SDL 2.0 Tutorial-06: Networking 3

In the last tutorial we setup a basic TCP server. In this tutorial we will start by modifying the server we setup and finish by writing a player client application. In order to write a client application with SDL 2.0 we will need a lot of the ideas which where presented in my first string of SDL tutorials (here). A lot of my personal preferences for setting up SDL have changed since writing those tutorials so instead of using code found within those tutorials I will be using a collection of C++ files I have settled into using (found here).

First let’s start with a few server-side modifications. Let’s start by adding a few extra packet flags:

//-----------------------------------------------------------------------------
#define FLAG_QUIT 0x0000
#define FLAG_WOOD_QUEST 0x0011
#define FLAG_WOOD_UPDATE 0x0010
#define FLAG_WOOD_GETTIME 0x0012

Now we have a packet flag for starting and completing a quest, as well as a packet flag for getting the amount of time left until the quest is completed. Next we will update our server-side “RecvData” function:

uint8_t* RecvData(int index, uint16_t* length) {
	uint8_t temp_data[MAX_PACKET];
	int num_recv = SDLNet_TCP_Recv(sockets[index], temp_data, MAX_PACKET);

	if(num_recv <= 0) {
		CloseSocket(index);

		const char* err = SDLNet_GetError();
		if(strlen(err) == 0) {
			printf("DB: client disconnectedn");
		} else {
			fprintf(stderr, "ER: SDLNet_TCP_Recv: %sn", err);
		}

		return NULL;
	} else {
		*length = num_recv;

		uint8_t* data = (uint8_t*) malloc(num_recv*sizeof(uint8_t));
		memcpy(data, temp_data, num_recv);

		return data;
	}
}

What I have done here is removed the work this function was doing to grab the packet flag from the start of the incoming socket data. This is because when a socket is ready for us to process it may have several packets all lined together. Therefore the data buffer returned by “RecvData” may contain several packets depending on how quickly the client is sending packets to us; so any work “RecvData” does to grab the flag from the first packet doesn’t help us identify the packet flags for all the other packets possibly following the first. Therefore the extra work being done isn’t necessary at this level.

Next we will add a new function called “ProcessData.” This function will continuously run on each packet we find within the data buffer returned by “RecvData.”

void ProcessData(int index, uint8_t* data, uint16_t* offset) {
	if(data == NULL) return;

	uint16_t flag = *(uint16_t*) &data[*offset];
	*offset += sizeof(uint16_t);

	switch(flag) {
		case FLAG_WOOD_UPDATE: {
			uint16_t send_offset = 0;
			uint8_t send_data[MAX_PACKET];

			memcpy(send_data+send_offset, &clients[index].amt_wood, sizeof(uint8_t));
			send_offset += sizeof(uint8_t);

			SendData(index, send_data, send_offset, FLAG_WOOD_UPDATE);
		} break;

		case FLAG_WOOD_GETTIME: {
			uint16_t send_offset = 0;
			uint8_t send_data[MAX_PACKET];

			uint32_t time_left;
			if(clients[index].questing) {
				time_left = WOOD_WAIT_TIME-(SDL_GetTicks()-clients[index].timer_wood);
			} else time_left = 0;

			memcpy(send_data+send_offset, &time_left, sizeof(uint32_t));
			send_offset += sizeof(uint32_t);

			SendData(index, send_data, send_offset, FLAG_WOOD_GETTIME);
		} break;

		case FLAG_WOOD_QUEST: {
			if(!clients[index].questing) {
				clients[index].questing = 1;
				clients[index].timer_wood = SDL_GetTicks();
			}
		} break;

		case FLAG_QUIT: {
			running = 0;
			printf("DB: shutdown by client id: %dn", index);
		} break;
	}
}

This is where we do the work and grab the packet’s flag. Everything here should look similar to what we where doing before when the server found client sockets which were ready with information to be processed – except for the fact that we have added two extra flags. The “GETTIME” flag checks if the player is currently questing; if not then the time left on the quest is 0, otherwise the server sends the time left on the quest to the player. The other flag is the “QUEST” flag; this flag checks whether the client is currently questing and starts the quest if they aren’t currently in a quest.

