malloc error when trying to assign the returned pointer to a struct field

[Steven Schveighoffer]

On Friday, 8 September 2023 at 07:59:37 UTC, rempas wrote:
> I do have the following struct:
>
...
> That's some minimal code that I do have just to showcase it.

This is not ideal. Why? Because 99% of the time, a poster has 
come here with a problem they don't know how to solve, and have 
focused in on where they *think* the problem is. However, the 
problem isn't there. But us reading the description can only see 
what the poster sees, and either don't see a problem ("I'm just 
as confused as you are!") or know there is more to the story.

Not only that, but frequently not-complete code is... not 
complete. And people tend to focus on problems they can see (e.g. 
where is that `_len` defined?), frustrating the poster with 
"trivial" problems that are solved "in the real code".

Inevitably, there is a subsequent post with the real code, and 
that contains the problem.

The best thing to post is a minimally reproducing example. The 
next best thing is a link to a complex reproducing example. Which 
you have done later (I will take a look). I just wanted to point 
this out because it's a frequent problem on these forums.

> So, some times, this work will works, some others, it will give 
> me the following error:
>
> `Fatal glibc error: malloc.c:2594 (sysmalloc): assertion 
> failed: (old_top == initial_top (av) && old_size == 0) || 
> ((unsigned long) (old_size) >= MINSIZE && prev_inuse (old_top) 
> && ((unsigned long) old_end & (pagesize - 1)) == 0)`

This is an internal message from glibc. It seems the malloc 
structure is corrupted.

> Is there any possible that there is a compiler bug? I do use 
> ldc2 and `betterC`!

There is always a chance...

Now, critiquing your original code, I see red flags here:

> ```d
>   u64 _cap = 0;   // Total amount of elements (not bytes) we 
> can store
> ```

and then later:

> ```d
>   this(i64 size) {
>     this._len = 0;
>     this._cap = size;
> ```

ok, so `size` must mean the number of elements, not the number of 
bytes.

> ```d
>     static if (is(T == char)) { size += 1; } // Additional 
> space for the null terminator
>     this._ptr = cast(T*)malloc(size);
> ```

Here, you have allocated `size` bytes for the array. Is this what 
is intended? Your comments suggest otherwise! If `T.sizeof` is 2 
or more, then you still only allocate e.g. 20 bytes for a `_cap` 
of 20. but an array of 20 T's would require 40 bytes.

-Steve