How do I call this function in inline ASM? (MSVC++)

Question

void __usercall sub_101A7850@<eax>(int a1@<edx>, int a2@<ecx>, int a3, int a4, int a5, int a6)

My first attempt (crashes):

__declspec(naked) void __stdcall callit(const int& a1, const int& a2, unsigned int a3, const int *a4, int a5, int *a6)
    {
        // void __usercall sub_101A7850@<eax>(int a1@<edx>, int a2@<ecx>, int a3, int a4, int a5, int a6)
        __asm
        {
            mov ecx, [esp + 4] // a1
            mov edx, [esp + 8] // a2
            push [esp + 12] // a3
            push [esp + 16] // a4
            push [esp + 20] // a5
            push [esp + 24] // a6
            call funcaddr
            retn 24
        }
    }

I have verified funcaddr is valid. Pretty sure its a __fastcall

Answer 1

You have exchanged ecx and edx: your func signature says a1<edx> but you put a1 into ecx and a2 to edx.

Moreover: After pushing a3 the esp relative offset for a4 becomes 4 bytes farther. After pushing a4 the esp relative offset for a5 becomes 8 bytes farther, and so on... (so the correct offsets are: a4: [esp+20], a5: [esp+28], a6: [esp+36])