The finial server-side update we make will be to the main processing loop. First let’s crank up how often we check for ready sockets:

int num_rdy = SDLNet_CheckSockets(socket_set, 50);

If num_rdy <= 0 then we do not have any sockets ready for processing so we enter the server’s idle tasks. Previously we were simply adding four wood to the client’s resource count continuously. This is quite a silly thing to do! Instead let’s check for whether a client has completed any quests and if so we’ll send out a quest completed packet and a wood update packet. In addition, we’ll continuously send out “GETTIME” packets to keep the client updated with the server’s timer.

int ind;
for(ind=0; ind<MAX_SOCKETS; ++ind) {
	if(!clients[ind].in_use) continue;

	if(clients[ind].questing &&
		(SDL_GetTicks()-clients[ind].timer_wood)>WOOD_WAIT_TIME
	) {
		clients[ind].questing = 0;
		clients[ind].amt_wood += 4;
		SendData(ind, NULL, 0, FLAG_WOOD_QUEST);

		uint16_t send_offset = 0;
		uint8_t send_data[MAX_PACKET];

		memcpy(send_data+send_offset, &clients[ind].amt_wood, sizeof(uint8_t));
		send_offset += sizeof(uint8_t);

		SendData(ind, send_data, send_offset, FLAG_WOOD_UPDATE);
	}

	uint16_t send_offset = 0;
	uint8_t send_data[MAX_PACKET];

	uint32_t time_left;
	if(clients[ind].questing) {
		time_left = WOOD_WAIT_TIME-(SDL_GetTicks()-clients[ind].timer_wood);
	} else time_left = 0;

	memcpy(send_data+send_offset, &time_left, sizeof(uint32_t));
	send_offset += sizeof(uint32_t);

	SendData(ind, send_data, send_offset, FLAG_WOOD_GETTIME);
}

And for the final server touch we’ll update the code which runs if there are sockets ready to be processed. Notice that how we handle the server socket (for new connections) isn’t changing but how we handle the client sockets is changing.

if(SDLNet_SocketReady(server_socket)) {
	int got_socket = AcceptSocket(next_ind);
	if(!got_socket) {
		num_rdy--;
		continue;
	}

	// NOTE: get a new index
	int chk_count;
	for(chk_count=0; chk_count<MAX_SOCKETS; ++chk_count) {
		if(sockets[(next_ind+chk_count)%MAX_SOCKETS] == NULL) break;
	}

	next_ind = (next_ind+chk_count)%MAX_SOCKETS;
	printf("DB: new connection (next_ind = %d)n", next_ind);

	num_rdy--;
}

int ind;
for(ind=0; (ind<MAX_SOCKETS) && num_rdy; ++ind) {
	if(sockets[ind] == NULL) continue;
	if(!SDLNet_SocketReady(sockets[ind])) continue;

	uint8_t* data;
	uint16_t length;
				
	data = RecvData(ind, &length);
	if(data == NULL) {
		num_rdy--;
		continue;
	}

	int num_processed = 0;

	uint16_t offset = 0;
	while(offset < length) {
		num_processed++;
		ProcessData(ind, data, &offset);
	}

	printf("num_processed from ID: %d was %dn", ind, num_processed);
	fflush(stdout);

	free(data);
	num_rdy--;
}

I am also keeping track of the number of packets which the server processes for each of the ready clients for debugging purposes. Now the client-side stuff isn’t that different when it comes to the network side of things. A lot of the helper functions we have server-side will have analogous client-server versions. Here is the client-side code in its entirety:

/*
*/

#include "AdBase.h"
#include "AdLevel.h"
#include "AdScreen.h"
#include "AdSpriteManager.h"

//-----------------------------------------------------------------------------
#define MAX_PACKET 0xFF

//-----------------------------------------------------------------------------
#define FLAG_QUIT 0x0000
#define FLAG_WOOD_QUEST 0x0011
#define FLAG_WOOD_UPDATE 0x0010
#define FLAG_WOOD_GETTIME 0x0012

//-----------------------------------------------------------------------------
TCPsocket socket;
SDLNet_SocketSet socket_set;

