DllStructs strange behavior

[Andreik]

Can someone explain this behavior?

If I run this code

For $i = 1 To 15
    Test()
Next

Func Test()
    Local $pStruct = DllStructGetPtr(CreateStruct())
    ConsoleWrite($pStruct & @CRLF)
    Local $tStruct = DllStructCreate('char Test[4];', $pStruct)
    ConsoleWrite('Value: ' & $tStruct.Test & @CRLF & @CRLF)
EndFunc

Func CreateStruct()
    Local $tStruct = DllStructCreate('char Test[4];')
    ConsoleWrite(DllStructGetPtr($tStruct) & @CRLF)
    DllStructSetData($tStruct, 'Test', 'test')
    Return $tStruct
EndFunc

the result is quite strange

expandcollapse popup

0x016C5DF0
0x016C5DF0
Value: test

0x016C5F90
0x016C5F90
Value: 

0x016C5E70
0x016C5E70
Value: test

0x016C5E50
0x016C5E50
Value: 

0x016C5FC0
0x016C5FC0
Value: test

0x016C5FB0
0x016C5FB0
Value: 

0x016C5E50
0x016C5E50
Value: 

0x016C5EB0
0x016C5EB0
Value: 

0x016C5DF0
0x016C5DF0
Value: test

0x016C5E00
0x016C5E00
Value: test

0x016C5ED0
0x016C5ED0
Value: 


0x016C5FD0
0x016C5FD0
Value: test

0x016C5EB0
0x016C5EB0
Value:  

0x016C5EF0
0x016C5EF0
Value: test

0x016C5F90
0x016C5F90
Value:

Sometimes seems to work fine but sometimes I get unexpected values.

If I change the code as below (changed just first two lines of code in Test function) seems to work fine.

For $i = 1 To 15
    Test()
Next

Func Test()
    $temp = CreateStruct()
    Local $pStruct = DllStructGetPtr($temp)
    ConsoleWrite($pStruct & @CRLF)
    Local $tStruct = DllStructCreate('char Test[4];', $pStruct)
    ConsoleWrite('Value: ' & $tStruct.Test & @CRLF & @CRLF)
EndFunc

Func CreateStruct()
    Local $tStruct = DllStructCreate('char Test[4];')
    ConsoleWrite(DllStructGetPtr($tStruct) & @CRLF)
    DllStructSetData($tStruct, 'Test', 'test')
    Return $tStruct
EndFunc

 

[spudw2k]

I feel like the DLLStructCreate in Test() is stomping over the struct you create in the CreateStruct() function, but I can't explain what is happening.

Can't you just use the struct as returned from CreateStruct()?  

For $i = 1 To 15
    Test()
Next

Func Test()
    Local $tStruct = CreateStruct()
    ConsoleWrite('Value: ' & $tStruct.Test & @CRLF & @CRLF)
EndFunc

Func CreateStruct()
    Local $tStruct = DllStructCreate('char Test[4];')
    DllStructSetData($tStruct, 'Test', 'test')
    Return $tStruct
EndFunc

What is the bigger picture?

[JockoDundee]

The problem in the code below is that you are losing the value that CreateStruct() returns because you are feeding it to DllStructGetPtr().  But after that you can’t reference it, only the pointer to it.

Func Test()
    Local $pStruct = DllStructGetPtr(CreateStruct())
    ConsoleWrite($pStruct & @CRLF)
    Local $tStruct = DllStructCreate('char Test[4];', $pStruct)
    ConsoleWrite('Value: ' & $tStruct.Test & @CRLF & @CRLF)
EndFunc

Isn’t this what you want:

For $i = 1 To 15
    Test()
Next

Func Test()
    Local $pStruct, $tStruct  
    
    CreateStruct($pStruct, $tStruct)
    
    ConsoleWrite('Value: ' & $tStruct.Test & @CRLF & @CRLF)
EndFunc

Func CreateStruct(ByRef $tStruct, ByRef $pStruct)
    $tStruct = DllStructCreate('char Test[4];')
    DllStructSetData($tStruct, 'Test', 'test')
    $pStruct = DllStructGetPtr($tStruct)
EndFunc

(Not tested)

Edit: Or as @spudw2k suggests, do you even want to use a pointer, since your allocating the memory yourself anyway?

Edited September 6, 2021 by JockoDundee

[Andreik]

I thought that this might be the issue but I don't get it why the first example doesn't fail every time.

PS: the actual script it's more complex, this was just a simplified script to reproduce this behavior.

[JockoDundee]

1 hour ago, Andreik said:

but I don't get it why the first example doesn't fail every time.

IMHO, and without knowing AutoIt internal Implementation details,  I think what’s happening is due to the $tStruct going out of scope - with its reference count=0, that’s why saving it works.  The randomness may just be due to the timing of garbage collection or whatever memory cleanup AutoIt does on $tStruct. If it runs async, then anything is possible.

[Andreik]

It makes sense.

[jchd]

That's the problem with pointers: whatever rock-solid vault you save it in, the "thing" the pointer points to (a block of memory) has to be kept alive until further use of the pointer, else anything bad can happen.

I don't recall that AutoIt uses a garbage collector. I believe the allocated block is just returned to the system. If you allocate a much larger block, an exception almost always results because the system is more likey to require an allocation fitting the hole made by releasing the initial block.

For $i = 1 To 5
    Test()
Next

Func Test()
    Local $pStruct = DllStructGetPtr(CreateStruct())
    ConsoleWrite($pStruct & @CRLF)
    Local $tStruct = DllStructCreate('wchar Test[1000000];', $pStruct)
    ConsoleWrite('Value: ' & $tStruct.Test & @CRLF & @CRLF)
EndFunc

Func CreateStruct()
    Local $tStruct = DllStructCreate('wchar Test[1000000];')
    ConsoleWrite(DllStructGetPtr($tStruct) & @CRLF)
    DllStructSetData($tStruct, 'Test', 'test')
    Return $tStruct
EndFunc

Yields an access violation at the second loop every time I ran it:

0x05F97020
0x05F97020
!>13:40:26 AutoIt3.exe ended.rc:-1073741819

 

[Danyfirex]

what AutoIt version are you using? I ran your code in a loop of 1000000 and it never failed. 3.3.14.5

I'm using Version 3.3.14.5

 

Saludos

[JockoDundee]

Maybe worth checking @error after the call to DllStructGetPtr().

[Andreik]

On 9/7/2021 at 3:15 AM, Danyfirex said:

what AutoIt version are you using? I ran your code in a loop of 1000000 and it never failed. 3.3.14.5

I'm using Version 3.3.14.5

 

Saludos

I'm using 3.3.14.5 as well.

[Andreik]

@jchd Since I used relative small buffers I didn't get the strange behavior but with your example it's kinda obvious what's going on.