Why do thread functions need to be declared as '__cdecl'?

Question

Sample code that shows how to create threads using MFC declares the thread function as both static and __cdecl. Why is the latter required? Boost threads don't bother with this convention, so is it just an anachronism?

For example (MFC):

static __cdecl UINT MyFunc(LPVOID pParam)
{
...
}

CWinThread* pThread = AfxBeginThread(MyFunc, ...);

Whereas Boost:

static void func()
{
...
}

boost::thread t;
t.create(&func);

(the code samples might not be 100% correct as I am nowhere near an IDE).

What is the point of __cdecl? How does it help when creating threads?

Answer 1

__cdecl tells the compiler to use the C calling convention (as opposed to the stdcall, fastcall or whatever other calling convention your compiler supports). I believe, VC++ uses stdcall by default.

The calling convention affects things such as how arguments are pushed onto the stack (or registers, in the case of fastcall) and who pops arguments off the stack (caller or callee).

In the case of Boost. I believe it uses template specialization to figure out the appropriate function type and calling convention.

Answer 2

Look at the prototype for AfxBeginThread():

CWinThread* AfxBeginThread(
   AFX_THREADPROC pfnThreadProc,
   LPVOID pParam,
   int nPriority = THREAD_PRIORITY_NORMAL,
   UINT nStackSize = 0,
   DWORD dwCreateFlags = 0,
   LPSECURITY_ATTRIBUTES lpSecurityAttrs = NULL 
);

AFX_THREADPROC is a typedef for UINT(AFX_CDECL*)(LPVOID). When you pass a function to AfxBeginThread(), it must match that prototype, including the calling convention.

The MSDN pages on __cdecl and __stdcall (as well as __fastcall and __thiscall) explain the pros and cons of each calling convention.

The boost::thread constructor uses templates to allow you to pass a function pointer or callable function object, so it doesn't have the same restrictions as MFC.