//-----------------------------------------------------------------------------
int questing;
uint8_t amt_wood;
uint32_t timer_wood;

//-----------------------------------------------------------------------------
void CloseSocket(void) {
	if(SDLNet_TCP_DelSocket(socket_set, socket) == -1) {
		fprintf(stderr, "%sn", SDLNet_GetError());
		system("pause");
		exit(-1);
	}

	SDLNet_FreeSocketSet(socket_set);
	SDLNet_TCP_Close(socket);
}

//-----------------------------------------------------------------------------
void SendData(uint8_t* data, uint16_t length, uint16_t flag) {
	uint8_t temp_data[MAX_PACKET];

	int offset = 0;
	memcpy(temp_data+offset, &flag, sizeof(uint16_t));
	offset += sizeof(uint16_t);
	memcpy(temp_data+offset, data, length);
	offset += length;

	int num_sent = SDLNet_TCP_Send(socket, temp_data, offset);
	if(num_sent < offset) {
		fprintf(stderr, "ER: SDLNet_TCP_Send: %sn", SDLNet_GetError());
		CloseSocket();
	}
}

//-----------------------------------------------------------------------------
uint8_t* RecvData(uint16_t* length) {
	uint8_t temp_data[MAX_PACKET];
	int num_recv = SDLNet_TCP_Recv(socket, temp_data, MAX_PACKET);

	if(num_recv <= 0) {
		CloseSocket();

		const char* err = SDLNet_GetError();
		if(strlen(err) == 0) {
			printf("DB: server shutdownn");
		} else {
			fprintf(stderr, "ER: SDLNet_TCP_Recv: %sn", err);
		}

		return NULL;
	} else {
		*length = num_recv;

		uint8_t* data = (uint8_t*) malloc(num_recv*sizeof(uint8_t));
		memcpy(data, temp_data, num_recv);

		return data;
	}
}

//-----------------------------------------------------------------------------
void ProcessData(uint8_t* data, uint16_t* offset) {
	if(data == NULL) return;

	uint16_t flag = *(uint16_t*) &data[*offset];
	*offset += sizeof(uint16_t);

	switch(flag) {
		case FLAG_WOOD_UPDATE: {
			amt_wood = *(uint8_t*) &data[*offset];
			*offset += sizeof(uint8_t);
		} break;

		case FLAG_WOOD_GETTIME: {
			timer_wood = *(uint32_t*) &data[*offset];
			*offset += sizeof(uint32_t);
		} break;

		case FLAG_WOOD_QUEST: {
			// NOTE: quest completed
			questing = 0;
		} break;
	}
}

//-----------------------------------------------------------------------------
void InitNetwork(const char* pIP, int iPort) {
	IPaddress ip;
	if(SDLNet_ResolveHost(&ip, pIP, iPort) == -1) {
		fprintf(stderr, "%sn", SDLNet_GetError());
		system("pause");
		exit(-1);
	}

	socket = SDLNet_TCP_Open(&ip);
	if(socket == NULL) {
		fprintf(stderr, "%sn", SDLNet_GetError());
		system("pause");
		exit(-1);
	}

	socket_set = SDLNet_AllocSocketSet(1);
	if(socket_set == NULL) {
		fprintf(stderr, "%sn", SDLNet_GetError());
		system("pause");
		exit(-1);
	}

	if(SDLNet_TCP_AddSocket(socket_set, socket) == -1) {
		fprintf(stderr, "%sn", SDLNet_GetError());
		system("pause");
		exit(-1);
	}
}

//-----------------------------------------------------------------------------
bool CheckSocket(void) {
	if(SDLNet_CheckSockets(socket_set, 0) == -1) {
		fprintf(stderr, "%sn", SDLNet_GetError());
		system("pause");
		exit(-1);
	}

	return SDLNet_SocketReady(socket);
}

//-----------------------------------------------------------------------------
int SDL_main(int argc, char* argv[]) {
	if(AdBase::Init(8*40, 8*30, 3) == false) {
		fprintf(stderr, "ERROR: Failed to initiate.n");
		system("pause");
		return -1;
	}

