Call assembly function from C++

Question

Added a bit more assembly on top of the function and below it to get a clearer image

00427F38   . 50             PUSH EAX
00427F39   . 8975 08        MOV DWORD PTR SS:[EBP+8],ESI
00427F3C   . E8 0FFE0200    CALL Test.00457D50
00427F41   . 8B4D 08        MOV ECX,DWORD PTR SS:[EBP+8]
00427F44   . 51             PUSH ECX                                 ; /Arg1
00427F45   . 8D4D E8        LEA ECX,DWORD PTR SS:[EBP-18]            ; |
00427F48   . E8 13FE0200    CALL Test.00457D60                       ; \Test.00457D60
00427F4D   . 8B55 08        MOV EDX,DWORD PTR SS:[EBP+8]
00427F50   . 8D4D E8        LEA ECX,DWORD PTR SS:[EBP-18]
00427F53   . 52             PUSH EDX

IDA Pro produced this function declaration

void *__userpurge sub_457D60<eax>(void **a1<ecx>, int a2<ebx>, int a3)

Here is what I tried, doesn't work.

int callAddress = (*This is calculated by me 100% correct*)

//void *__userpurge sub_457D60<eax>(void **a1<ecx>, int a2<ebx>, int a3)
__declspec(naked) void stepOneWrapped(int a1, char* a2, int a3)
{
    __asm{
        push ebp
        mov ebp, esp
        push a3
        mov ebx, [a2]
        mov ecx, a1
        call [callAddress]
        leave
        ret
    }
}

Special note: this is like a dll injection so the Test program is loaded with this program altogether.

Answer 1

you need to preserve ebx, as its a non-volatile register:

__declspec(naked) void stepOneWrapped(int a1, char* a2, int a3)
{
    __asm{
        push ebp
        mov ebp, esp
        push ebx
        push a3
        mov ebx, [a2]
        mov ecx, a1
        call [callAddress]
        pop ebx
        leave
        ret
    }
}

but according to you IDA dump, your params are wrong, so it should be like this (to match IDA):

__declspec(naked) void stepOneWrapped(void** a1, int a2, int a3)
{
    __asm{
        push ebp
        mov ebp, esp
        push ebx
        push a3
        mov ebx, a2
        mov ecx, a1
        call [callAddress]
        pop ebx
        leave
        ret
    }
}