

        printf("Done? %s\n", buffer);



        if(strcmp(buffer, "CO"))
        {
            done = 1;
        }
    }

    done = 0;
}

fclose(file_ptr);
n = write(newsockfd, "DN", 2);

CLIENT code snippet:

while(noSupplies != 1)
{
    //RECEIVE MSG from server about supplies
    bzero(buffer,2); 
    n = read(sockfd,buffer,2);

    if (n < 0) 
    {
        error("ERROR reading from socket");
    }

    printf("%s\n",buffer);

    if(strcmp(buffer, "BC") == 0 || strcmp(buffer, "CB") == 0)
    {
        //SEND server msg that you are using supply

        n = write(sockfd,"A", 1);
        if (n < 0)
        {
            error("ERROR writing to socket"); 
        }


        printf("Client A has received components %s during round %d.\n", buffer, round);


        n = write(sockfd,"CO", 2);
        if (n < 0)
        {
            error("ERROR writing to socket"); 
        }

    }
    else if(strcmp(buffer, "DN"))
    {
        noSupplies = 1;
    }

    round++;
}

I get the following from the server (before killing):

BC
Who is smoking? A
Done smoking? CO

And the following from the client (before killing):

BC
Client A has received components BC during round 1.

Then after killing from the client.. I get the following from the server:

BC
Who is using supply? A
Done? CO
Done? 
CB
Who is using supply? 
Done? 
CB
Who is using supply? 
Done? 
BC
Broken pipe

Does anyone understand what I am doing wrong? (looking for code fix!)

Just an FYI.. I will be changing the server code eventually to handle listening to mulitple clients (TCP) but one hurdle at a time right? :S

Thanks!

Answer 1

Not sure if this is all of your problem, but

bzero(buffer, 2);
n = read(socket, buffer, 2);

...

printf("%s", buffer);

is not correct, since if the read actually did get 2 bytes and neither of them were 0 then buffer may not be a null terminated string (unless there is something earlier that made that so).

Several calls to strcmp should also always fail for the same reason -- the read in string is not 0 terminated so strcmp thinks that it keeps on going and therefore is not the same as the compared to string, even if the first letters are the same.

edit

 n = read(socket, buffer, 2);
 if (n<0) {
     die_horrible_death();
 }
 buffer[n] = 0; // since arrays are 0 indexed this should be after last read character
 printf("I just read: \"%s\"\n", buffer);

Answer 2

Your server code writes only once something to the client, at the beginning. Later, in the loop - it just reads.

OTOH your client reads and writes in the loop.

Since you're working with sockets in a blocking mode - this means that your client eventually will become blocked in a call to recv.

P.S. No offenses - but you have a lot to learn.

1. TCP is a tunnel. When your server sends 2 bytes of data in a single call to send/write - there's no guarantee your client will read this within a single call to recv.
2. Hence - reading and parsing always must be done in a sort of a loop.
3. You can't write a really robust client/server with ability to timeout/interrupt on demand using only blocking I/O.
4. Learn, learn, learn