What is the minimum value of a pointer?

[Peter Stephensen]

What is the minimum value of a pointer?

The reason for this question is that i would like an element in a UDT to be an integer ID OR a pointer to an ID-string:

type MYTYPE
    ....
    ID as long   'Integer ID OR a pointer to an ID-string
    ...
end type

In the function that has the UDT as input i have to have a way to analyze the ID. If i know %MIN_POINTER_VALUE i can do this:

function MyFunc(mt as MYTYPE) as long

    select case mt.ID
    case < %MIN_POINTER_VALUE
        ' The integer-id case
    case else        
        ' The string-id case     
    end select

end function

Regards
Peter

[This message has been edited by Peter Stephensen (edited June 18, 2000).]

[Semen Matusovski]

Probably, I'm wrong, but I don't beleive that Ptr could be < 1 Mb (at least, 640K ... 1Mb)

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[Florent Heyworth]

Peter

I don't think you'll find a reliable way to do what you want using a minumum pointer value. Why don't you do something like:

#COMPILE EXE

%LONG_LONG = 1
%LONG_PTR = 2
%DOUBLE_PTR = 3
%INTEGER_PTR = 4
%ASCIIZ_PTR = 5


UNION U_TYPES
      lValue AS LONG
      plValuePtr AS LONG PTR
      pdValuePtr AS DOUBLE PTR
      piValuePtr AS INTEGER PTR
      pszValuePtr AS ASCIIZ PTR
END UNION

TYPE T_TYPES
     lValueType AS LONG '%LONG_LONG, %LONG_PTR, %DOUBLE_PTR, etc
     uType AS U_TYPES
END TYPE
      

FUNCTION PBMAIN() AS LONG
   LOCAL sTest AS STRING
   LOCAL tTest AS T_TYPES
   
   
   sTest = "HELLO THERE"
   
   tTest.uType.lValue = STRPTR(sTest)
   tTest.lValueType = %ASCIIZ_PTR
   
   SELECT CASE tTest.lValueType
          CASE %LONG_LONG
               PRINT tTest.uType.lValue
          CASE %LONG_PTR
               PRINT [email protected]
          CASE %DOUBLE_PTR
               PRINT [email protected]
          CASE %INTEGER_PTR
               PRINT [email protected]
          CASE %ASCIIZ_PTR
               PRINT [email protected]
   END SELECT
   
   
   WAITKEY$
END FUNCTION

Florent

------------------

[Peter Lameijn]

There is a simple solution:

#Include "WIN32API.INC"
type MYTYPE    ....    
  ID as long   'Integer ID OR a pointer to an ID-string    
End type

Function MyFunc(mt as MYTYPE) as long    
If IsBadCodePtr (mt.ID) Then
' do whatever....(it's not a pointer or an invalid one)
Else
' do whatever....(it's a pointer)
End If

------------------
Kind regards,
Peter.



[This message has been edited by Peter Lameijn (edited June 18, 2000).]

[Semen Matusovski]

Florent --
From one side you are right, but from another side look, what Windows does.
There is a lot API functions similar LoadIcon.

HICON LoadIcon(
  HINSTANCE hInstance, // handle to application instance
  LPCTSTR lpIconName   // name string or resource identifier
);

Parameters
hInstance
[in] Handle to an instance of the module whose executable file contains the icon to be loaded. This parameter must be NULL when a standard icon is being loaded.
lpIconName
[in] Pointer to a null-terminated string that contains the name of the icon resource to be loaded. Alternatively, this parameter can contain the resource identifier in the low-order word and zero in the high-order word. Use the MAKEINTRESOURCE macro to create this value.

This means that Windows automatic accepts that a value <= &H0000FFFF is ID.


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[Florent Heyworth]

Peter, good one - that'll do the trick!

Semen, interesting comment - I'll have a think about that one 8-)

Cheers

Florent

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[Peter Stephensen]

Peter --
That's it !

Semen --
Your comment suggests an alternative method:

function MyFunc(mt as MYTYPE) as long
 
    if hiwrd(mt.ID) then
        ' The string-id case     
    else        
        ' The integer-id case
    end if
 
end function

Regards
Peter

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[This message has been edited by Peter Stephensen (edited June 18, 2000).]

[Semen Matusovski]

Guys --
IsBadCodePtr checks the read access at the specified address.
Nothing more.
I agree than to use it much better than the minimum value of a pointer from the sky, but test following
(reduce - if necessary %Max)

#Compile Exe
#Include "win32api.inc"
%Max = 100000000 '&HFFFFFFFF&
Function PbMain
   Dim i As Dword, j As Dword
   Do
      i = j
      Do
         If i = %Max Then Exit Do
         If IsBadCodePtr(i) = %False Then MsgBox Format$(i),, "Start": Exit Do
         Incr i
      Loop
      If i = %Max Then Exit Do
      j = i
      Do
         If j = %Max Then Exit Do
         If IsBadCodePtr(j) = %True Then MsgBox Format$(j - 1),, "Finish": Exit Do
         Incr j
      Loop
      If j = %Max Then Exit Do
   Loop

End Function

On my PC under Win95b this code returns a lot of ranges.
(I guess, including DLL memory, drivers and so on).

------------------

[Tom Hanlin]

A "pointer" is a blob that points to an actual data item. It is not a good idea to construe a pointer as something for which a "minimum value" has any meaning. It's a wrong question!

There are no guarantees as to the range of values that a pointer may have. It's arbitrary and may change depending on the operating system, other programs that are loaded, or the phase of the moon.

The IsBadxxxPtr functions are not going to do the trick for you here. They only check to see whether the pointer points to a currently-valid area of memory. Obviously, some data values may randomly turn out to point to valid areas of memory. Worse, of course, the valid areas will change as your program allocates and deallocates memory, so the same value may be differently interpreted at different points in the program!

This is just not a right way of going about it. I'd suggest adding a flag byte or some other explicit "mode" value to your Type.

------------------
Tom Hanlin
PowerBASIC Staff