stack out of scope ?

[Steven Schveighoffer]

On 5/16/21 12:32 PM, Alain De Vos wrote:
> 
> Why doesn't this program dups core ?
> Is s not recycled ?
> ```
> import std.stdio:writeln;
> void main(){
> //I point to the heap
> int[] p=[1,2,3];
>      {
>          int[3]s=[1,2,3];
>          //I point to the stack
>          p=s;
>      }
>      //Why do I still live ?
>      writeln(p[0]);
> }
> 
> ```

To answer your question, why does this not dump core, it's because you 
didn't access memory that was invalid. Stack memory, like most computer 
memory, is allocated in chunks to the program. The OS typically 
allocates pages at a time per request for it. Which means, even though 
the memory isn't technically in scope, it's not going to cause a 
segmentation fault (this is a hardware fault saying you accessed an 
address that doesn't exist in your process).

In order for a dangling pointer (which is what this is) to cause a seg 
fault, you have to release that memory back to the OS, and make that 
address invalid before accessing. D very rarely gives memory back to the OS.

The more common problem for dangling pointers is that you access a piece 
of memory that has since been reallocated to something else, and screw 
up that something (generally a pointer).

-Steve