StrPtr

[Thomas Noelker]

How are you guys doing?

Don't kill me for this but I don't get it!

The following lines drive me nuts!
It might be discussed already but I can't
remember.
'---------------------------------------
#COMPILE EXE
#INCLUDE "tlxdebug.inc"

FUNCTION PBMAIN
DIM a$
DIM i AS LONG



a$ = a$ + SPACE$(524219)
debugprint str$(STRPTR(a$))
NEXT i


END FUNCTION
'---------------------------------------

RESULT = 1.192757E+7

If I just write a$ = Space$(524219) the world is ok and the size
can be as big as it wants.

Another step:
For i = 1 to 524219
a$ = a$ + " "
Next i
This will perform the same overflow

Everything is fine as long the string size is not bigger than 524218
if I expand an existing string.

Ok, what is that?
I mean it looks like a bug to me.

Oh, I forgot: Ram size is 64MB. Language is PBDLL 6.0
and tlxdebug is not needed. The Msgbox will do the same.

Regards
--- Tom.




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[Semen Matusovski]

Tom --
Lance already explained this (stack size).
Use Static/Global String (Asciiz) instead of Local.


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E-MAIL: [email protected]

[Thomas Noelker]

It doesn't work with global strings either.

Regards
---Tom


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[Semen Matusovski]

Tom --
Doesn't work ? What ?
a$ = a$ + " " one million times ?
It's clear that such operations
1) in any case take a lot of time
2) if you operate with dynamic strings I also expect serious troubles, because each time you ask bigger memory block.
Meanwhile in some OSes there are problems with memory management.

Try this

  #Compile Exe
  #Register None
  Function PbMain
     %n = 5000000
     %m = 3000000
     Global a As Asciiz * (%n + 1), i As Long
     ReDim b(1 To %n) As Byte At VarPtr(a)
     For i = 1 To %m
        b(i) = 32 ' instead of a = a + " "
     Next
     MsgBox Str$(Len(a$))
  End Function

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E-MAIL: [email protected]

[Thomas Noelker]

Semen--

I do well understand that it makes no sense to add one char
1000000 times but thanks for clarifying. ;-)

My question was just another:
If I allocate memory like that a$ = SPACE$(2^20) then I get a
Strinpointer by using STRPTR()

If I allocate memory like that a$ = "" : a$ = a$ + SPACE$(2^20)
Then I get an Overflow. If I use a predifined String instead of
Space$() then it is not gonna work either.

This technic is not working also with Global Strings.

I just want to know why?
What is going on internally when I do a$ = a$ + BigString
How is the memory allocated
What is different to just: a$ = anything


Ok.
---Tom




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[Semen Matusovski]

Tom --
I really do not understand about what you are talking.
BTW, about local stack. I remembered that Lance told about local pre-allocated strings (asciiz).
Here I agree - in your case Global/Static/Local has no relation to your problem.

Well, you wrote
"If I allocate memory like that a$ = SPACE$(2^20) then I get a
Strinpointer by using STRPTR()
If I allocate memory like that a$ = "" : a$ = a$ + SPACE$(2^20)
Then I get an Overflow."

You need to give full sample, which doesn't work, because on my PC similar code works under Win2000 and 95 too.

   #Compile Exe
   Function PbMain
      Dim a As String
      a$ = a$ + Space$(2^25)
      #Debug Print Str$(StrPtr(a$))
      MsgBox Str$(StrPtr(a$))
   End Function

I don't think that there is difference between a$ = a$ + BigString
and a$ = BigString, if initial a$ = "".

Another question that statements similar For i = 1 To x: a$ = a$ + " ": Next are able to touch any PC.

------------------
E-MAIL: [email protected]

[Thomas Noelker]

Semen --

It looks like that it is a problem on my machine. Thats the only
explanation I'm having.

Every value which is bigger than approx. 1/2 Meg makes this trouble.

But you know whats funny the length of the string is exactly the
length I've allocated. All the regular string operations are working
pretty well but not STRPTR() there I'm getting an overflow.

Regards.
---Tom


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

[Thomas Noelker]

Semen --

Now I found out that when I' doing this:
================================
DIM strp AS STRING PTR

strp = VARPTR(a$)
================================

that everything is ok and I can live with that.

But I still don't understand that STRPTR() is not working
correctly on my machine but on yours.

Regards.
--- Tom.


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

[Steve Hutchesson]

Tom,

I ran the following code and apart from it taking a very long time to
complete, it ran OK. Over 20 minutes on a PIII 600 with half gig ram.

    LOCAL a$
    LOCAL i AS LONG

    For i = 1 to 524219
    a$ = a$ + " "                           ' very slow operation
    SetWindowText hWin,ByCopy str$(len(a$)) ' nearly as slow
    Next i

    MsgBox "Done"

A basic dynamic string is not dependent on the stack size at all, if you
make a LOCAL variable of the type string, it takes 4 bytes for the string
handle so dynamic string is not your problem here. Basic string uses the
OLE string pool so it has no dependency on local or global memory.

The STRPTR() function gives you the address of the start of a dynamic
string. Where you have to be careful is in changing the string with normal
basic string functions as it may change the address of the dynamic string
after the initial address was obtained.

If you can give us a clear definition of the problem you have had, we may
be able to do something with it.

Regards,

[email protected]

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

[Thomas Noelker]

Hi Steve,

I'll try to clarify and here is the not working snippet first:
'==============================================================
#COMPILE EXE

GLOBAL a$

FUNCTION PBMAIN
a$ = ""
a$ = a$ + SPACE$(1000000)
MSGBOX STR$(STRPTR(a$))
END FUNCTION
'==============================================================
Result here is something like: 1.192757E+7
Which is an overflow isn't it?

Here is the working version:
'==============================================================
#COMPILE EXE

GLOBAL a$

FUNCTION PBMAIN
a$ = SPACE$(1000000)
MSGBOX STR$(STRPTR(a$))
END FUNCTION
'==============================================================
Result here is a pointer to the data.

Is this just on my machine or has somebody else ever had a problem
like that. The value of 1000000 may vary on other machines.

It is not that just STRPTR() gives me that result
this one here CVL( PEEK$( VARPTR(a$), 4)) comes out with the same.

And the funny thing is that the string *IS* allocated. I can do
operations with it but I need the pointer to the data.

Any ideas?

Regards
---Tom

[email protected]


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[Semen Matusovski]

Tom --
> Result here is something like: 1.192757E+7
> Which is an overflow isn't it?
No, it is not. This is normal address in virtual memory.
Instead of Str$ better to use Hex$.

> STRPTR() gives me that result CVL(PEEK$(VARPTR(a$), 4))

It's correct for dynamic strings.
At address VarPtr is located a POINTER (StrPtr) to string data.
See PB manual (assembler's samples).

Below is simple sample of direct usage StrPtr

   #Compile Exe
   Global a$
   Function PbMain
      a$ = "": a$ = a$ + Space$(1000000)
      ReDim b(1 : 1) As String * 1 At StrPtr(a$)
      b(24) = "A": b(25) = "B": b(26) = "C"
      MsgBox "/" + Mid$(a$, 24, 3) + "/",, Hex$(StrPtr(a$))
   End Function

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E-MAIL: [email protected]

[Thomas Noelker]

Alright Semen --

Now we finally got it.
This is what I want to know!

You see me like on that overflow thing.

Thanks a lot.

Regards.
---Tom.


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[email protected]