Winsock Tutorial 5

Error handling with Winsock

Previously we purposely omitted too much detail in error handling so as not to overwhelm newcomers. Now we examine error handling in greater detail. When we are finished, we should be able to troubleshoot our applications so we can pinpoint where a problem may exist with greater efficiency.


For this tutorial we will be working with the final code used back in tutorial 2 (the blocking server).

Prerequisites

Project type: Console
Include files: winsock2.h
Library files: ws2_32.lib

Error Handling

You would remember in our previous tutorials that first we must initialize Winsock. What if this fails for some reason? How would you know what went wrong?

Fortunately, if WSAStartup fails, it will return a Winsock error code. A list of winsock error codes can be found here and also at the bottom of this tutorial for your convenience.

If we modify our Winsock initialization code slightly...

WSADATA WsaDat;
int nResult=WSAStartup(MAKEWORD(2,2),&WsaDat);
if(nResult!=0)
{
	std::cout<<"WSA Initialization failed: "<<nResult<"\r\n";
	WSACleanup();
	system("PAUSE");
	return 0;
}

...we will be able to catch the reason (if any) for our initialization failure.

We can test this by inducing an error into our code. Try changing the following line containing the WSAStartup() to this;

int nResult=WSAStartup(MAKEWORD(0,0),&WsaDat);
We have accidentaly instructed winsock to operate using version 0.0 within the wsock32.dll at runtime. This version doesn't exist. So, if we run the application it will report the following error in our console.

WSA Initialization failed: 10092
Press any key to continue . . .

The error code 10092 translates to WSAVERNOTSUPPORTED. This means, the version we are trying to use doesn't exist. Which is the error we intended to get, with our little test.


Once we are happy that Winsock has initialized correctly, we can now use a handy function to address any further failures.

WSAGetLastError()
The next step was to set up a socket. Again we can modify our code (slightly) to display why socket creation has failed.

SOCKET Socket=socket(AF_INET,SOCK_STREAM,IPPROTO_TCP);
if(Socket==INVALID_SOCKET)
{
	int nError=WSAGetLastError();
	std::cout<<"Socket creation failed: "<<nError<<"\r\n";
	WSACleanup();
	system("PAUSE");
	return 0;
}

When we attempt to create our socket the socket will be checked to see if it is valid. If it is not valid we can use WSAGetLastError() to find what has gone amiss.

We can, once again, test this to see that it actually works. Change the following line to;

SOCKET Socket=socket(AF_INET,SOCK_STREAM,IPPROTO_UDP);

What we have done here is mix and match TCP/IP elements (SOCK_STREAM) and UDP elements (IPPROTO_UDP), which is a big no no. It just wont work like that. We will explore UDP in later tutorials.

Running the application with this change will give you error 10043 (WSAEPROTONOSUPPORT) which means 'protocol not supported'. As we expected.

Now we can catch any errors during socket creation. Great!

We chose to go with the server code in this tutorial as there are more things to go wrong, as you would appreciate. After filling out our SOCKADDR_IN struct (refer to tutorial 2) we bind the socket, remember?

If we add the same error checking code we get this;

if(bind(Socket,(SOCKADDR*)(&serverInf),sizeof(serverInf))==SOCKET_ERROR)
{
	int nError=WSAGetLastError();
	std::cout<<"Unable to bind socket: "<<nError<<"\r\n";
	WSACleanup();
	system("PAUSE");
	return 0;
}

We might get 'bind' problems if there is another service using the same port, for example. Don't believe me? Run two instances of the server (with our updated code) and see what happens.

Error 10048 (WSAEADDRINUSE) right? You cant have two servers using the same port number, as we have just found out!

As you see WSAGetLastError() is an essential function to use, if you want to know why your application might be failing.

You should now begining to see a pattern forming with our error checking. Try to modify the rest of the code from tutorial 2 to check for potential problems with the 'listen' and 'shutdown' commands.

Otherwise you can skip to the full source code below.

The Full Code

#include <iostream>
#include <winsock2.h>
#pragma comment(lib,"ws2_32.lib")

int main()
{
	WSADATA WsaDat;
	int nResult=WSAStartup(MAKEWORD(2,2),&WsaDat);
	if(nResult!=0)
	{
		std::cout<<"WSA Initialization failed: "<<nResult<<"\r\n";
		WSACleanup();
		system("PAUSE");
		return 0;
	}

	SOCKET Socket=socket(AF_INET,SOCK_STREAM,IPPROTO_TCP);
	if(Socket==INVALID_SOCKET)
	{
		int nError=WSAGetLastError();
		std::cout<<"Socket creation failed: "<<nError<<"\r\n";
		WSACleanup();
		system("PAUSE");
		return 0;
	}

	SOCKADDR_IN serverInf;
	serverInf.sin_family=AF_INET;
	serverInf.sin_addr.s_addr=INADDR_ANY;
	serverInf.sin_port=htons(8888);

	if(bind(Socket,(SOCKADDR*)(&serverInf),sizeof(serverInf))==SOCKET_ERROR)
	{
		int nError=WSAGetLastError();
		std::cout<<"Unable to bind socket: "<<nError<<"\r\n";
		WSACleanup();
		system("PAUSE");
		return 0;
	}

	if(listen(Socket,1)==SOCKET_ERROR)
	{
		int nError=WSAGetLastError();
		std::cout<<"Unable to listen: "<<nError<<"\r\n";
		WSACleanup();
		system("PAUSE");
		return 0;
	}

