Wrong pointer calculation without casting on struct

Baz via Digitalmars-d-learn digitalmars-d-learn at puremagic.com
Thu Feb 5 20:34:52 PST 2015


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: 7FFFF74F5030
>> With   : 140737342558228
>>
>> 0x7FFFF74F5030 => 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: 7FFFF74F5010
> Base Cast  : 140737342558224
>
> 0x7FFFF74F5010 => 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.


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