C winsock function to send all message data

I'm writing a server in c using WSA that will handle multiple clients. The protocol is something I defined on my own, and the part I'm having trouble with is how to make sure the whole message is actually being sent to the client.

I send my message once, and than I check how many bytes where actually transferred. Then, if not, I send again, and as the length of the data that I will now send I use unsentBytes (see in code). My problem is that when I try to send the extra bytes that weren't sent, I am currently sending again the entire message. How can I send only the remaining part of the message?

I know I can send only 1 char at a time, and stop recieving on the client side when I get to the end of the message, but I think I can do it like this as well and that it's better.

Is this the right logic to use?

int send_msg(SOCKET s, char *msg, int msg_len)
{
    int unsentBytes = msg_len;
    int bytesResult = send(s, msg, msg_len, SEND_FLAGS);
    unsentBytes -= bytesResult;
    while (unsentBytes != 0)
    {
        bytesResult = send(s, msg, unsentBytes, SEND_FLAGS); // ### msg is the problem
        unsentBytes -= bytesResult;
    }
}

Here's working code from one of my own projects. Note data + count to offset into the data.

int sendall(int sd, char *data, int length) {
    int count = 0;
    while (count < length) {
        int n = send(sd, data + count, length, 0);
        if (n == -1) {
            return -1;
        }
        count += n;
        length -= n;
    }
    return 0;
}