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Back in January this year I was due to be teaching some diploma level programming (roughly equivalent to the UK A/S level for any Brits), and part of that had to deal with network programming with sockets and stuff, so I duly did my research and put together a simple chat server and client in SDL_net. And then my classes changed and I got moved on to teach other stuff. I wasn’t too upset though – I’d learnt a lot, and I’d put up the code to help people out who might be in a similar situation, so it was all good.

But now in November I’m back on programming duty, so I dug up my code, looked it over, and thought Naah – I can’t use that, it’s unweildy, and complex, and it would be a real pain to try to re-use the code. So I’ve gone back to the drawing board and refactored it all into something I hope is a lot more palletable and both easy to use and re-purpose. In effect, I’ve refactored it into two wrappers which now consist of a ServerSocket class and a ClientSocket class.

Check it out…

Cross platform socket connectivity? That'll be a yes, then...

Socket Server

The old chat server was 250 lines, it’s now down to 77:

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// Re-written simple SDL_net socket server example | Nov 2011 | r3dux
// Library dependencies: libSDL, libSDL_net
 
#include <iostream>
#include <string>
#include <SDL/SDL_net.h>
#include "ServerSocket.h"
 
int main(int argc, char *argv[])
{
	// Initialise SDL_net
	if (SDLNet_Init() == -1)
	{
		std::cerr << "Failed to intialise SDL_net: " << SDLNet_GetError() << std::endl;
		exit(-1);
	}
 
	// Create a pointer to a ServerSocket object
	ServerSocket *ss;
 
	try
	{
		// Try to instantiate the server socket
		// Parameters: port number, buffer size (i.e. max message size), max sockets
		ss = new ServerSocket(1234, 512, 3);
	}
	catch (SocketException e)
	{
		std::cerr << "Something went wrong creating a SocketServer object." << std::endl;
		std::cerr << "Error is: " << e.what()   << std::endl;
		std::cerr << "Terminating application." << std::endl;
		exit(-1);
	}
 
	try
	{
		// Specify which client is active, -1 means "no client is active"
		int activeClient = -1;
 
		// Main loop...
		do
		{
			// Check for any incoming connections to the server socket
			ss->checkForConnections();
 
			// At least once, but as many times as necessary to process all active clients...
			do
			{
				// ...get the client number of any clients with unprocessed activity (returns -1 if none)
				activeClient = ss->checkForActivity();
 
				// If there's a client with unprocessed activity...
				if (activeClient != -1)
				{
					// ...then process that client!
					ss->dealWithActivity(activeClient);
				}
 
			// When there are no more clients with activity to process, continue...
			} while (activeClient != -1);
 
		// ...until we've been asked to shut down.
		} while (ss->getShutdownStatus() == false);
 
	}
	catch (SocketException e)
	{
		cerr << "Caught an exception in the main loop..." << endl;
		cerr << e.what() << endl;
		cerr << "Terminating application." << endl;
	}
 
	// Shutdown SDLNet - our ServerSocket will clean up after itself on destruction
	SDLNet_Quit();
 
	return 0;
}

Socket Client

And the old chat client was 313 lines, which is now down to 83:

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// Re-written simple SDL_net socket client example | Nov 2011 | r3dux
// Library dependencies: libSDL, libSDL_net
 
#include <iostream>
#include <string>
#include <SDL/SDL_net.h>
 
#include "ClientSocket.h"
 
int main(int argc, char *argv[])
{
	// Initialise SDL_net (Note: We don't initialise or use normal SDL at all - only the SDL_net library!)
	if (SDLNet_Init() == -1)
	{
		std::cerr << "Failed to intialise SDL_net: " << SDLNet_GetError() << std::endl;
		exit(-1);
	}
 
	// Create a pointer to a ServerSocket object
	ClientSocket *cs;
 
	try
	{
		// Try to instantiate the client socket
		// Parameters: server address, port number, buffer size (i.e. max message size)
		// Note: You can provide the serverURL as a dot-quad ("1.2.3.4") or a hostname ("server.foo.com")
		cs = new ClientSocket("127.0.0.1", 1234, 512);
	}
	catch (SocketException e)
	{
		std::cerr << "Something went wrong creating a ClientSocket object." << std::endl;
		std::cerr << "Error is: " << e.what()   << std::endl;
		std::cerr << "Terminating application." << std::endl;
		exit(-1);
	}
 
	try
	{
		// Attempt to connect to the server at the provided address and port
		cs->connectToServer();
 
		string receivedMessage = "";
 
		cout << "Use /quit to disconnect or /shutdown to shutdown the server." << endl;
 
		// Display the initial prompt
		cs->displayPrompt();
 
		// Run the main loop...
		do
		{
			// Check if we've received a message
			receivedMessage = cs->checkForIncomingMessages();
 