	// TESTING
	InitNetwork("PUT_SERVER_IP_HERE!!!", 8099); // arg #2 is the port

	AdLevel* testLvl = new AdLevel();
	//

	SDL_Event sdlEvent = {};
	while(sdlEvent.type != SDL_QUIT) {
		SDL_PollEvent(&sdlEvent);

		AdScreen::Clear();

		// TESTING
		if(CheckSocket()) {
			uint16_t length, flag;
			uint8_t* data = RecvData(&length);

			uint16_t offset = 0;
			while(offset < length) {
				ProcessData(data, &offset);
			}

			free(data);
		}
		//

		// TESTING
		testLvl->Update(&sdlEvent);

		char string[0xFF];
		sprintf(string, "Wood: %d", amt_wood);

		SDL_Point pnt1 = {0, 0};
		SDL_Color color1 = {0xFF, 0x00, 0x00, 0x00};
		SDL_Surface* pSurf = AdSpriteManager::BuildSprite(string, color1);
		AdScreen::DrawSprite(pnt1, pSurf);
		SDL_FreeSurface(pSurf);

		pnt1.y += 8;
		sprintf(string, "Timer: %d", (int) ceil((double)timer_wood/1000.0f));
		pSurf = AdSpriteManager::BuildSprite(string, color1);
		AdScreen::DrawSprite(pnt1, pSurf);
		SDL_FreeSurface(pSurf);
		//

		//
		if(timer_wood == 0) {
			SDL_Point pnt2 = {128, 8};
			SDL_Rect rec2 = {pnt2.x, pnt2.y, 5*8, 8};

			if(
				testLvl->m_iMouseX>=rec2.x && testLvl->m_iMouseX<=(rec2.x+rec2.w) &&
				testLvl->m_iMouseY>=rec2.y && testLvl->m_iMouseY<=(rec2.y+rec2.h)
			) {
				SDL_Color color2 = {0x00, 0xFF, 0x00, 0x00};
				pSurf = AdSpriteManager::BuildSprite("Quest", color2);

				if(testLvl->m_bMouseLeft && !questing) {
					questing = 1;
					SendData(NULL, 0, FLAG_WOOD_QUEST);
				}
			} else {
				SDL_Color color2 = {0x00, 0x00, 0xFF, 0x00};
				pSurf = AdSpriteManager::BuildSprite("Quest", color2);
			}
			
			AdScreen::DrawSprite(pnt2, pSurf);
			SDL_FreeSurface(pSurf);
		}
		//

		AdScreen::Present();
	}

	// TESTING
	delete testLvl;

	SendData(NULL, 0, FLAG_QUIT);
	CloseSocket();
	//

	AdBase::Quit();

	return 0;
}

All the SDL graphics processing and input handling is being wrapped with the custom files I wrote and linked at the beginning of this post. If you find these posts helpful or would like me to dive deeper into a particular topic covered here please send me an email at “info@stephenmeier.net”.

SDL 2.0 Tutorial-05: Networking 2

In the last tutorial we finished with a hallow if-statement within our processing loop.

int running = 1;
while(running) {
	int num_rdy = SDLNet_CheckSockets(socket_set, 1000);

	if(num_rdy <= 0) {
		// NOTE: none of the sockets are ready
	} else {
		// NOTE: some number of the sockets are ready
	}
}

If there aren’t any sockets which we need to process (i.e. num_rdy <= 0 ) then we’ll loop over all the currently connected client sockets and perform any tasks which may be pending. This is basically the server’s chance to catch up with game state and perform any security checks. This will work well for a simple TCP game where the server and client need not be in sync the entire time. For something more real-time we would prioritize game state and perhaps use UDP instead.

For now let’s add four wood to the resource count for each connected client.

int ind;
for(ind=0; ind<MAX_SOCKETS; ++ind) {
	if(!clients[ind].in_use) continue;
	clients[ind].amt_wood += 4;
}

Remember this will run every second assuming there aren’t any sockets to be processed (i.e. num_rdy <= 0 ). Why am I saying it’ll run every second? It is because of the following function call:

SDLNet_CheckSockets(socket_set, 1000);

The second argument to this function is 1000, which is the number of milliseconds this function will block while waiting for a socket connection.

