Question

I’m trying to get a UDP connection working for a game I’m making, the input for the socket is read via a second thread, so the game can continue running while it’s waiting for messages. I got it to work in a separate project, but when porting it over to my game code I get an Unresolved External Symbol error:

LNK2019: unresolved external symbol “void __cdecl receiveThread(void *)” (?receiveThread@@YAXPAX@Z) referenced in function “public: bool __thiscall Network::setupServerUDP(void)” (?setupServerUDP@Network@@QAE_NXZ)).

I’ve googled this and everyone says the Runtime Library should be set to Multi-threaded, which I did (In MVS 2010 I don’t think it’s even possible to set it to single-threaded) but it doesn’t resolve the error.
I’m sure it’s just some stupid option set to the wrong thing somewhere. But I just can’t think of what it is.

I’ve included ws2tcpip.h and I’ve linked to the correct library: #pragma comment(lib, “Ws2_32.lib”).

The only difference between my test project and the game project is that the test project was a Console application and the other is a Windows application, but I don’t see why that should be any problem. Thoughts?

Thanks for your help!

Cheers,

Maxim

Answer 1

The unresolved function is not _beginthread, it’s receiveThread. That’s your function – the thread entry point that you provide as a parameter to _beginthread. Did you write that function with the right calling convention? Is it really cdecl? If you declare it as cdecl but implement without an explicit calling convention, that would cause a link error like this. For example, this code would cause a link error:

extern "C" { void receiveThread(void*); } //There's a cdecl function somewhere

//...

_beginthread(receiveThread, 0, 0); //Let's refer to it

//...

void receiveThread(void *p) //But this is not it! It's stdcall.
{
     //Do something
}

To fix, make sure your implementation of receiveThread uses the cdecl convention or is declared as extern "C".

For the record, _beginthreadex expects a stdcall function, not cdecl. With _beginthreadex, you don’t have to worry about conventions.