So the your code is like this:
var TitleStr: string = 'abc';
procedure test;
var a: pChar;
begin
a := @ TitleStr [1];
a[1] := 'A';
end;
The reason it works is :
Though the string 'abc' is still defined in a reserved memory area when
OS allocates memory for the executable,
Delphi deals with a[1] := 'A' differently.
It logic is : apply a piece of memory from OS, copy 'abc' in reserved
memory area to this new memory area.
Then change the value of a[1] to 'A';
As var TitleStr: string = 'abc', means that TitleStr is a global
variable with initial value of 'abc', which you can change its value
later some time when running, so Delphi translates the code :
procedure test;
var a: pChar;
begin
a := @ TitleStr [1];
a[1] := 'A';
end;
to the following assembly code :
mov eax, $0044b9bc // $0044b9bc is the actual address im my
computer where 'abc' is defined.
// It is in reserved memory
area which is read only.
Call UniqueString // inside the UniqueString, there
is a line : call @NewAnsistring, then make a copy of 'abc' to this
memory.
Mov byte ptr [eax + $01], 'A' // eax now becomes $009432B0, it is the
pointer to the newly applied memory in the call UniqueString.
Thus no AV complains from OS.
Instead, if you define
const TitleStr = 'abc';
As it means that TitleStr is a global constant with value of 'abc',
which you can not change its value later some time when running,
so Delphi now translates the above code to the following assembly code
:
mov eax, $0044a91c // $0044a91c is the actual address of my
computer where 'abc' is defined. It is in reserved memory area.
Mov byte ptr [eax + $01], 'A' // eax points to the reserved memory
Thus AV complains from OS jumps out.
________________________________
From: delphi-boun...@listserver.123.net.nz
[mailto:delphi-boun...@listserver.123.net.nz] On Behalf Of Ross Levis
Sent: Monday, 13 May 2013 3:30 p.m.
To: 'NZ Borland Developers Group - Delphi List'
Subject: Re: [DUG] StrCopy problem - Reason : Const value notallowed
tobe changed
Yep.
Ross.
From: delphi-boun...@listserver.123.net.nz
[mailto:delphi-boun...@listserver.123.net.nz] On Behalf Of Jianming Lin
(FMI)
Sent: Monday, 13 May 2013 3:09 PM
To: NZ Borland Developers Group - Delphi List
Subject: Re: [DUG] StrCopy problem - Reason : Const value not allowed
tobe changed
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-boun...@listserver.123.net.nz
[mailto:delphi-boun...@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-boun...@listserver.123.net.nz
[mailto:delphi-boun...@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-boun...@listserver.123.net.nz
[mailto:delphi-boun...@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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