[DUG] StrCopy problem - Reason : Const value not allowed tobe changed

[Jianming Lin (FMI)]

Hi, Ross,

 

a := 'abc';

a[1] := 'A';

 

Certainly it will fail. 'abc' is defined in a reserved memory area when
OS allocates memory for the executable.

 

As for 

a := @ConsStr[1];

a[1] := 'A';

 

I am wondering how did you define ConsStr? 

var ConsStr : string = 'abc'; ?

 

 

________________________________

From: delphi-bounces at listserver.123.net.nz
[mailto:delphi-bounces at listserver.123.net.nz] On Behalf Of Ross Levis
Sent: Monday, 13 May 2013 2:55 p.m.
To: 'NZ Borland Developers Group - Delphi List'
Subject: Re: [DUG] StrCopy problem - Reason : Const value not allowed
tobe changed

 

The String and pChar in my case are global vars and the code is
executing in a library dpr between begin..end.  There are no units or
objects or procedures in use.  This works fine when using global vars, I
just tried it...

 

a := @ConsStr[1];

a[1] := 'A';

 

But this fails...

 

a := 'abc';

a[1] := 'A';

 

Ross.

 

From: delphi-bounces at listserver.123.net.nz
[mailto:delphi-bounces at listserver.123.net.nz] On Behalf Of Jianming Lin
(FMI)
Sent: Monday, 13 May 2013 10:35 AM
To: NZ Borland Developers Group - Delphi List
Subject: Re: [DUG] StrCopy problem - Reason : Const value not allowed to
be changed

 

It seems that we all focus on the StrCopy. That's misleading.

 

Let's look at this code:

 

procedure test;

var  a: pChar;

begin

   a := 'abc';

   a[1] := 'A';

end;

 

 

procedure TForm1.Button1Click(Sender: TObject);

begin

  test;

end;

 

The line : a[1] will cause exactly same AV as your example.

 

As CPU window shows cpu instruction in assembly language, not many
people can understand it, I do further experiment:

 

 

 

procedure test;

var  a: pChar;

const ConsStr  = 'abc';

begin

   a := @ConsStr[1];

   a[1] := 'A';

end;

 

 

The same AV will happen. And that's the key problem where your code
actually is.

 

Now we can see that the reason of your code crash:

 

Delphi compile 'abcdefghi' as the local const value. 

Certainly constant value is not allowed to change.

If you put a line : ConsStr := 'def'; Compiler won't let you go.

If you purposely use the trick of pointer : 

   a := @ConsStr[1];

   a[1] := 'A';

to bypass compiler checking, the  operating system then has to activate
the last protection by showing the AV and stop your program to run. 

 

 

Bevan is right : the area of memory for const value is protected while
the application is running.

 

 

 

 

________________________________

From: delphi-bounces at listserver.123.net.nz
[mailto:delphi-bounces at listserver.123.net.nz] On Behalf Of Ross Levis
Sent: Sunday, 12 May 2013 2:39 a.m.
To: 'NZ Borland Developers Group - Delphi List'
Subject: [DUG] StrCopy problem

 

var

  a: pChar;

  b: pChar;

begin

  a := 'abcdefghi';

  b := 'jklmnopqr';

  StrCopy(a,b);

end;

 

Question:  Why does this code crash?

 

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