CHAPTER 64. ARGUMENTS PASSING METHODS (CALLING CONVENTIONS) CHAPTER 64. ARGUMENTS PASSING METHODS (CALLING CONVENTIONS)

Chapter 64

Arguments passing methods (calling conventions)

64.1 cdecl

This is the most popular method for passing arguments to functions in the C/C++ languages.

The glscaller also must return the value of thestack pointer(ESP) to its initial state after thecalleefunction exits.

Listing 64.1: cdecl

push arg3

push arg2

push arg1

call function

add esp, 12 ; returns ESP

64.2 stdcall

It’s almost the same ascdecl, with the exception that thecalleemust setESPto the initial state by executing theRET x

instruction instead ofRET, wherex = arguments number * sizeof(int)^1. Thecalleris not adjusting thestack

pointer, there are noadd esp, xinstruction.

Listing 64.2: stdcall

push arg3

push arg2

push arg1

call function

function:

... do something ...

ret 12

The method is ubiquitous in win32 standard libraries, but not in win64 (see below about win64).

For example, we can take the function from8.1 on page 88and change it slightly by adding the__stdcallmodifier:

int __stdcall f2 (int a, int b, int c)

{

return a*b+c;

};

It is to be compiled in almost the same way as8.2 on page 88, but you will seeRET 12instead ofRET.SPis not update in

thecaller.

As a consequence, the number of function arguments can be easily deduced from theRETN ninstruction: just dividenby

4.

Listing 64.3: MSVC 2010

_a$ = 8 ; size = 4

(^1) The size of aninttype variable is 4 in x86 systems and 8 in x64 systems