Now, if there are sockets ready to be processed we’ll have to check whether it is a client socket or the server socket. If the server socket is ready then we likely have a new client connection. For the client connections we’ll have to check each one until we find the ones which are ready and then process the data that we have received from them.

First let’s handle the case when the server socket is ready:

if(SDLNet_SocketReady(server_socket)) {
	int got_socket = AcceptSocket(next_ind);
	if(!got_socket) {
		num_rdy--;
		continue;
	}

	// NOTE: get a new index
	int chk_count;
	for(chk_count=0; chk_count<MAX_SOCKETS; ++chk_count) {
		if(sockets[(next_ind+chk_count)%MAX_SOCKETS] == NULL) break;
	}

	next_ind = (next_ind+chk_count)%MAX_SOCKETS;
	printf("DB: new connection (next_ind = %d)n", next_ind);

	num_rdy--;
}

Here the function int AcceptSocket(int index); is a shorthand for accepting a new socket connection using sockets[index] and clients[index] . This function returns whether or not the connection was successfully accepted. Here is the function itself:

int AcceptSocket(int index) {
	if(sockets[index]) {
		fprintf(stderr, "ER: Overriding socket at index %d.n", index);
		CloseSocket(index);
	}

	sockets[index] = SDLNet_TCP_Accept(server_socket);
	if(sockets[index] == NULL) return 0;

	clients[index].in_use = 1;
	if(SDLNet_TCP_AddSocket(socket_set, sockets[index]) == -1) {
		fprintf(stderr, "ER: SDLNet_TCP_AddSocket: %sn", SDLNet_GetError());
		exit(-1);
	}

	return 1;
}

The function void CloseSocket(int index); was presented in the previous part of this tutorial. After we accept the new client connection we also update the next_ind variable so that we are ready for the next client connection. As it’s currently written, if a new client connects when the server is full then another client will be kicked to make room for the new connection.

Now let’s process any clients which might have data for us.

int ind;
for(ind=0; (ind<MAX_SOCKETS) && num_rdy; ++ind) {
	if(sockets[ind] == NULL) continue;
	if(!SDLNet_SocketReady(sockets[ind])) continue;

	uint8_t* data;
	uint16_t flag;
	uint16_t length;
				
	data = RecvData(ind, &length, &flag);
	if(data == NULL) {
		num_rdy--;
		continue;
	}

	switch(flag) {
		case FLAG_WOOD_UPDATE: {
			uint16_t offset = 0;
			uint8_t send_data[MAX_PACKET];

			memcpy(send_data+offset, &clients[ind].amt_wood, sizeof(uint8_t));
			offset += sizeof(uint8_t);

			SendData(ind, send_data, offset, FLAG_WOOD_UPDATE);
		} break;

		case FLAG_QUIT: {
			running = 0;
			printf("DB: shutdown by client id: %dn", ind);
		} break;
	}

	free(data);
	num_rdy--;
}

What we are doing here is looping over all the client sockets and checking to see whether a client is connected and whether that connected client has some information ready for us. We also want to make sure that we aren’t working harder than we have to by breaking out of the for-loop when num_rdy == 0 ; because at that point we have processed all the ready client sockets so there is no point in checking any remaining client connections for information.

Once we know that we have a connected socket and that the socket have information ready for us, we fill a buffer with the data they sent us. This is what the uint8_t* RecvData(int index, uint16_t* length, uint16_t* flag); function does for us:

uint8_t* RecvData(int index, uint16_t* length, uint16_t* flag) {
	uint8_t temp_data[MAX_PACKET];
	int num_recv = SDLNet_TCP_Recv(sockets[index], temp_data, MAX_PACKET);

	if(num_recv <= 0) {
		CloseSocket(index);
		const char* err = SDLNet_GetError();
		if(strlen(err) == 0) {
			printf("DB: client disconnectedn");
		} else {
			fprintf(stderr, "ER: SDLNet_TCP_Recv: %sn", err);
		}

		return NULL;
	} else {
		int offset = 0;
		*flag = *(uint16_t*) &temp_data[offset];
		offset += sizeof(uint16_t);

		*length = (num_recv-offset);

		uint8_t* data = (uint8_t*) malloc((num_recv-offset)*sizeof(uint8_t));
		memcpy(data, &temp_data[offset], (num_recv-offset));

		return data;
	}
}

This function gets data from sockets[index] and returns a pointer to a new buffer. The in/out function arguments length and flag are the length of the newly allocated buffer and the packet flag for the information we are receiving from the client.

