Transferring an Image using TCP Sockets in Linux

Question

I am trying to transfer an image using TCP sockets using linux. I have used the code many times to transfer small amounts but as soon as I tried to transfer the image it only transfered the first third. Is it possible that there is a maximum buffer size for tcp sockets in linux? If so how can I increase it? Is there a function that does this programatically?

Answer 1

I would guess that the problem is on the receiving side when you read from the socket. TCP is a stream based protocol with no idea of packets or message boundaries.

This means when you do a read you may get less bytes than you request. If your image is 128k for example you may only get 24k on your first read requiring you to read again to get the rest of the data. The fact that it's an image is irrelevant. Data is data.

For example:

int read_image(int sock, int size, unsigned char *buf) {
   int bytes_read = 0, len = 0;
   while (bytes_read < size && ((len = recv(sock, buf + bytes_read,size-bytes_read, 0)) > 0)) {
       bytes_read += len;
   }
   if (len == 0 || len < 0) doerror();
   return bytes_read;
}

Answer 2

The maximum size for 1 single IP packet is 65535, which is extremely close to the number you are hitting. I doubt that is a coincidence.

Answer 3

TCP sends the data in pieces, so you're not guaranteed to get it all at once with a single read (although it's guaranteed to stay in the order you send it). You basically have to read multiple times until you get all the data. It also doesn't know how much data you sent on the receiver side. Normally, you send a fixed size "length" field first (always 8 bytes, for example) so you know how much data there is. Then you keep reading and building a buffer until you get that many bytes.

So the sender would look something like this (pseudocode)

int imageLength;
char *imageData;

// set imageLength and imageData

send(&imageLength, sizeof(int));
send(imageData, imageLength);

And the receiver would look like this (pseudocode)

int imageLength;
char *imageData;

guaranteed_read(&imageLength, sizeof(int));
imageData = new char[imageLength];
guaranteed_read(imageData, imageLength);

void guaranteed_read(char* destBuf, int length)
{
    int totalRead=0, numRead;
    while(totalRead < length)
    {
        int remaining = length - totalRead;
        numRead = read(&destBuf[totalRead], remaining);
        if(numRead > 0)
        {
            totalRead += numRead;
        }
        else
        {
            // error reading from socket
        }
    }
}

Obviously I left off the actual socket descriptor and you need to add a lot of error checking to all of that. It wasn't meant to be complete, more to show the idea.