	SOCKET TempSock=SOCKET_ERROR;
	while(TempSock==SOCKET_ERROR)
	{
		std::cout<<"Waiting for incoming connections...\r\n";
		TempSock=accept(Socket,NULL,NULL);
	}
	Socket=TempSock;
	std::cout<<"Client connected!\r\n\r\n";

	char *szMessage="Welcome to the server!\r\n";
	send(Socket,szMessage,strlen(szMessage),0);

	// Shutdown our socket
	if(shutdown(Socket,SD_SEND)==SOCKET_ERROR)
	{
		int nError=WSAGetLastError();
		std::cout<<"Unable to shutdown socket: "<<nError<<"\r\n";
		closesocket(Socket);
		WSACleanup();
		system("PAUSE");
		return 0;
	}

	// Close our socket
	closesocket(Socket);

	// Cleanup Winsock
	WSACleanup();
	system("PAUSE");
	return 0;
}

Things to try

Try thinking of other ways to find errors and handle them. Some errors do not always mean death for the program. So, quitting (the application) at the first sign of trouble may not be the answer.

Winsock error codes

10004	WSAEINTR	Interrupted function call
10009	WSAEBADF	WSAEBADF
10013	WSAEACCES	WSAEACCES
10014	WSAEFAULT	Bad address
10022	WSAEINVAL	Invalid argument
10024	WSAEMFILE	Too many open files
10035	WSAEWOULDBLOCK	Operation would block
10036	WSAEINPROGRESS	Operation now in progress
10037	WSAEALREADY	Operation already in progress
10038	WSAENOTSOCK	Socket operation on non-socket
10039	WSAEDESTADDRREQ	Destination address required
10040	WSAEMSGSIZE	Message too long
10041	WSAEPROTOTYPE	Protocol wrong type for socket
10042	WSAENOPROTOOPT	Bad protocol option
10043	WSAEPROTONOSUPPORT	Protocol not supported
10044	WSAESOCKTNOSUPPORT	Socket type not supported
10045	WSAEOPNOTSUPP	Operation not supported
10046	WSAEPFNOSUPPORT	Protocol family not supported
10047	WSAEAFNOSUPPORT	Address family not supported by protocol family
10048	WSAEADDRINUSE	Address already in use
10049	WSAEADDRNOTAVAIL	Cannot assign requested address
10050	WSAENETDOWN	Network is down
10051	WSAENETUNREACH	Network is unreachable
10052	WSAENETRESET	Network dropped connection on reset
10053	WSAECONNABORTED	Software caused connection abort
10054	WSAECONNRESET	Connection reset by peer
10055	WSAENOBUFS	No buffer space available
10056	WSAEISCONN	Socket is already connected
10057	WSAENOTCONN	Socket is not connected
10058	WSAESHUTDOWN	Cannot send after socket shutdown
10059	WSAETOOMANYREFS	WSAETOOMANYREFS
10060	WSAETIMEDOUT	Connection timed out
10061	WSAECONNREFUSED	Connection refused
10062	WSAELOOP	WSAELOOP
10063	WSAENAMETOOLONG	WSAENAMETOOLONG
10064	WSAEHOSTDOWN	Host is down
10065	WSAEHOSTUNREACH	No route to host
10066	WSAENOTEMPTY	WSAENOTEMPTY
10067	WSAEPROCLIM	Too many processes
10068	WSAEUSERS	WSAEUSERS
10069	WSAEDQUOT	WSAEDQUOT
10070	WSAESTALE	WSAESTALE
10071	WSAEREMOTE	WSAEREMOTE
10091	WSASYSNOTREADY	Network subsystem is unavailable
10092	WSAVERNOTSUPPORTED	WINSOCK.DLL version out of range
10093	WSANOTINITIALISED	Successful WSAStartup() not yet performed
10101	WSAEDISCON	WSAEDISCON
10102	WSAENOMORE	WSAENOMORE
10103	WSAECANCELLED	WSAECANCELLED
10104	WSAEINVALIDPROCTABLE	WSAEINVALIDPROCTABLE
10105	WSAEINVALIDPROVIDER	WSAEINVALIDPROVIDER
10106	WSAEPROVIDERFAILEDINIT	WSAEPROVIDERFAILEDINIT
10107	WSASYSCALLFAILURE	WSASYSCALLFAILURE
10108	WSASERVICE_NOT_FOUND	WSASERVICE_NOT_FOUND
10109	WSATYPE_NOT_FOUND	WSATYPE_NOT_FOUND
10110	WSA_E_NO_MORE	WSA_E_NO_MORE
10111	WSA_E_CANCELLED	WSA_E_CANCELLED
10112	WSAEREFUSED	WSAEREFUSED
11001	WSAHOST_NOT_FOUND	Host not found
11002	WSATRY_AGAIN	Non-authoritative host not found
11003	WSANO_RECOVERY	This is a non-recoverable error
11004	WSANO_DATA	Valid name, no data record of requested type

Next tutorial

Tutorial 6 - Async sockets in TCP/IP (The Client)

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