Problem? VB string byRef to PB function modified via RTLMoveMemory

[Mike Joseph]

Hi
Im not sure if this is a problem or not. Could someone explain
this to me?

The following function is called from VB and the string
being passed in the Text argument = ""

Function PassStringByRef (ByRef Text as string) Export as Long
     Dim s1 as string
     Dim s2 as string
     
     s1 = "Hello"
     MoveMemory STRPTR(Text), STRPTR(s1), Len(s1)
     
     Msgbox Text  ' <-- Produces "Hello" in message box
     Text = Text & " added"  ' <-- Produces " added" in message box
     
     s2 = Text
     MSGBOX s2    ' <-- Produces "Hello" in message box
     s2 = s2 & " added"  
     Msgbox s2    ' <-- Produces " added" in message box
     
     Functon = 0  ' <-- The results of Text back on the VB side = "" 
  
End Function

Obviously the problem is a result of the dynamic string's length
not being updated when MoveMemory is used on it and can be
worked around by just using Space$() on the string ahead of time.
But is this normal behavior? I tested this scenario under VB and
the strings length is correctly updated after the MoveMemory
operation.

But it seems odd that Msgbox would show "Hello" at all. Sorta
like "now you see it, now you dont." It also seems odd that
when actually assigning Text to s1, that s1 takes on the exact
characteristics of Text.

Just to be clear, this isnt an issue for me since i now just
use Space$() to presize the string. Im just posting this message
for clarification whether this is a bug or by design.

Thanks
-Mike


[This message has been edited by Mike Joseph (edited August 28, 2000).]

[Lance Edmonds]

this is an faq question: http://www.powerbasic.com/support/pb...hread.php?t=26




------------------
lance
powerbasic support
mailto:[email protected][email protected]m</a>

[Mike Joseph]

I didnt really see anything at that link which seemed relevant
to this. This has nothing to do with passing args from VB to PB
and back. This is about using RLTMovememory on a dynamic string
in PB. I just found it odd that MsgBox shows the string yet when
you try to append to it, the original string is lost. Clearly though
its adviseable to just presize a string using Space$ or String$.
The following code causes a GPF when trying to append to the string.

#COMPILE EXE
#REGISTER NONE
#DIM ALL
#INCLUDE "win32api.inc"
FUNCTION PBMAIN()
    DIM s AS STRING
    DIM s1 AS STRING
    DIM s2 AS STRING
    
    s = ""
    s1 = STRING$(1000,"A")
    MoveMemory STRPTR(s), STRPTR(s1), LEN(s1)
   
    MSGBOX s
    MSGBOX s & " added"     <- GPF here
   
END FUNCTION

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

[Charles Dietz]

Mike,

To copy source bytes to a destination, you have to make sure that the
copied bytes don't step on important stuff. So I believe you have to
assign destination space before using MoveMemory or you risk getting
GPF errors. In your last post, you tried to copy bytes to a string
with no length.

As for your first post, assuming that you assigned string space in VB,
your problem is probably with the way VB passes strings. I don't know
that much about VB. I used PowerBasic to call your function and every
thing seems to work as expected, as demonstrated in the code below.

At least that's the way I see it. Hope this helps.

#COMPILE EXE
#DIM ALL
#INCLUDE "win32api.INC"
DECLARE FUNCTION PassStringByRef (BYREF TEXT AS STRING) AS LONG

FUNCTION PBMAIN
   LOCAL txt AS STRING
   txt = SPACE$(10)
   PassStringByRef txt
END FUNCTION

FUNCTION PassStringByRef (BYREF txt AS STRING) AS LONG
   DIM s AS STRING
   DIM s1 AS STRING
   DIM s2 AS STRING

   s1 = "Hello"
   MoveMemory STRPTR(txt), STRPTR(s1), LEN(s1)
   MSGBOX txt            ' <-- Produces "Hello"
   txt = txt + " added"  ' <-- Produces "hello      added"
   MSGBOX txt

   s2 = txt
   MSGBOX s2    ' <-- Produces "Hello      added"
   s2 = s2 & " added"
   MSGBOX s2    ' <-- Produces "Hello      added added"

   s = SPACE$(1000)  'Need to assign destination space here
   s1 = STRING$(1000,"A")
   MoveMemory STRPTR(s), STRPTR(s1), LEN(s1)
   MSGBOX s
   MSGBOX s & " added"     '<- no GPF here now

