How to store register values using assembly to C++ variables?

Question

int my_var;
void __declspec(naked) stuff()
{
    __asm
    {
        lea edx, [ecx + edi + 0x0000111]
    }
}

How to store the value from the address [ecx + edi + 0x0000111] in the c++ variable "my_var" above.

Answers [like this](https://stackoverflow.com/questions/15908835/how-to-store-a-c-variable-in-a-register) might help. — tadman, Feb 21 '21 at 03:38
I'd probably look at the MS docs, like [here](https://learn.microsoft.com/en-us/cpp/assembler/inline/accessing-c-or-cpp-data-in-asm-blocks?view=msvc-160). — David Wohlferd, Feb 21 '21 at 04:23
@tadman: That's the opposite problem, and not specific to MSVC's flavour of inline asm (which makes it trivial even for local vars that don't have a symbol in the object file or in stand-alone asm). — Peter Cordes, Feb 21 '21 at 05:11

kdcode · Answer 1 · 2021-02-21T05:15:22.180

2

From the Microsoft docs:

An __asm block can refer to any symbols, including variable names, that are in scope where the block appears.

Therefore you could do this:

int my_var;
void __declspec(naked) stuff()
{
    __asm
    {
        lea edx, [ecx + edi + 0x0000111]
        mov my_var, edx
        ret

    }
}

edited Feb 21 '21 at 05:15

answered Feb 21 '21 at 05:08

kdcode

Your answer should probably include the missing `ret` instruction this `naked` function needs to be callable without crashing. – Peter Cordes Feb 21 '21 at 05:13
@PeterCordes Did not noticed it. Thanks. – kdcode Feb 21 '21 at 05:15
I tried that before and that gave me the address instead of WHAT is in the address, so the actual value. – Emre Köycü Feb 21 '21 at 18:04
@EmreKöycü I assumed you wanted to have the "value" of the register. It might be address of something. To get the value you want, just define my_var as pointer of a suitable type then dereference it after the _asm block (remember to remove ret instruction) . – kdcode Feb 21 '21 at 18:38

1 Answers1