; // function prolog
push ebp
mov ebp,esp
sub esp,0C0h
push ebx
push esi
push edi
lea edi,[ebp-0C0h]
mov ecx,30h
mov eax,0CCCCCCCCh
rep stos dword ptr [edi]
; // return a + b;
mov eax,dword ptr [a]
add eax,dword ptr [b]
; // function epilog
pop edi
pop esi
pop ebx
mov esp,ebp
pop ebp
ret
; // put the arguments in the registers EDX and ECX
mov edx,3
mov ecx,2
; // call the function
call @fastcallSum@8
; // copy the return value from EAX to a local variable (int c)
mov dword ptr [c],eax
函数代码：
// function prolog
push ebp
mov ebp,esp
sub esp,0D8h
push ebx
push esi
push edi
push ecx
lea edi,[ebp-0D8h]
mov ecx,36h
mov eax,0CCCCCCCCh
rep stos dword ptr [edi]
pop ecx
mov dword ptr [ebp-14h],edx
mov dword ptr [ebp-8],ecx
; // return a + b;
mov eax,dword ptr [a]
add eax,dword ptr [b]
;// function epilog
pop edi
pop esi
pop ebx
mov esp,ebp
pop ebp
ret
原文：During the long, hard, but yet beautiful process of learning C++ programming for Windows, you have probably been curious about the strange specifiers that sometime appear in front of function declarations, like __cdecl, __stdcall, __fastcall, WINAPI, etc. After looking through MSDN, or some other reference, you probably found out that these specifiers specify the calling conventions for functions. In this article, I will try to explain different calling conventions used by Visual C++ (and probably other Windows C/C++ compilers). I emphasize that above mentioned specifiers are Microsoft-specific, and that you should not use them if you want to write portable code.
So, what are the calling conventions? When a function is called, the arguments are typically passed to it, and the return value is retrieved. A calling convention describes how the arguments are passed and values returned by functions. It also specifies how the function names are decorated. Is it really necessary to understand the calling conventions to write good C/C++ programs? Not at all. However, it may be helpful with debugging. Also, it is necessary for linking C/C++ with assembly code.
To understand this article, you will need to have some very basic knowledge of assembly programming.
No matter which calling convention is used, the following things will happen:
All arguments are widened to 4 bytes (on Win32, of course), and put into appropriate memory locations. These locations are typically on the stack, but may also be in registers; this is specified by calling conventions.
Program execution jumps to the address of the called function.
Inside the function, registers ESI, EDI, EBX, and EBP are saved on the stack. The part of code that performs these operations is called function prolog and usually is generated by the compiler.
The function-specific code is executed, and the return value is placed into the EAX register.
Registers ESI, EDI, EBX, and EBP are restored from the stack. The piece of code that does this is called function epilog, and as with the function prolog, in most cases the compiler generates it.
Arguments are removed from the stack. This operation is called stack cleanup and may be performed either inside the called function or by the caller, depending on the calling convention used.
As an example for the calling conventions (except for this), we are going to use a simple function:
Collapse | Copy Codeint sumExample (int a, int b)
{
return a + b;
}The call to this function will look like this:
Collapse | Copy Code int c = sum (2, 3);For __cdecl, __stdcall, and __fastcall calling conventions, I compiled the example code as C (not C++). The function name decorations, mentioned later in the article, apply to the C decoration schema. C++ name decorations are beyond the scope of this article.
C calling convention (__cdecl)
This convention is the default for C/C++ programs (compiler option /Gd). If a project is set to use some other calling convention, we can still declare a function to use __cdecl:
Collapse | Copy Codeint __cdecl sumExample (int a, int b);The main characteristics of __cdecl calling convention are:
Arguments are passed from right to left, and placed on the stack.
Stack cleanup is performed by the caller.
Function name is decorated by prefixing it with an underscore character '_' .
Now, take a look at an example of a __cdecl call:
Collapse | Copy Code; // push arguments to the stack, from right to left
push 3
push 2
; // call the function
call _sumExample
; // cleanup the stack by adding the size of the arguments to ESP register
add esp,8
; // copy the return value from EAX to a local variable (int c)
mov dword ptr [c],eaxThe called function is shown below:
Collapse | Copy Code; // function prolog
push ebp
mov ebp,esp
sub esp,0C0h
push ebx
push esi
push edi
lea edi,[ebp-0C0h]
mov ecx,30h
mov eax,0CCCCCCCCh
rep stos dword ptr [edi]
; // function epilog
pop edi
pop esi
pop ebx
mov esp,ebp
pop ebp
retStandard calling convention (__stdcall)
This convention is usually used to call Win32 API functions. In fact, WINAPI is nothing but another name for __stdcall:
Collapse | Copy Code#define WINAPI __stdcallWe can explicitly declare a function to use the __stdcall convention:
Collapse | Copy Codeint __stdcall sumExample (int a, int b);Also, we can use the compiler option /Gz to specify __stdcall for all functions not explicitly declared with some other calling convention.
