Trouble with packing and sending binary data over socket -Serialization

Question

I have for several hours tried, debugged and cried in hope to get my program to send/pack and receive/unpack my data correctly. My only hope is some good help!

To pack/unpack data i have implemented Beej's Guide to Network Programming example pack2.c

The code works pack/unpack works internally in the same program. But when i try to send the packed data over a socket to another program on the same computer, it will not work! My thoughts is that the recv() and send() functions in a way maybe makes the data corrupt or something like this, i have tried to debug this is several ways:

I need some help to get this working, i believe that the problem lays in the way buf is sent and received, since the pack()/unpack() functions work internally in a program.

Thanks

Client side:

unsigned char buf[1024];

    int16_t packetsize = pack(buf, "h", (int16_t)37);
    packi16(buf+1, packetsize);
    int len = packetsize;

    uint16_t header = htons(packetsize); //Convert from host -> network

    //Sends info to receiver about next pack coming.

    printf("HEADER SIZE %d\n", packetsize);

    if(send(sock, &header, packetsize, 0) == -1) {
        perror("sendall");
    }


    /*==========SEND PACKAGE======================*/

    int bytessendt = send(sock, buf, len, 0);//server_send_all(sock, buf, &len);

    if(bytessendt == -1) {
        perror("sendall");
    }

    printf("BYTES SENT OVER SOCKET: %d \n", bytessendt);


    close(sock);

Server side:

                uint16_t buf;
                int len = sizeof(buf);

                if(recv(i, &buf, len, 0) == -1) {
                    printf("****Error when receiving");
                }

                int header = ntohs(buf);

                printf("HEADER SIZE: %d\n", header);

                /*========REICIVE PACK AND UNPACK================*/

                unsigned char buf2[1024];
        int16_t monkeycount;

                int bytesreaded = recv(i, buf2, header, 0);

                if(bytesreaded == -1) {
                    printf("Feil ved mottakelse\n");
                }

                printf("BYTES READED: %d\n", bytesreaded);
                printf("BYTES SHOULD BE READED: %d\n", header);

                unpack(buf2, "h", &monkeycount);


                printf("CONTENT: %d\n", monkeycount);


                close(i);

int server_send_all(int socket, unsigned char *buf, int *len) {

    int total = 0;              //Total amout of data to send
    int bytesleft = *len;       //Amount of data left to send
    int n;                      //Holds return from send()

    printf("SOCKET -DATA TO SEND: %d\n", *len);

    while(total < *len) {
        n = send(socket, buf+total, bytesleft, 0);

        if(n == -1) { //send() returns error
            break;
        }

        total += n;
        bytesleft -= n;
    }

    *len = total; //Returns actually sent

    return n == -1?-1:0;
}

Sending string

From client:

int packetsize = 0;
packetsize += pack(buf + packetsize, "h", (int16_t)37);
packetsize += pack(buf + packetsize, "s", "Hello World");

    send(sock, &header, packetsize, 0)

To server:

     char s2[30];
                        unpack(buf2, "h30s", &monkeycount, s2);                  

                        printf("CONTENT: %d %s\n", monkeycount, s2);

Gives output

CONTENT: 37 Hello Wor

What is the error ? what is the expected behaviour ? saying it won't work isn't enough. — iabdalkader
The expected behavior is that printf("CONTENT: %d\n", monkeycount); should print 37, when unpacking monkeycount should hold 37. I believe that server_send_all() function takes care of sending all the data ? — user265767
Of course, I don't actually understand why we increase buf pointer :} The code now increments buf pointer, and the server side now removes the last two characters :( — user265767

iabdalkader iabdalkader · Accepted Answer · 2012-11-06T15:56:17

you pack an int16_t which is 2 bytes and then you increment buf by 1 byte only, that should be 2 bytes instead:

int16_t packetsize = pack(buf, "h", (int16_t)37);
packi16(buf+2, packetsize);

Each time you pack something into the buffer you should increment the buffer pointer by that amount when calling pack() again:

pack(buf, 2 bytes);
pack(buf+2, 4 bytes);
pack(buf+6, 2 bytes);
...

So I think you should do it like this:

int packetsize =0;
packetsize += pack(buf+packetsize, "h", (int16_t)37);
packetsize += pack(buf+packetsize, "h", (int16_t)38);
...

Note1: packetsize should be an int as pack() returns an int, and I don't see why you use packi16() the second time instead of the more generic pack() function.

Note2: Also the packet size is wrong, you forgot to add the size of packetsize. in fact, I don't think you mean to add the packet size to the packet at all, because you're sending the packet size and the packet separately

Trouble with packing and sending binary data over socket -Serialization

2 Answers