			// If so then...
			if (receivedMessage != "")
			{
				// Display the message and then blank it...
				cs->displayMessage(receivedMessage);
 
				// ...and then re-display the prompt along with any typed-but-not-yet-sent input
				cs->displayPrompt();
			}
 
			// Get and deal with input from the user in a non-blocking manner
			cs->getUserInput();
 
		// ... until we decide to quit or the server is shut down
		} while ( (cs->getShutdownStatus() == false));
 
	}
	catch (SocketException e)
	{
		cerr << "Caught an exception in the main loop..." << endl;
		cerr << e.what() << endl;
		cerr << "Terminating application." << endl;
	}
 
	// Shutdown SDLNet - our ClientSocket will clean up after itself on destruction
	SDLNet_Quit();
 
	return 0;
}

On top of all this we have try/catch exception handling, a nice encapsulated & easy to work-with/modify/extend design and debug flags to control whether the client/server should be verbose or run silently. Obviously the chat client itself can’t run completely silently – you wouldn’t be able to read the messages being sent back and forth! – but when debug is off it only ever outputs anything when it receives a message or when the user enters messages to send, so it’s pretty darn quiet.

Oh, and it now comes in Linux and Windows flavours =D

Overall I’m really happy with it – it’s taken a few days to properly redesign and test (not to mention the issues involved with porting to Windows) – but I think the next time I need to do some socket stuff with C++ I’d be able to grab this code, make whatever modifications I need and get something up and running in no time.

Awthome! =P

Download links

• Linux Client and Server Code (Code::Blocks projects, libs not included)
• Windows Client and Server Code (Visual Studio 2008 projects, libs included)

Notes on Building for Windows

The windows client and server projects have some important tweaks made for them to compile, and it’s worth mentioning what they are:

• The solution (which contains two projects) comes comes with a copy of the SDL and SDL_net libs and headers all merged together in the SDL folder (well, there are separate libs and include folders, but all the headers from both are in the include folder and all the libs from both are in the libs folder).
• Each project has the following libraries linked in: SDL.lib, SDLmain.lib and SDL_net.lib, these libraries point to…
• SDL.dll and SDL_net.dll which are in the solution’s Debug folder, so you can compile and build from Visual Studio, BUT you’lll need to copy these two dll files into the same folder as the SocketServer-Rewritten.exe or SocketClient-Rewritten.exe folders to run the executables “standalone” in other locations as opposed to build-and-run-from-visual-studio style!
• The projects are defined as console applications, and because SDLmain.lib defines the main function as int main(int argc, char *argv[]) and not just int main() you MUST keep the definition of main in-line with the SDLmain.lib definition.
• Finally, the projects will only build successfully in debug mode. Why? I’ve no idea. If you know how to fix it then go for it!

Update – Nov 2011: I’ve refactored this code into something significantly easier to work with, modify and extend – so you should probably try this instead: http://r3dux.org/2011/11/a-simple-csdl_net-chat-server-client-rewritten/.

I was due to be teaching some network programming in the new term, so I thought I’d try to find the simplest library for cross-platform socket programming and knock up some examples that I could teach from. The trouble is, there are lots of different socket libraries… Lots and lots. So I did a bit of investigating and came up with the following:

In the end I chose SDL_net as teh winnah, as I’d already done some SDL stuff with the class previously (before switching to GLFW to minimise code-bloat), and I managed to get SDL_net up and communicating pretty easily. So, for the next couple of days I put together some simple client/server examples, culminating in the code you’ll find below, which is a (very) simple IRC-esque chat server.

Have a look – you can tell what it’s doing from the output in the windows:

The server is in the middle, the clients connect in, and chat commences (click for larger, more legible version)

Although I’ve written this code in Linux, as SDL_net is cross-platform, this code should be cross platform – only there are two tweaks you’ll need to make for it to work in Windows: I’ve used some custom kbhit and getch functions to check for a keypress and read what key was pressed, if one was. These functions come natively with Windows, so you should use the native versions and strip the custom ones out.

With those small changes made this should work fine in Windows – but if it doesn’t please feel free to fix it yourself and send me the changes you made

As the code for all this is a couple of hundred lines for each for the client and server I’ve put all the source after the jump. Part of the reason it’s so large is that I’ve commented it to the hilt (no really, I’ve gone to town on it – even by my exceptionally verbose standards) as it was originally meant to be a teaching aid. Also, I’ve also left a lot of commented-out debug code in there, so you can uncomment it if you’d like to see exactly what’s going on behind the scenes.

Anyways, I hope this is of use to someone starting off socket programming with SDL_net – and if you have any issues or such please feel free to sling a comment in the article and I’ll do my best to help out.

Cheers!

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