X86 calling conventions

lastmod 2018-08-17created 2015-04-056 edits

Describes the calling conventions used on the x86 architecture.

stdcall vs cdecl

Difference between __stdcall and __cdecl!!! Endless Challenge¹<span style="color:red">(At the top of the post, the definitions of stdcall and cdecl are swapped)

A brief summary of the differences between __stdcall and __cdecl.

With cdecl, the calling function cleans up the stack.
With stdcall, the stack is cleaned up inside the called function.
- Using asm:ret is faster than cleaning up the SP (Stack Pointer) with ADD.
stdcall is faster than cdecl, but does not support variadic arguments.
- This is because, with variadic arguments, the number of arguments cannot be determined directly inside the function.
Use stdcall where possible for functions called by the OS or functions in a DLL.

cdecl

MSDN: __cdecl

__cdecl is the default calling convention for C and C++ programs. Because the stack is cleaned up by the caller, it can do vararg functions. The __cdecl calling convention creates larger executables than __stdcall, because it includes stack cleanup code in each function call. The following list shows the implementation of this calling convention.

Element	Implementation
Argument-passing order	Right to left
Argument-passing convention	Calling function pops the arguments from the stack.
Stack-maintenance responsibility	Called function pops its own arguments from the stack.
Name-decoration convention	Underscore character (_) is prefixed to names, except when __cdecl functions that use C linkage are exported.
Case-translation convention	No case translation performed.

stdcall

MSDN: __stdcall

The __stdcall calling convention is used to call Win32 API functions. The callee cleans the stack, so the compiler makes vararg functions __cdecl. Functions that use this calling convention require a function prototype.

Element	Implementation
Argument-passing order	Right to left
Argument-passing convention	By value, unless a pointer or reference type is passed
Stack-maintenance responsibility	Called function pops its own arguments from the stack.
Name-decoration convention	An underscore (_) is prefixed to the name. The name is followed by the at sign (@), followed by the number of bytes (in decimal) in the argument list. Therefore, the function declared as int func( int a, double b ) is decorated as _func@12.
Case-translation convention	None

fastcall

MSDN: __fastcall

The __fastcall calling convention specifies that arguments to functions are to be passed in registers, when possible. This calling convention only applies to the x86 architecture. The following list shows the implementation of this calling convention.

Element	Implementation
Argument-passing order	The first two DWORD or smaller arguments that are found in the argument list from left to right are passed in ECX and EDX registers; all other arguments are passed on the stack from right to left.
Stack-maintenance responsibility	Called function pops the arguments from the stack.
Name-decoration convention	At sign (@) is prefixed to names. An at sign followed by the number of bytes (in decimal) in the parameter list is suffixed to names.
Case-translation convention	No case translation performed.

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References

Egloos.zum.com-qufl1999-stdcall_vs_cdecl.pdf ↩
C_and_Cpp_calling_convention_-_cdecl_vs_pascal_vs_stdcall.pdf ↩

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