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Help with scope issues for NumPut/NumGet

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rulfzid

Post subject: Help with scope issues for NumPut/NumGet

Posted: May 30th, 2009, 4:20 am

Joined: November 27th, 2008, 9:44 am
Posts: 62

See the code examples. The three functions ostensibly all do the same thing, but func3 ends up yielding a different result. I don't understand why. When I compared the the return value before and after returning, they were the same. Why does it freak out about the zeroth offset?

Code:

func1(a, 1234, 5678)
msgbox % NumGet(a, 0) " : " NumGet(a, 4) ; displays "1234 : 5678"

func2(1234, 5678)
msgbox % NumGet(n, 0) " : " NumGet(n, 4) ; displays "1234 : 5678"

b := func3(1234, 5678)
msgbox % NumGet(b+0, 0) " : " NumGet(b+0, 4) ; displays "1024 : 5678"

func1(ByRef n, x=0, y=0)
{
   VarSetCapacity(n, 8)
   NumPut(x, n, 0)
   NumPut(y, n, 4)
}

func2(x=0, y=0)
{
   global n
   VarSetCapacity(n, 8)
   NumPut(x, n, 0)
   NumPut(y, n, 4)
}

func3(x=0, y=0)
{
   VarSetCapacity(n, 8)
   NumPut(x, n, 0)
   NumPut(y, n, 4)
   return &n
}

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SKAN

Post subject:

Posted: May 30th, 2009, 4:37 am

Joined: December 26th, 2005, 4:40 pm
Posts: 8776

Unless you use Static n / Global n in Func3() the variable content is lost when the function returns.

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rulfzid

Post subject:

Posted: May 30th, 2009, 4:58 am

Joined: November 27th, 2008, 9:44 am
Posts: 62

SKAN wrote:

Unless you use Static n / Global n in Func3() the variable content is lost when the function returns.

Then how does it get the right content at offset 4, and "kind of close" with the offset at 0:

Code:

b := func3(1234, 5678)
msgbox % NumGet(b+0, 0) " : " NumGet(b+0, 4) ; displays "1024 : 5678"

b := func3(2345, 5678)
msgbox % NumGet(b+0, 0) " : " NumGet(b+0, 4) ; displays "2304 : 5678"

b := func3(3456, 5678)
msgbox % NumGet(b+0, 0) " : " NumGet(b+0, 4) ; displays "3328 : 5678"

b := func3(4567, 5678)
msgbox % NumGet(b+0, 0) " : " NumGet(b+0, 4) ; displays "4352 : 5678"


func3(x=0, y=0)
{
   VarSetCapacity(n, 8)
   NumPut(x, n, 0)
   NumPut(y, n, 4)
   return &n
}

The behavior just seems really weird to me.

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SKAN

Post subject:

Posted: May 30th, 2009, 5:09 am

Joined: December 26th, 2005, 4:40 pm
Posts: 8776

rulfzid wrote:

Then how does it get the right content at offset 4, and "kind of close" with the offset at 0:

The first byte is overwritten with a Null and here is the proof:

Code:

mAdd := Func()

Loop 7
 MsgBox, 0, % "Offset:" A_Index-1, % DllCall("MulDiv", Int,mAdd+A_Index-1, Int,1, Int,1, Str )

Func() {
 Var := "rulfzid"
 Return &Var
}

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Lexikos

Post subject:

Posted: May 30th, 2009, 3:20 pm

Joined: October 17th, 2006, 4:15 pm
Posts: 7502
Location: Australia

That behaviour is undocumented and should in no way be relied upon. Additionally, it only happens if the variable was allocated "persistent" memory (< 64 bytes). The important point is that it is not logical or advisable to use a local variable's address after the function returns.

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rulfzid

Post subject:

Posted: May 31st, 2009, 12:11 am

Joined: November 27th, 2008, 9:44 am
Posts: 62

@SKAN & @ Lexikos

Thanks for the insights. I think I've figured out how to get around what I want - I'm using the winapi heap memory functions.

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