Question

I’ve got a piece of code:

while(1) {
    if(recvfrom(*val, buffer, 1024, MSG_PEEK, NULL, NULL)==-1) {    
        perror("recv");
        exit(1);
    } else printf("recv msgpeek\n");
    if(*(int*)buffer>5) {
        if(recvfrom(*val, buffer, 1024, 0, NULL, NULL)==-1) {   
            perror("recv");
            exit(1);
        } else printf("recv\n");
        if(*(int*)buffer==6) {
                    printf("%d\n", *(int*)(buffer+sizeof(int)+30));
                    printf("%s\n", (char*)buffer+sizeof(int));
                }
   }

This is part of a client programme. I’m sending messages from server to this client and I’ve noticed that when client receives this messages, they’re connected. I’m using SOCK_STREAM type sockets. Anyone know how to avoid connecting messages?

Answer 1

If I understood you correctly, you are reading from TCP socket, and expecting to get exactly same number of bytes as you “sent” from the other side. Here you assumption is wrong. TCP socket is a bi-directional stream, i.e. it does not preserve boundaries of the application messages you send through it. A “write” on one side of the connection could result in multiple “reads” on the other side, and the other way around, multiple “writes” could be received together. That last case is what you are seeing. It is your responsibility to keep track of message boundaries.

Related question – Receiving data in TCP.