How to handle pointers?

[joemc]

This is the relevant portion in MASM.  It does not work and i have tried a few variations but have not succeeded:

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.ELSEIF uMsg==WM_GETMINMAXINFO
     
      mov [lParam + 24],300 ; beginning of ptMinTrackSize
      mov [lParam + 28],300 ;
        mov eax, NULL
        ret

the code above crashes.  If i do lParam +12 and +16 it does not crash but that was just done as a test bc it is not the variable i need.

This does work to desired effect in VC:

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case WM_GETMINMAXINFO:
((MINMAXINFO*)lparam)->ptMinTrackSize.x=300;
((MINMAXINFO*)lparam)->ptMinTrackSize.y=300;
return 0;


MSDN defines MINMAXINFO structure as:

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typedef struct {
    POINT ptReserved;   
    POINT ptMaxSize;     
    POINT ptMaxPosition;
    POINT ptMinTrackSize;
    POINT ptMaxTrackSize;
} MINMAXINFO;


MSDN defines POINT as:

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typedef struct tagPOINT {
  LONG x;
  LONG y;
}POINT, *PPOINT;


[redskull]

lParam+24 is probably not the beggining of ptMinTrackSize

lParam is a pointer to a structure, so the line "[lParam + 24]" is "the value pointed to by the value in the memory address located 24 bytes above lParam", which is certainly not valid.  Try:

mov eax, lParam       ;move the value stored in lParam into eax (the address of ptMinTrackSize)
mov eax, [eax+24]   ;move the value stored 24 bytes above the address of ptMinTrackSize into eax

-r

[dedndave]

when you try this

        mov     [lParam + 24],300

which is the same as this

        mov     lParam[24],300

the assembler does not know whether lParam[24] is a word or a dword
i think points are dwords, so

        mov dword ptr lParam[24],300

WM_MINMAXINFO is defined in windows.inc as

WM_GETMINMAXINFO                     equ 24h

someplace, you have to declare a structure

MyMinMaxInfo MINMAXINFO <>

if you look in masm32\windows.inc

MINMAXINFO STRUCT
  ptReserved        POINT <>
  ptMaxSize         POINT <>
  ptMaxPosition     POINT <>
  ptMinTrackSize    POINT <>
  ptMaxTrackSize    POINT <>
MINMAXINFO ENDS

i think what you want is

        .DATA?
MyMinMaxInfo MINMAXINFO <>

        .CODE
.
.
.
        mov dword ptr MyMinMaxInfo.ptMinTrackSize.x,300
        mov dword ptr MyMinMaxInfo.ptMinTrackSize.y,300

now, the assembler knows the size (maybe - lol), so the "dword ptr" may not be required

        mov     MyMinMaxInfo.ptMinTrackSize.x,300
        mov     MyMinMaxInfo.ptMinTrackSize.y,300

[sinsi]

Doesn't this code tell C that lparam contains a pointer?

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((MINMAXINFO*)lparam)->ptMinTrackSize.x=300;
Whereas your code is the following

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((MINMAXINFO)lparam)->ptMinTrackSize.x=300;
I don't know C but isn't the * a de-reference or something? I am curious...

redskull has it right

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mov eax, lParam       ;move the value stored in lParam into eax (the address of ptMinTrackSize)

[joemc]

i figured it out:

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      .ELSEIF uMsg==WM_GETMINMAXINFO
      mov ecx, lParam;
    mov eax,300;
    mov [ecx+24], eax 
    mov [ecx+28], eax 


works correctly!!! thanks for all the points(lol) in the right direction.  i have to say pointers in MASM are fairly confusing.  I guess you can move based on a register but not a memory location? idk  the assembler gets mad at :

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    mov [lParam+24], 300

and even

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    mov [ecx+24], 300

apparently it has to be register to memory location based on register

[joemc]

Doesn't this code tell C that lparam contains a pointer?

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((MINMAXINFO*)lparam)->ptMinTrackSize.x=300;
Whereas your code is the following

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((MINMAXINFO)lparam)->ptMinTrackSize.x=300;
I don't know C but isn't the * a de-reference or something? I am curious...

my code is the top portion.  not sure what you are saying.
* is inside the () above. what i am doing is saying cast lparam into a pointer to a MINMAXINFO structure.

* can be a dereference pointer as well.  it actualy has many uses. the code above can also be written as:

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(*((MINMAXINFO*)lparam)).ptMinTrackSize.x=300;


in this example the first * is a derefernce operator so you can use . instead of the pointer form ->

it can also be written C++ only format of :

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reinterpret_cast<MINMAXINFO*>(lparam)->ptMinTrackSize.x=300;


or using dereference

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(*reinterpret_cast<MINMAXINFO*>(lparam)).ptMinTrackSize.x=300;


[BogdanOntanu]

I do not think that Sinsi was really expecting an answer in C/C++... instead he was using the questions as hints to point you in the right direction.

You are sending a pointer as a parameter in lparam and because of this you have to load it into a register because this CPU can not directly use a memory location to reference another memory location. C compiler will do that for you behind the scene but in ASM you must do it yourself.

As said above ... You could use something like this:

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  mov esi,[lparam]                       ; esi now points to MINMAXINFO
  mov [esi + MINMAXINFO.ptMinTrackSize.x],300    ; access structure members by name and not by offsets
  mov [esi + MINMAXINFO.ptMinTrackSize.y],300

THis does not look confusing when compared to the C alternative...

[Ficko]

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  mov esi,[lparam]                       ; esi now points to MINMAXINFO
  mov [esi + MINMAXINFO.ptMinTrackSize.x],300    ; access structure members by name and not by offsets
  mov [esi + MINMAXINFO.ptMinTrackSize.y],300


if you like C syntax you can write:

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  mov [esi.MINMAXINFO.ptMinTrackSize.x],300   
  mov [esi.MINMAXINFO.ptMinTrackSize.y],300


OR

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  mov [esi].MINMAXINFO.ptMinTrackSize.x,300   
  mov [esi].MINMAXINFO.ptMinTrackSize.y,300


[joemc]

Reading back his post makes more sense. he was pointing out what my ASM code looks like.  and his post is what helped me figure it out even though i did not understand him. :)   and both of your responses make the code more readable. Once again sorry for such a basic question.  i found the goASM website that answeres alot of these questions in the code provided there (http://www.jorgon.freeserve.co.uk/)