Example of Calling Convention

The following program illustrates how to use the calling convention.

C++

// C++ Program to demonstrate the calling convention 
#include <iostream> 
  
// __cdecl calling convention 
int __cdecl cdeclAdd(int a, int b) 
{ 
    int c = a + b; 
    return c; 
} 
  
// __stdcall calling convention 
int __stdcall stdcallAdd(int a, int b) 
{ 
    int c = a + b; 
    return c; 
} 
  
// __fastcall calling convention 
int __fastcall fastcallAdd(int a, int b, int c, int d) 
{ 
    int e = a + b + c + d; 
    return e; 
} 
  
// __thiscall calling convention 
class Temp { 
public: 
    int __thiscall thiscallAdd(int a, int b) 
    { 
        int c = a + b; 
        return c; 
    } 
}; 
  
// driver code 
int main() 
{ 
    int result; 
    Temp obj; 
  
    // Function calls and output 
    result = cdeclAdd(1, 2); 
    std::cout << "Result: " << result << std::endl; 
  
    result = stdcallAdd(3, 4); 
    std::cout << "Result: " << result << std::endl; 
  
    result = fastcallAdd(7, 8, 9, 10); 
    std::cout << "Result: " << result << std::endl; 
  
    result = obj.thiscallAdd(5, 6); 
    std::cout << "Result: " << result << std::endl; 
}

Output

Result: 3
Result: 7
Result: 34
Result: 11

Here we have 4 functions: cdeclAdd(), stdcallAdd(), fastcallAdd(), and thiscallAdd() with calling conventions __cdecl, __stdcall, __fastcall, and __thiscall respectively. They are being called by the caller i.e. main(). Let’s understand each of them one by one.

Calling Conventions in C/C++

In C/C++ programming, a calling convention is a set of rules that specify how a function will be called. You might have seen keywords like __cdecl or __stdcall when you get linking errors. For example:

error LNK2019: unresolved external symbol "void __cdecl A(void)" (?A@@YAXXZ) referenced in function _main

Here, we see __cdecl being used. It is one of the calling conventions in C/C++. You can also see code like this in 3rd party libraries.

For example:

extern __m128i __cdecl _mm256_mask_cvtepi32_epi16(__m128i, __mmask8, __m256i);

What are caller and callee?

When we call a function, a stack frame is allocated for that function, arguments are passed to the function, and after the function does its work, the allocated stack frame is deallocated and control is passed to the calling function.

The function that calls the subroutine is called the caller. The function that gets called(i.e. subroutine) by the caller is called the callee.

C++

// C++ Program to illustrate the caller and callee 
#include <iostream> 
  
// callee 
void func() { std::cout << "Geeks"; } 
  
// caller 
int main() 
{ 
  
    // function call 
    func(); 
  
    return 0; 
}

Output

Geeks

In the above code, main() is the caller and func() is the callee.

Tags:

#C Language #C++ #CPP

Calling Conventions

__cdecl

Example of Calling Convention

C++

Calling Conventions in C/C++

What are caller and callee?

C++

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