[krapp]

You have to allocate the size for m and n, because C arrays decay bare pointers.

But it seems to work, which I didn't expect[0]. It also seems like you have to de-reference it to use it which is weird.

[0]https://onlinegdb.com/HwX-WTL5t

[uecker]

Double indirection arrays with multiple allocations are 25 years obsolete (ok, there are some use cases) but since C99 we prefer to do it like the parent.

In your code link you over allocate memory, sizeof *arr is enough and you need to dereference like with (*arr)[i][j]. You need to dereference it because it is a pointer to an array, if you dereference you get an array. You can also let the first dimensions decay then it looks like:

  double (*arr)[m] = malloc(n * sizeof *arr);
  arr[i][j] = ...

but this is not as safe because the bound of the outermost dimension is lost. (Edited)