How to invoke a pointer to a function.

[tilman]

So I'm trying to invoke a pointer to a function, but I'm failing. Could somebody please help?

I'm storing the function pointer like so:

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mov pFunction,offset Function

I can actually get the call to work by doing this:

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call pFunction

But when I try this:

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invoke pFunction

I get "error A2190: INVOKE requires prototype for procedure".

And if I try this:

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invoke Function pFunction

I get "error A2206: missing operator in expression".

Any ideas?

[dedndave]

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FuncName EQU <pr0 PTR pFunction>


        INVOKE  FuncName

if it has parameters, chage the "pr0" to "pr1", "pr2", and so on

Erol gives a nice example where he actually creates a jump table entry for it
http://www.masm32.com/board/index.php?topic=11772.msg89003#msg89003

although, the jump should not be required

[tilman]

Hi dedndave,

Thanks for the speedy reply. Although this assembles it doesn't do what I want it to, i.e. it doesn't call Function, it calls 'offset pFunction'. Or am I misunderstanding something?

Here is the whole listing:

.386
.model flat,stdcall
   option       casemap: none
   include      d:\masm32\include\windows.inc
   include      d:\masm32\include\kernel32.inc
   includelib    d:\masm32\lib\kernel32.lib
   
   Function proto
.const
   FuncName EQU <pr0 PTR pFunction>
.data?
   pFunction   dd      ?
.code
start:
   mov pFunction,offset Function
   INVOKE  FuncName
   
   push   0
   call   ExitProcess
   
Function proc
   ret
Function endp
end start

[dedndave]

no - my mistake
it looks like the jump is required, although Erol's example may be dereferenced

let me play with it for a minute....

[dedndave]

well - i know there is a better way to do it - lol
but, for now, use Erol's method
i will play with it later today - or someone else will show us before then   :P

[tilman]

Thanks a lot for your help, I just found the answer here

http://webster.cs.ucr.edu/Page_TechDocs/MASMDoc/ProgrammersGuide/Chap_07.htm

although it seems a bit crackpot to me...

This is what you have to do:

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.386
.model flat,stdcall
option casemap: none
include d:\masm32\include\windows.inc
include d:\masm32\include\kernel32.inc
includelib d:\masm32\lib\kernel32.lib

FUNCPROTO       TYPEDEF PROTO
FUNCPTR         TYPEDEF PTR FUNCPROTO
.data?
pFunction FUNCPTR ?
.code
start:
mov pFunction,offset Function
mov eax,offset pFunction
invoke FUNCPTR ptr [eax]

push 0
call ExitProcess

Function proc
ret
Function endp
end start

[dedndave]

well - that is less than ideal - you don't want to do all that every time - lol

the problem i was having is, there is no near absolute direct JMP instruction
but, with minimal effort, we can make a relative JMP work for us
we just have to add a SUB instruction to make the address relative

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        .DATA

FuncBranch db 0E9h
pFunc      dd ?

FuncName    EQU <pr0 PTR FuncBranch>

        .CODE

;EAX = FunctionAddress

        sub     eax,offset pFunc+4
        mov     pFunc,eax

;then, to INVOKE the function...

        INVOKE  FuncName

you may be able to eliminate the SUB like this...

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        mov     pFunc,offset Function-offset pFunc-4

[dedndave]

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        .DATA

FuncBranch db 0E9h
pFunc      dd ?

FuncName    EQU <pr0 PTR FuncBranch>

        .CODE

;initialization

        mov     pFunc,offset Function-offset pFunc-4

;then, to INVOKE the function...

        INVOKE  FuncName

[dedndave]

because you can grab "offset Function", this should work - with no init code...

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        .DATA

FuncBranch db 0E9h
pFunc      dd offset Function-offset pFunc-4

FuncName    EQU <pr0 PTR FuncBranch>

        .CODE

;then, to INVOKE the function...

        INVOKE  FuncName

[dedndave]

i am not sure why you are doing all this   :lol
unless you want to be able to change the function address in code
if that's the case, use the previous 2 posts

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Function PROTO

        .CODE

        INVOKE  Function
;
;
;
Function PROC
        ret
Function ENDP

[tilman]

What I actually want to do is pass a callback to a function. (Isn't that a common task?) The example here is just simplification which demonstrated my problem.

I actually discovered another simplification of the official solution:

Instead of

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mov pFunction,offset Function
mov eax,offset pFunction
invoke FUNCPTR ptr [eax]


i can just do

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mov pFunction,offset Function
invoke FUNCPTR ptr pFunction


Now my main complaint is that I may have to define function prototypes twice. Oh vell...

Some questions about your solution:

a) Where does the magic 0e9h come from?
b) Does your code still do type checking on parameters?

Cheers!

[dedndave]

well - type checking is a matter of the "prN" definitions in windows.inc (or wherever they are defined)
for most 32-bit applications, they are always DWORD's of some sort

i am assuming that you have more than 0 parameters for this function
otherwise, a simple CALL would be easier

so - you want to pass the address of a callback and store it in a global variable ?

[tilman]

well - type checking is a matter of the "prN" definitions in windows.inc (or wherever they are defined)
for most 32-bit applications, they are always DWORD's of some sort

I actually switched to WinInc, because MASM32 was missing some COM structures. They use all sorts of types... ;)

so - you want to pass the address of a callback and store it in a global variable ?

That is correct. I've got the caller in a seperate file and I want to keep it general, so it doesn't know what to call at assemble time.

[Vortex]

Hi tilman,

You can also use a macro simulating invoke :

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include     FuncPointer.inc

.data

capt        db 'Hello',0
message     db 'Function invoked through pointer',0


.data?

pFunction   dd ?


.code

start:

    mov     pFunction,OFFSET Function

    _invoke pFunction,ADDR message

    invoke  ExitProcess,0


Function PROC msg:DWORD

    invoke  MessageBox,0,msg,ADDR capt,MB_OK
    ret

Function ENDP


END start


_invoke mimics Masm's internal macro invoke

[tilman]

Hey Vortex,

Thanks for that. I'm not that hot on macros (yet), but does the macro check that the correct number and type of parameters were passed? Because, as I understand it, that's the whole point of using invoke over call (and yes, I tried to get by with just call and I failed and invoke saved my bacon  :bg)