If the client sends us a request for the amount of wood they current possess (i.e. FLAG_WOOD_UPDATE ) then we reply by sending them a packet with the information they are requesting. To send information to the connected client we use the following shorthand function:

void SendData(int index, uint8_t* data, uint16_t length, uint16_t flag) {
	uint8_t temp_data[MAX_PACKET];

	int offset = 0;
	memcpy(temp_data+offset, &flag, sizeof(uint16_t));
	offset += sizeof(uint16_t);
	memcpy(temp_data+offset, data, length);
	offset += length;

	int num_sent = SDLNet_TCP_Send(sockets[index], temp_data, offset);
	if(num_sent < offset) {
		fprintf(stderr, "ER: SDLNet_TCP_Send: %sn", SDLNet_GetError());
		CloseSocket(index);
	}
}

For debugging purposes the client can also shutdown the server with a FLAG_QUIT . In the next tutorial we will write a simple client which will display the amount of wood the client current has. This amount will continue to increase because the server currently adds four wood each time it enters idle processing. Once we have achieved this we’ll add a “quest” button and timer on the client-side and a few more packet flags allowing the client to request the amount of time left on a quest and to start a quest. The server will check for when a quest completes and will increase the amount of wood for that client as a result.

Till next time!

SDL 2.0 Tutorial-04: Networking 1

For the next few posts I plan to run through some of the basics of TCP communication using the SDLNet library. The goal here is to write a simple “resource” game where the client sends a “quest” request and, after some amount of time has passed, the quest is considered complete and the client’s resource count increases a little. This series will provide an excellent setup for developing a player driven economy type game.

Our first goal will be to write a server which manages all the client connections. To keep things simple we will be working with one single resource; here we are using wood as the resource. We’ll start with a few #define ‘s

//-----------------------------------------------------------------------------
#define MAX_PACKET 0xFF
#define MAX_SOCKETS 0x10

//-----------------------------------------------------------------------------
#define WOOD_WAIT_TIME 5000

//-----------------------------------------------------------------------------
#define FLAG_QUIT 0x0000
#define FLAG_WOOD_UPDATE 0x0010

Here we have three different type of #define ‘s

  • MAX_PACKET and MAX_SOCKETS define the maximum size of the TCP packets we are willing to process and the maximum number of clients we will allow online at once.
  • WOOD_WAIT_TIME is the amount of time in milliseconds to wait before the quest for obtaining additional wood units is complete.
  • FLAG_QUIT and FLAG_WOOD_UPDATE are 16 bit packet flags used to determine the type of packet we are receiving and we then process the rest of the packet’s data based on this flag.

Next we need a structure which we can use to keep all the client’s information separate from one another.

typedef struct {
	int in_use;
	int questing;
	uint8_t amt_wood;
	uint32_t timer_wood;
} Client;

Here we have a few important variables which we need in order to efficiently work with each of the clients which connect to the server. in_use allows us to determine whether the current client is being used by an active socket connection or not. This will prevent us from running checks on client structures which don’t have corresponding network connections. questing is a simple boolean which tells us whether the current client is running a wood fetching quest or not. amt_wood is the amount of the wood resource which the current client has. timer_wood is a timestamp for when the wood fetching quest for this particular client was started – this will help us determine when the wood fetching quest has been completed.

Next we need some global state variables which will make writing our little server a bit easier.

int next_ind = 0;
TCPsocket server_socket;
Client clients[MAX_SOCKETS];
SDLNet_SocketSet socket_set;
TCPsocket sockets[MAX_SOCKETS];

next_ind is the index into the sockets and clients arrays for the next player that will connect to the server. When one client connects next_ind gets updated so that we never overwrite a connected clients information.

