Have the loop break when a line's complete. Read one character at a time. This means send_message will need to write a \n after every message so the receiver can tell where it ends.

Code:

int pos=0;
bzero(buffer,MAXDATASIZE);

// Stop once we have an entire single line
while((buffer[pos] != '\n') && (pos < MAXDATASIZE))
{
        // Put new data 'pos' bytes ahead, so we don't overwrite the old
        ssize_t b=read(socket_fd, buffer+pos, 1); 
        if(b > 0) pos+=b;
        else break; // b<=0 means socket closed or socket error, so give up.
}

The receiving end isn't told how much the sending end sent. The socket doesn't necessarily send all the data at once, either. Plus there's travel time too. It's easily possible to read less than the sender wrote.

I see you've posted the client code now, I'll give it a quick look over.