is this code legal???

[jacfrosty]

Hi, could someone please tell me if this code is legal, if so then whats going on with it and if not - why not.

I was messing with pointers to structures and some how seem to have myself lost in the woods - so to speak.

Any info would be really appreciated.

Code: Select all

Structure testyItem
  name.s{30}
  x.l
  y.l
  len.l
  wid.l
  type.b
EndStructure

Structure testyStruct
  name.s{30}
  location.s{70}
  *item.testyItem[0]
EndStructure

t.testyStruct

For a = 0 To 999
  t\item[a] = AllocateMemory(SizeOf(testyItem))
Next

t\name="parts"
t\location="c:\DatabaseDir"
t\item[0]\name = "partno"
t\item[1]\name = "partdesc"


Debug SizeOf(t)
Debug SizeOf(t\name)
Debug SizeOf(t\item)

The results of the debugs are interesting as the last one is zero..

Kind regards

David

[Pupil]

In a bigger program you'll probably owerwite quite a lot of other variables using that code. The problem is the '*item.testyitem[0]' member of the structure 'testyStruct'. You're not allocating any space for the pointers so when you use 't\item[a]' you're actually accessing the memory '@t+sizeof(testyStruct)+4*a', which is bad

[jacfrosty]

I did think that that should be the case, but as it compiles and runs without errors I thought that something magic may have been going on, i.e. PureBasic may be allocating memory for the pointers or something.

I still can't get my head around why this works without instantly blowing up my computer as it would appear to be so very very dangerous..

[Pupil]

If you increase maxvalue in the FOR-loop you'll probably have a crash. I think windows allocates a whole page or something and wont complain until you try to poke outside this memory chunk.

[jacfrosty]

Thanks for that Pupil,

Ok then, is there a way that I can allocate memory for the array of pointers before I allocate the space that each pointer points to??

Basically I want to have a structure that can point to an array of structures that are of different sizes (when I say sizes I mean the no of elements in the array, not a different physical array size) - I hope I have explained what I mean correctly there..

So I was thinking something along these lines

Code: Select all

Structure testyItem
  name.s{30}
  x.l
  y.l
  len.l
  wid.l
  type.b
EndStructure

Structure testyStruct
  name.s{30}
  location.s{70}
  *item.testyItem[0]
EndStructure

t.testyStruct

t\item = AllocateMemory(4*1000)

For a = 0 To 999
  t\item[a] = AllocateMemory(SizeOf(testyItem))
Next

t\name="parts"
t\location="c:\DatabaseDir"
t\item[0]\name = "partno"
t\item[1]\name = "partdesc"


Debug SizeOf(t)
Debug SizeOf(t\name)
Debug SizeOf(t\item) 

Would that be OK??

Kind regards

David.

[Pupil]

No, i think that code would have the same problem as the previous one.

I've made two small examples that you should be able to use to solve your problem:

Code: Select all

; Example of one way to solve problem:
Structure testyItem
  name.s{30}
  x.l
  y.l
  len.l
  wid.l
  type.b
EndStructure

Structure ItemArray
  *item.testyItem[0]
EndStructure

Structure testyStruct
  name.s{30}
  location.s{70}
  *array.ItemArray
EndStructure

t.testyStruct

t\array = AllocateMemory(4*1000)

For a = 0 To 999
  t\array\item[a] = AllocateMemory(SizeOf(testyItem))
Next



;----------------------
; Another way to do it:

Structure testyStruct1
  name.s{30}
  location.s{70}
  *item.testyItem[0]
EndStructure

arraysize.l = 1000
*tmp.testyStruct1 = AllocateMemory(SizeOf(testyStruct1)+ 4*arraysize)

For a = 0 To 999
  *tmp\item[a] = AllocateMemory(SizeOf(testyItem))
Next

[jacfrosty]

Thanks very much for your help Pupil, greatly appreciated..

I think I understand where I was going wrong.

I am from a C background and things are done a little different when it comes to pointers and PureBasic.

Thanks for the examples.