Calling Convention – Part II (__stdcall)
Make sure you have read “Calling Convention – Part I” of this article.
Standard calling convention ( __stdcall )
This convention is usually used to call Win32 API functions.
WINAPI is nothing but another name for
#define WINAPI __stdcall
We can explicitly declare a function to use the
int __stdcall Add( int nValue1, int nValue2 );
The main characteristics of
__stdcall calling convention are:
- Arguments are passed from right to left, and placed on the stack.
- Stack cleanup is performed by the called function.
Function Name Decoration For stdcall
Function name is decorated by prefixing an underscore character ‘_’ and postfixing a ‘@’ character and number of bytes of stack space required by the arguments at end of the function name.
[email protected] //underscore before function name & @ and number of bytes space required on stack
Now, take a look at an example of a __stdcall call:
; // push arguments to the stack, from right to left push 3 push 2 ; // call the function call [email protected] ; // copy the return value from EAX to a local variable (int nResult) mov DWORD PTR [nResult], eax
The called function is shown below:
; // function prolog push ebp mov ebp, esp push ebx push esi push edi ; // return nValue1 + nValue2; mov eax, DWORD PTR [nValue1] add eax, DWORD PTR [nValue2] ; // function epilog pop ebx pop esi pop edi mov esp, ebp pop ebp ;//Stack cleanup and return ret 8
__stdcall is default calling convention for Win32 API’s.
__stdcall calling convention is that it creates smaller executables than
__cdecl, in which the code for stack cleanup will be cleaned by called function.
Disadvantage of __stdcall calling convention is that functions with variable number of arguments (like
printf()) can’t use __stdcall. Instead they must use
__cdecl, because it is the only calling convention who knows the number of arguments in each function call; therefore only the caller can perform the stack cleanup.