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 is decorated by prepending an underscore character and appending a '@' character and the number of bytes of stack space required.
The example follows:
Collapse | Copy Code; // push arguments to the stack, from right to left
push 3
push 2
; // call the function
call _sumExample@8
; // copy the return value from EAX to a local variable (int c)
mov dword ptr [c],eaxThe function code is shown below:
Collapse | Copy Code; // function prolog goes here (the same code as in the __cdecl example)
; // function epilog goes here (the same code as in the __cdecl example)
; // cleanup the stack and return
ret 8Because the stack is cleaned by the called function, the __stdcall calling convention creates smaller executables than __cdecl, in which the code for stack cleanup must be generated for each function call. On the other hand, functions with the variable number of arguments (like printf()) must use __cdecl, because only the caller knows the number of arguments in each function call; therefore only the caller can perform the stack cleanup.
Fast calling convention (__fastcall)
Fast calling convention indicates that the arguments should be placed in registers, rather than on the stack, whenever possible. This reduces the cost of a function call, because operations with registers are faster than with the stack.
We can explicitly declare a function to use the __fastcall convention as shown:
Collapse | Copy Codeint __fastcall sumExample (int a, int b);We can also use the compiler option /Gr to specify __fastcall for all functions not explicitly declared with some other calling convention.
The main characteristics of __fastcall calling convention are:
The first two function arguments that require 32 bits or less are placed into registers ECX and EDX. The rest of them are pushed on the stack from right to left.
Arguments are popped from the stack by the called function.
Function name is decorated by by prepending a '@' character and appending a '@' and the number of bytes (decimal) of space required by the arguments.
Note: Microsoft have reserved the right to change the registers for passing the arguments in future compiler versions.
Here goes an example:
Collapse | Copy Code; // put the arguments in the registers EDX and ECX
mov edx,3
mov ecx,2
; // call the function
call @fastcallSum@8
; // copy the return value from EAX to a local variable (int c)
mov dword ptr [c],eaxFunction code:
Collapse | Copy Code; // function prolog
push ebp
mov ebp,esp
sub esp,0D8h
push ebx
push esi
push edi
push ecx
lea edi,[ebp-0D8h]
mov ecx,36h
mov eax,0CCCCCCCCh
rep stos dword ptr [edi]
pop ecx
mov dword ptr [ebp-14h],edx
mov dword ptr [ebp-8],ecx
; // return a + b;
mov eax,dword ptr [a]
add eax,dword ptr
;// function epilog
pop edi
pop esi
pop ebx
mov esp,ebp
pop ebp
retHow fast is this calling convention, comparing to __cdecl and __stdcall? Find out for yourselves. Set the compiler option /Gr, and compare the execution time. I didn't find __fastcall to be any faster than other calling conventons, but you may come to different conclusions.
Thiscall
Thiscall is the default calling convention for calling member functions of C++ classes (except for those with a variable number of arguments).
The main characteristics of thiscall calling convention are:
Arguments are passed from right to left, and placed on the stack. this is placed in ECX.
Stack cleanup is performed by the called function.
The example for this calling convention had to be a little different. First, the code is compiled as C++, and not C. Second, we have a struct with a member function, instead of a global function.
Collapse | Copy Codestruct CSum
{
int sum ( int a, int b) {return a+b;}
};The assembly code for the function call looks like this:
Collapse | Copy Code push 3
push 2
lea ecx,[sumObj]
call ?sum@CSum@@QAEHHH@Z ; CSum::sum
mov dword ptr [s4],eaxThe function itself is given below:
Collapse | Copy Code push ebp
mov ebp,esp
sub esp,0CCh
push ebx
push esi
push edi
push ecx
lea edi,[ebp-0CCh]
mov ecx,33h
mov eax,0CCCCCCCCh
rep stos dword ptr [edi]
pop ecx
mov dword ptr [ebp-8],ecx
mov eax,dword ptr [a]
add eax,dword ptr
pop edi
pop esi
pop ebx
mov esp,ebp
pop ebp
ret 8Now, what happens if we have a member function with a variable number of arguments? In that case, __cdecl is used, and this is pushed onto the stack last.
惭愧惭愧啊。。。
