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POProblems parsing a TCP-based TLV protocol in C++
 

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  1. POProblems parsing a TCP-based TLV protocol in C++
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    <p>I am trying to implement a (T)LV protocol over TCP sending protocol buffers from a python client and receiving with a C++ server.</p> <p>My code looks more or less like this:</p> <pre><code>char* buffer[RCVBUFSIZE]; int recvsize= 0; // Filling my buffer with as much as possible. while(true) { if(recvsize == RCVBUFSIZE) { break; } else if(recvsize+= recv(sock, buffer+recvsize, sizeof(buffer)-recvsize, 0) &lt; 1) { break; } } //Parsing LV protocol while(true) { unsigned short protosize= 0; //Copy first two bytes into protosize memcpy((char *) &amp;protosize, buffer, sizeof(unsigned short)); if(protosize == 0) { break; } // Protocol indicates EOM be setting length to "0" void* protomsg[protosize]; memcpy(protomsg, buffer+sizeof(unsigned short), protosize); int msglength= sizeof(unsigned short)+protosize; //Now I'll move the whole buffer to the left so that I don't have to keep track of where I'm at. memmove(buffer, buffer+msglength, RCVBUFSIZE-msglength); protoMsg pm; if(!pm.ParseFromArray(protomsg, protosize)) { break; } // Parsing failed. // Do stuff with parsed message. } </code></pre> <p>Now I have several problems:</p> <ul> <li>The while loop receiving the message never terminates. I suspect that the recv call blocks when there isn't any data left anymore while I expected it to return with an error. I have found the select function to check whether there's something to read. I will give that a try. But when I call receive only once to skip this problem (The message received comes in at ~10 bytes, so I expect all to be collected in one call.) I get another problem:</li> <li>memcpy and memmove don't seem to be working as expected. On the first loop the short is processed as expected (I receive the same value I send on the other side), but then everything from parsing of the protocol buffer fails. Have I misunderstood something?</li> </ul> <p><strong>Edit:</strong> Regarding the comment about ntohs -- I'm transmitting the short as little-endian currently, forgot to mention that. (I will change this still, btw.)</p> <p><strong>Third edit:</strong> The code now works, but I had to change the following:</p> <pre><code>char* buffer[RCVBUFSIZE]; int recvsize= 0; // Filling my buffer with as much as possible. while(true) { if(recvsize == RCVBUFSIZE) { break; } else if((recvsize+= recv(sock, buffer+recvsize, sizeof(buffer)-recvsize, 0)) &lt; 1) { break; } else if(recvsize &gt; 1) { unsigned short msglength= 0; memcpy((char *) &amp;msglength, buffer+recvsize-sizeof(unsigned short), sizeof(unsigned short)); if(msglength == 0) { break; } // Received a full transmission. } } </code></pre> <p>So first I needed to add brackets around the <code>recvsize+= recv()</code> statement and second as the non-blocking method didn't work for some reason I am now checking whether the last two bytes that were transmitted translate to a 0 when read a unsigned short. This probably leads to a problem if I read a 0 by chance that is not the length field. I'll start another question about this probably.</p> <p>I also changed <code>protomsg</code> to <code>char[]</code>, but I don't think this really changed anything. (I had parsing working with a void array already.. )</p>
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    1. COYou should be calling `ntohs` on the received `short` (and call `htons` when sending it - "Network TO Host, Short"/"Host TO Network, Short")
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      1. POProblems parsing a TCP-based TLV protocol in C++
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