First thing we do in int main(int argc, char** argv); is initialize a few SDL components and the SDLNet library.

if(SDL_Init(SDL_INIT_TIMER|SDL_INIT_EVENTS) != 0) {
	fprintf(stderr, "ER: SDL_Init: %sn", SDL_GetError());
	exit(-1);
}

if(SDLNet_Init() == -1) {
	fprintf(stderr, "ER: SDLNet_Init: %sn", SDLNet_GetError());
	exit(-1);
}

Next we’ll want to open the server’s TCP socket on a specific port, here I’m using port 8099. Any function specific information can be found within the SDLNet online API.

IPaddress ip;
if(SDLNet_ResolveHost(&ip, NULL, 8099) == -1) {
	fprintf(stderr, "ER: SDLNet_ResolveHost: %sn", SDLNet_GetError());
	exit(-1);
}

server_socket = SDLNet_TCP_Open(&ip);
if(server_socket == NULL) {
	fprintf(stderr, "ER: SDLNet_TCP_Open: %sn", SDLNet_GetError());
	exit(-1);
}

Finally, before the main processing loop, we will setup an SDLNet socket set and add the server_socket to the socket set so that we may be informed when a net connection is looking to be accepted by the server.

socket_set = SDLNet_AllocSocketSet(MAX_SOCKETS+1);
if(socket_set == NULL) {
	fprintf(stderr, "ER: SDLNet_AllocSocketSet: %sn", SDLNet_GetError());
	exit(-1);
}

if(SDLNet_TCP_AddSocket(socket_set, server_socket) == -1) {
	fprintf(stderr, "ER: SDLNet_TCP_AddSocket: %sn", SDLNet_GetError());
	exit(-1);
}

It’s worth noting that the size of the socket set is MAX_SOCKETS+1 , the plus one accounts for the addition of the server socket. The main processing loop is setup like the following (currently hollow):

int running = 1;
while(running) {
	int num_rdy = SDLNet_CheckSockets(socket_set, 1000);

	if(num_rdy <= 0) {
		// NOTE: none of the sockets are ready
	} else {
		// NOTE: some number of the sockets are ready
	}
}

While the server is running we check for any sockets from the socket set which might have packets for us to process; we check the socket set for a second (1000 milliseconds). If there aren’t any sockets ready to be processed then we use this time to run other various server activities like checking is quests have been completed, etc. If there is some number of sockets ready to be processed then we process those sockets.

In the next tutorial we will fill in these two parts of the if-statement above; this is where the real magic is! To clean up everything after the server is done running we will run the following code.

if(SDLNet_TCP_DelSocket(socket_set, server_socket) == -1) {
	fprintf(stderr, "ER: SDLNet_TCP_DelSocket: %sn", SDLNet_GetError());
	exit(-1);
} SDLNet_TCP_Close(server_socket);

int i;
for(i=0; i<MAX_SOCKETS; ++i) {
	if(sockets[i] == NULL) continue;
	CloseSocket(i);
}

SDLNet_FreeSocketSet(socket_set);
SDLNet_Quit();
SDL_Quit();

The missing piece here is the void CloseSocket(int index); function which is basically just a shorthand for closing a client socket and is stated below:

void CloseSocket(int index) {
	if(sockets[index] == NULL) {
		fprintf(stderr, "ER: Attempted to delete a NULL socket.n");
		return;
	}

	if(SDLNet_TCP_DelSocket(socket_set, sockets[index]) == -1) {
		fprintf(stderr, "ER: SDLNet_TCP_DelSocket: %sn", SDLNet_GetError());
		exit(-1);
	}

	memset(&clients[index], 0x00, sizeof(Client));
	SDLNet_TCP_Close(sockets[index]);
	sockets[index] = NULL;
}

For the next tutorial we’ll pick up from here and fill in the if-statement mentioned earlier. Once the server-side code has been setup to process a simple player request to perform a quest to gather wood, we’ll start working on a simple client-side quest button, quest timer, and resource counter. If there is time or interest we will add persistence to the resource count for each player using SQLite.