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POUse stringstream to read from TCP socket
 

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  1. POUse stringstream to read from TCP socket
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    <p>I am using a socket library (I'd rather not not use it) whose <code>recv</code> operations works with <code>std::string</code>, but is just a wrapper for one call of the <code>recv</code> socket function, so it is probably that I only got some part of the message I wanted. My first instinct was to go in a loop and append the received string to another string until I get everything, but this seems inefficient. Another possibility was to do the same with a char array, but this seems messy. (I'd have to check the strings size before adding into the array and if it overflowed I need to store the string somewhere until the array is empty again.. )</p> <p>So I was thinking about using a stringstream. I use a TLV protocol, so I need to first extract two bytes into an unsigned short, then get a certain amount of bytes from the stringstream and then loop again until I reach a delimiter field.</p> <p>Is there any better way to do this? Am I completely on the wrong track? Are there any best practices? So far I've always only seen direct use of the socket library with char arrays so I'm curious why using `std::string`` with stringstreams could be a bad idea.. </p> <p><strong>Edit:</strong> Replying to the comment below: The library is one we use internally, its not public (its nothing special though, mostly just a wrapper around the socket library to add exceptions, etc.).</p> <p>I should mention that I have a working prototype using the socket library directly. </p> <p>This works something like:</p> <pre><code>int lengthFieldSize = sizeof(unsigned short); int endOfBuffer= 0;//Pointer to last valid position in buffer. while(true) { char buffer[RCVBUFSIZE]; while(true) { int offset= endOfBuffer; int rs= 0; rs= recv(sock, buffer+offset, sizeof(buffer)-offset, 0); endOfBuffer+= rs; if(rs &lt; 1) { // Received nothing or error. break; } else if(endOfBuffer == RCVBUFSIZE) { // Buffer full. break; } else if(rs &gt; 0 &amp;&amp; endOfBuffer &gt; 1) { unsigned short msglength= 0; memcpy((char *) &amp;msglength, buffer+endOfBuffer-lengthFieldSize, lengthFieldSize); if(msglength == 0) { break; // Received a full transmission. } } } unsigned int startOfData = 0; unsigned short protosize= 0; while(true) { // Copy first two bytes into protosize (length field) memcpy((char *) &amp;protosize, buffer+startOfData, lengthFieldSize); // Is the last length field the delimiter? // Then reply and return. (We're done.) // Otherwise: Is the next message not completely in the buffer? // Then break. (Outer while will take us back to receiving) if(protosize == 0) { // Done receiving. Now send: SendReplyMsg(sock, lengthFieldSize); // Clean up. close(sock); return; } else if((endOfBuffer-lengthFieldSize-startOfData) &lt; protosize) { memmove(buffer, buffer+startOfData, RCVBUFSIZE-startOfData); //Adjust endOfBuffer: endOfBuffer-=startOfData; break; } startOfData+= lengthFieldSize; gtControl::gtMsg gtMessage; if(!gtMessage.ParseFromArray(buffer+startOfData, protosize)) { cerr &lt;&lt; "Failed to parse gtMessage." &lt;&lt; endl; close(sock); return; } // Move position pointer forward by one message (length+pbuf) startOfData+= protosize; PrintGtMessage(&amp;gtMessage); } } </code></pre> <p>So basically I have a big loop which contains a receiving loop and a parsing loop. There's a character array being passed back and forth as I can't be sure to have received everything until I actually parse it. I'm trying to replicate this behaviour using "proper" C++ (i.e. std::string)</p>
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    1. CO(If I took your explamation right.) I'm pretty sure that string::append is no less efficient then operator<<(ostringstream&, const char*). It would be helpfull if you gave some code samples illustrating your ideas and probably name the library under consideration.
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      1. USArtem Pelenitsyn
 

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