Having issues with sockets and telnet - c++

Question

I've been learning sockets, and I have created a basic server where you can telnet into and type messages, then press enter and the message is printed on the server.
Since it's telnet, every key press gets sent to the server. So I basically hold all sent bytes in a buffer, and then when a carriage return ("\r\n") is received, I discard that, and print out the clients current buffer. Then I clear the clients buffer.
My problem is that every once in a while (and I'm not quite sure how to replicate it), the first "line" of data I send in gets an extra space tacked onto each character. For example, I'll type "Test" on the telnet client, but my server will receive it as "T e s t ". I always clear the receiving buffer before receiving any data. One obvious solution is just to remove all spaces serverside, but then that messes up my ability to send more than one word. Is this just an issue with my telnet, or is there something I can do on the server to fix this?
I am using the WinSock2 API and Windows 10 Telnet.
EDIT:
I have checked the hex value of the extra character, and it is 0x20.
EDIT:
Here is the code that receives and handles the incoming telnet data.
// This client is trying to send some data to us
memset(receiveBuffer, sizeof(receiveBuffer), 0);
int receivedBytes = recv(client->socket, receiveBuffer, sizeof(receiveBuffer), 0);
if (receivedBytes == SOCKET_ERROR)
{
FD_CLR(client->socket, &masterFDSet);
std::cerr << "Error! recv(): " << WSAGetLastError() << std::endl;
closesocket(client->socket);
client->isDisconnected = true;
continue;
}
else if (receivedBytes == 0)
{
FD_CLR(client->socket, &masterFDSet);
std::cout << "Socket " << client->socket << " was closed by the client." << std::endl;
closesocket(client->socket);
client->isDisconnected = true;
continue;
}
// Print out the hex value of the incoming data, for debug purposes
const int siz_ar = strlen(receiveBuffer);
for (int i = 0; i < siz_ar; i++)
{
std::cout << std::hex << (int)receiveBuffer[i] << " " << std::dec;
}
std::cout << std::endl;
std::string stringCRLF = "\r\n"; // Carraige return representation
std::string stringBS = "\b"; // Backspace representation
std::string commandBuffer = receiveBuffer;
if (commandBuffer.find(stringCRLF) != std::string::npos)
{
// New line detected. Process message.
ProcessClientMessage(client);
}
else if (commandBuffer.find(stringBS) != std::string::npos)
{
// Backspace detected,
int size = strlen(client->dataBuffer);
client->dataBuffer[size - 1] = '\0';
}
else
{
// Strip any extra dumb characters that might have found their way in there
commandBuffer.erase(std::remove(commandBuffer.begin(), commandBuffer.end(), '\r'), commandBuffer.end());
commandBuffer.erase(std::remove(commandBuffer.begin(), commandBuffer.end(), '\n'), commandBuffer.end());
// Add the new data to the clients data buffer
strcat_s(client->dataBuffer, sizeof(client->dataBuffer), commandBuffer.c_str());
}
std::cout << "length of data buffer is " << strlen(client->dataBuffer) << std::endl;

Answer 1

You have two major problems.
First, you have a variable, receivedBytes that knows the number of bytes you received. Why then do you call strlen? You have no guarantee that the data you received is a C-style string. It could, for example, contain embedded zero bytes. Do not call strlen on it.
Second, you check the data you just received for a \r\n, rather than the full receive buffer. And you receive data into the beginning of the receive buffer, not the first unused space in it. As a result, if one call to recv gets the \r and the next gets the \n, your code will do the wrong thing.
You never actually wrote code to receive a message. You never actually created a message buffer to hold the received message.

Answer 2

Your code, my comments:
memset(receiveBuffer, sizeof(receiveBuffer), 0);
You don't need this. You shouldn't need this. If you do there is a bug later in your code.
int receivedBytes = recv(client->socket, receiveBuffer, sizeof(receiveBuffer), 0);
if (receivedBytes == SOCKET_ERROR)
{
FD_CLR(client->socket, &masterFDSet);
std::cerr << "Error! recv(): " << WSAGetLastError() << std::endl;
closesocket(client->socket);
client->isDisconnected = true;
continue;
You mean 'break'. You got an error. You closed the socket. There is nothing to continue.
}
else if (receivedBytes == 0)
{
FD_CLR(client->socket, &masterFDSet);
std::cout << "Socket " << client->socket << " was closed by the client." << std::endl;
closesocket(client->socket);
client->isDisconnected = true;
continue;
Ditto. You mean 'break'. You got an error. You closed the socket. There is nothing to continue.
}
// Print out the hex value of the incoming data, for debug purposes
const int siz_ar = strlen(receiveBuffer);
Bzzzzzzzzzzzzt. There is no guarantee there is a null anywhere in the buffer. You don't need this variable. The correct value is already present, in receivedBytes.
for (int i = 0; i < siz_ar; i++)
That should be `for (int i = 0; i < receivedBytes; i++)
{
std::cout << std::hex << (int)receiveBuffer[i] << " " << std::dec;
}
std::cout << std::endl;
std::string stringCRLF = "\r\n"; // Carraige return representation
No. That is a carriage return (\r) followed by a line feed (\n), often called CRLF as indeed you have yourself in the variable name. This is the standard line terminator in Telnet.
std::string stringBS = "\b"; // Backspace representation
std::string commandBuffer = receiveBuffer;
Bzzt. This copy should be length-delimited by receivedBytes.
if (commandBuffer.find(stringCRLF) != std::string::npos)
As noted by #DavidShwartz you can't assume you got the CR and the LF in the same buffer.
{
// New line detected. Process message.
ProcessClientMessage(client);
}
else if (commandBuffer.find(stringBS) != std::string::npos)
{
// Backspace detected,
int size = strlen(client->dataBuffer);
client->dataBuffer[size - 1] = '\0';
This doesn't make any sense. You are using strlen() to tell you where the trailing null is, and then you're putting a null there. You also have the problem that there may not be a trailing null. In any case what you should be doing is removing the backspace and the character before it, which requires different code. You're also operating on the wrong data buffer.
}
else
{
// Strip any extra dumb characters that might have found their way in there
commandBuffer.erase(std::remove(commandBuffer.begin(), commandBuffer.end(), '\r'), commandBuffer.end());
commandBuffer.erase(std::remove(commandBuffer.begin(), commandBuffer.end(), '\n'), commandBuffer.end());
// Add the new data to the clients data buffer
strcat_s(client->dataBuffer, sizeof(client->dataBuffer), commandBuffer.c_str());
}