   FUNCTION = 0  ' <-- The results of Text back on the VB side = ""
END FUNCTION

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

[Fred Oxenby]

I dont belive that you can use RTLMoveMemory in Visualbasic, the way you
describe it.
This is the Powerbasic code

Function PbMain()As Long
  a$="123456789012345678901234567890"
  b$="Hello"
  MoveMemory StrPtr(a$), StrPtr(b$), Len(b$)
  Print a$
  waitkey$
End Function

This is the code you will need in VB to do the same thing.

Private Sub Form_Load()
  a$ = "123456789012345678901234567890"
  b$ = "Hello"
  MoveMemory StrPtr(a$), StrPtr(b$), 2 * Len(b$)

End Sub

The result will be the same => Hello6789012345678901234567890
Because either in VB or in PB RTLMoveMemory updates the dynamic strings
it only copies bytes from one area in memory to another..
If the destination string is too short, you will owerwrite something important.....

PS: Why do you need to use MoveMemory ??


------------------
Fred
mailto:[email protected][email protected]</A>
http://www.oxenby.se



[This message has been edited by Fred Oxenby (edited August 29, 2000).]

[Mike Joseph]

Thanks for the comments.

Fred, i need to use MoveMemory because Im copying data from a portion
of a page of virtual memory. I track the address offsets and
lengths of the data I need to read back in.

I suppose I could just use Peek$. (still need to get used to
all these handy, built in PB functions)

-Mike


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

[Fred Oxenby]

Well, this is how I should do it...

#Compile DLL
#Include "Win32Api.inc"

Function GetStringByRef()Export as string
Local TmpStr     as string
Local VirtAdress as long  
Local VirtLen    as long
  ..get your virtual adress and length...
  TmpStr = PEEK$(VirtAdress,VirtLen)
  ..manipulate your 'virtual data'
  TmpStr = TmpStr & " This is added to your virtual page"
  Function = TmpStr
End Function
  
..or
 
Function PassStringByRef(ByRef Text$) Export as long
Local VirtAdress as long  
Local VirtLen    as long
  ..get your virtual adress and length...
  Text$ = PEEK$(VirtAdress,VirtLen)
  'or if you use MoveMemory
  'Text$=Space$(VirtLen)
  'MoveMemory StrPtr(Text$),VirtAdress,VirtLen
  ..manipulate your 'virtual data'
  Text$ = Text$ & " This is added to your virtual page"
  Function = 0
End Function

VB-example:

 MyVirtPage$ = GetStringByRef()
  
 MyVirtPage$ = ""
 if PassStringByRef(MyVirtPage$)= 0 then
  Debug.Print "Alles OK"
 end if

------------------
Fred
mailto:[email protected][email protected]</A>
http://www.oxenby.se

[Tom Hanlin]

When you use MoveMemory, the amount of information that gets copied is based on the length you specify. When you copy to a string that isn't large enough to hold that many bytes, you'll either get a GPF or end up overwriting something else in memory.

The reason the message box works is presumably that there happened to be a CHR$(0) at the end of the string. The MessageBox API call is presumably looking for the CHR$(0) marker rather than the string length count, which won't be set by MoveMemory.


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

[Lance Edmonds]

Exactly... hence the reason I pointed out the FAQ on VB strings.

------------------
Lance
PowerBASIC Support
mailto:[email protected][email protected]</A>

[Wayne Diamond]

mike, this thread may be of use: http://www.powerbasic.com/support/pb...ead.php?t=2620
(re: inter-process communications using copymemory (rtlmovememory))