Hello everyone! I've encountered the problem which I already encountered before. Unfortunately, I had no time in the previous time to report and to talk about it. So I decided to play making my own "malloc" function in pure D (betterC) at this time. And I encountered the issue one more time. `this` can be null. How? Please take a look at my project [0]. It gets the current heap break and tries to increase via a free list. So the segfault I meet happens there [1]. Tell me, please. What do I wrong?

[0] https://github.com/ANtlord/deadmemory
[1] https://github.com/ANtlord/deadmemory/blob/master/src/deadmemory/freelist.d#L56

On Saturday, 9 March 2019 at 19:18:38 UTC, ANtlord wrote:
> Hello everyone! I've encountered the problem which I already encountered before. Unfortunately, I had no time in the previous time to report and to talk about it. So I decided to play making my own "malloc" function in pure D (betterC) at this time. And I encountered the issue one more time. `this` can be null. How? Please take a look at my project [0]. It gets the current heap break and tries to increase via a free list. So the segfault I meet happens there [1]. Tell me, please. What do I wrong?
>
> [0] https://github.com/ANtlord/deadmemory
> [1] https://github.com/ANtlord/deadmemory/blob/master/src/deadmemory/freelist.d#L56

You can end up with a null `this` reference if you dereference a null pointer to a struct and then call a method on the result. For example:

struct S
{
    bool isThisNull() { return &this is null; }
}

void main()
{
    import.std.stdio;
    S* p = null;
    writeln((*p).isThisNull); // true
}

Interactive version: https://run.dlang.io/is/fgT2rS

On Saturday, 9 March 2019 at 20:04:53 UTC, Paul Backus wrote:
>
> You can end up with a null `this` reference if you dereference a null pointer to a struct and then call a method on the result. For example:
>

I can but my reference is not null before calling. Take a look at the line of code [0]. There is a check before the line.

https://github.com/ANtlord/deadmemory/blob/master/src/deadmemory/mem.d#L20 [0]

On Saturday, 9 March 2019 at 20:04:53 UTC, Paul Backus wrote:
>
> struct S
> {
>     bool isThisNull() { return &this is null; }
> }
>
> void main()
> {
>     import.std.stdio;
>     S* p = null;
>     writeln((*p).isThisNull); // true
> }
>
> Interactive version: https://run.dlang.io/is/fgT2rS

Anyway, thank you! I didn't know about the feature.

On 03/09/2019 12:10 PM, ANtlord wrote:
> On Saturday, 9 March 2019 at 20:04:53 UTC, Paul Backus wrote:
>>
>> You can end up with a null `this` reference if you dereference a null pointer to a struct and then call a method on the result. For example:
>>
> 
> I can but my reference is not null before calling. Take a look at the line of code [0]. There is a check before the line.
> 
> https://github.com/ANtlord/deadmemory/blob/master/src/deadmemory/mem.d#L20 [0]

I haven't run the code but which pointer is null? Try adding this check as well:

  auto node = this.list.getFisrtFreeOrAdd(memViewLen);
  assert(node !is null);

Ali

On Saturday, 9 March 2019 at 21:00:51 UTC, Ali Çehreli wrote:
> I haven't run the code but which pointer is null? Try adding
I mean `this` by "this" word. You can see that `this` is null if you run gdb and before that line make `p/x this` [0]
> this check as well:
>
>   auto node = this.list.getFisrtFreeOrAdd(memViewLen);
>   assert(node !is null);
>
I get segfault in `getFisrtFreeOrAdd` method. Before the line I have an assertion [1]. It looks like the program misses (I have no idea how) `list` object while calling its method `getFisrtFreeOrAdd`.

[0] https://github.com/ANtlord/deadmemory/blob/master/src/deadmemory/freelist.d#L56
[1] https://github.com/ANtlord/deadmemory/blob/master/src/deadmemory/mem.d#L19

On 09/03/2019 21:10, ANtlord via Digitalmars-d-learn wrote:
> On Saturday, 9 March 2019 at 20:04:53 UTC, Paul Backus wrote:
>>
>> You can end up with a null `this` reference if you dereference a null pointer to a struct and then call a method on the result. For example:
>>
> 
> I can but my reference is not null before calling. Take a look at the line of code [0]. There is a check before the line.
> 
> https://github.com/ANtlord/deadmemory/blob/master/src/deadmemory/mem.d#L20 [0]

There is a typo in this instruction:

T* ptr = this.list.getFisrtFreeOrAdd(memViewLen).getPtr!T();
                        ^^
                        rs
(may this explain your null? the compiler should complain)

diniz

On Sunday, 10 March 2019 at 14:25:56 UTC, spir wrote:

> There is a typo in this instruction:
>
> T* ptr = this.list.getFisrtFreeOrAdd(memViewLen).getPtr!T();
>                         ^^
>                         rs
> (may this explain your null? the compiler should complain)
>
> diniz

Good catch! But I have the same typo within the definition of the method. I believe DMD screams about undefined method if it happens.