Re: Wrong pointer calculation without casting on struct

[Ali Çehreli via Digitalmars-d-learn]

On 02/05/2015 07:59 PM, tcak wrote:

 writeln( Without: , (this + id.offsetof) );

In pointer arithmetic, the increment value means that many *objects* 
away, not than many bytes away.


Since id.offsetof is 4, you are calculating 4 MessageBase objects away 
(4*8==32 bytes away).


 writeln( With   : , (cast(size_t)this + id.offsetof) );

That is not pointer arithmetic, so the increment is just 4.

32 - 4 == 28.

Ali

Re: Wrong pointer calculation without casting on struct

[tcak via Digitalmars-d-learn]

On Friday, 6 February 2015 at 03:59:51 UTC, tcak wrote:

I am on 64-bit Linux.

I defined a struct that it 8 bytes in total.

align(1) struct MessageBase{
align(1):
ushort qc;
ushort wc;
ushort id;
ushort contentLength;
void[0] content;
}


I defined a function in this struct that tries to set a pointer 
to contentLength field.


writeln( Without: , (this + id.offsetof) );
writeln( With   : , (cast(size_t)this + id.offsetof) );

Results:
Without: 774F5030
With   : 140737342558228

0x774F5030 = 140737342558256


As it is seen, there is 28 bytes of difference between them. 
What is this behaviour exactly?


By the way,

writeln(Base Normal: , this);
writeln(Base Cast  : , cast(size_t)this);

Result:
Base Normal: 774F5010
Base Cast  : 140737342558224

0x774F5010 = 140737342558224

These are same. So, the issue is about addition process.

Re: Wrong pointer calculation without casting on struct

[Baz via Digitalmars-d-learn]

On Friday, 6 February 2015 at 04:10:08 UTC, tcak wrote:

On Friday, 6 February 2015 at 03:59:51 UTC, tcak wrote:

I am on 64-bit Linux.

I defined a struct that it 8 bytes in total.

align(1) struct MessageBase{
align(1):
ushort qc;
ushort wc;
ushort id;
ushort contentLength;
void[0] content;
}


I defined a function in this struct that tries to set a 
pointer to contentLength field.


writeln( Without: , (this + id.offsetof) );
writeln( With   : , (cast(size_t)this + id.offsetof) );

Results:
Without: 774F5030
With   : 140737342558228

0x774F5030 = 140737342558256


As it is seen, there is 28 bytes of difference between them. 
What is this behaviour exactly?


By the way,

writeln(Base Normal: , this);
writeln(Base Cast  : , cast(size_t)this);

Result:
Base Normal: 774F5010
Base Cast  : 140737342558224

0x774F5010 = 140737342558224

These are same. So, the issue is about addition process.


I've tried this;

import std.stdio;

struct MessageBase{
align(1):
ushort qc;
ushort wc;
ushort id;
ushort contentLength;
void[0] content;

void a(){
writefln(%.8X,(cast(void*)this + id.offsetof));
writefln(%.8X,(cast(size_t)this + id.offsetof));
}
}


void main()
{
MessageBase mb;
mb.a;
}

and got the same value for both. Your problem is:

typeof(this).stringof is MessageBase* so with ptr arithmetic 
when you add 1 you actually shift by 1 * MessageBase.sizeof.


typeof(cast(void*)this) is void* so with ptr arithmetic you 
shift by a certain number of bytes.

Wrong pointer calculation without casting on struct

[tcak via Digitalmars-d-learn]

I am on 64-bit Linux.

I defined a struct that it 8 bytes in total.

align(1) struct MessageBase{
align(1):
ushort qc;
ushort wc;
ushort id;
ushort contentLength;
void[0] content;
}


I defined a function in this struct that tries to set a pointer 
to contentLength field.


writeln( Without: , (this + id.offsetof) );
writeln( With   : , (cast(size_t)this + id.offsetof) );

Results:
Without: 774F5030
With   : 140737342558228

0x774F5030 = 140737342558256


As it is seen, there is 28 bytes of difference between them. What 
is this behaviour exactly?