Creating a DLL, and have one of the functions return a chunk

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Foxxy

Joined: 14 Jul 2014
Posts: 42
Location: Somewhere over the rainbow...

Foxxy 16 Jul 2014, 05:13

Hello, I am new to assembly, and I got tired of rewriting my functions. So I am going to include them all in a single DLL. One of these functions needs to return something larger than can be contained inside of a 32 bit register. For example, in the following code, I want to return 'ThisVariable'. I think I need to pass the pointer to 'ThisVariable' to the program. But where do I go from there?

Code:

format PE GUI 4.0 DLL
entry DllEntryPoint

include 'win32a.inc'

section '.text' code readable executable

proc DllEntryPoint hinstDLL,fdwReason,lpvReserved
        mov     eax,TRUE
        ret
endp 

proc AFunction thisthat:DWORD bytePtr:DWORD
;return ThisVariable (256 bytes) to calling program
endp

16 Jul 2014, 05:13

baldr

Joined: 19 Mar 2008
Posts: 1651

baldr 16 Jul 2014, 06:35

Foxxy,

You may pass pointer to that memory as an argument to AFunction() (along with its size for sanity check), or you can develop some kind of a protocol, in which returned pointer is expected to be allocated using predefined method, with a clear method to free that memory (e.g. HeapAlloc()/HeapFree()).

16 Jul 2014, 06:35

AsmGuru62

Joined: 28 Jan 2004
Posts: 1661
Location: Toronto, Canada

AsmGuru62 16 Jul 2014, 16:50

Just put all your functions into .ASM files and then include the files as needed.

16 Jul 2014, 16:50

Foxxy

Joined: 14 Jul 2014
Posts: 42
Location: Somewhere over the rainbow...

Foxxy 16 Jul 2014, 18:56

Okay, can you point me to somewhere where I can learn how to do this? Also, @baldr how would I write to the memory that the pointer is passed to the DLL function? Preferably without using an outside function. I'm sorry if this seems rude or needy, but I can't seem to find anything on the internet regarding DLL creation and large variables.

16 Jul 2014, 18:56

baldr

Joined: 19 Mar 2008
Posts: 1651

baldr 16 Jul 2014, 20:28

Foxxy,

That's simply a pointer, use it to access memory:

Code:

proc AFunction, pointer
        mov     eax, [pointer]
        mov     byte [eax], 42
        ret
endp

16 Jul 2014, 20:28

Foxxy

Joined: 14 Jul 2014
Posts: 42
Location: Somewhere over the rainbow...

Foxxy 17 Jul 2014, 03:50

Wonderful! Just as I was hoping it would work. Thank you.

17 Jul 2014, 03:50

Roman

Joined: 21 Apr 2012
Posts: 1840

Roman 17 Jul 2014, 04:33

Code:

proc AFunction, pointer 
        mov     eax, [pointer]

pointer like

Code:

 mov     eax, [esp+8]

Code:

mov     eax, [esp+8] the same like  mov     eax, [pointer]

17 Jul 2014, 04:33

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