[Avalon]

Just create a .ASM file, change the build rule to MASM, define the subroutine and call it from the C file.


masm.asm

.CODE

PUBLIC MyAsmRoutine
PUBLIC ChangeRaxRoutine

MyAsmRoutine PROC
push rbp
mov rbp, rsp
call qword ptr [rcx]
mov rsp, rbp
pop rbp
ret
MyAsmRoutine ENDP

ChangeRaxRoutine PROC
mov rax, 0x4141
ChangeRcxRoutine ENDP

END

file.c

void MyAsmRoutine(PVOID pFunc);
void __declspec(naked) ChangeRaxRoutine();

int main()
{
PVOID pNtDirectCall = ....
MyAsmRoutine(pNtDirectCall);
ChangeRaxRoutine();
//now your program will return 0x4141 as RAX is the return code
}