[Dlang-internal] Modifying vtbl in ClassInfo

[Jacob Carlborg via Dlang-internal]

On 2017-06-27 00:19, Jean-Louis Leroy wrote:
> I realize I am doing bad things, anyway...
>
> I'd like to modify the vtbl inside ClassInfo. My goal is to stick a
> pointer inside each instance of a series of types, and I thought I'd
> reserve a slot in the vtbl and put the pointer in place of the function,
> like this:
>
> import std.stdio;
>
> class MethObject
> {
>    void methSlotTable() { }
> }
>
> int[] a_slots = [ 0, 1 ];
>
> void main()
> {
>    int firstUserVFunc = 5;
>    assert(MethObject.classinfo.vtbl[firstUserVFunc] ==
> &MethObject.methSlotTable);
>    writeln(MethObject.classinfo.vtbl);
>    MethObject.classinfo.vtbl[firstUserVFunc] = cast(void*) a_slots.ptr;
> }
>
> Not so surprisingly, I get a segfault:
> ldc2 -unittest --run abi.d
> [624230, 40CC50, 40C9E0, 40CC70, 40CB70, 40CED0]
> Error: abi failed with status: -2
> Error: message: Segmentation fault (core dumped)
> Error: program received signal 2 (Interrupt)
>
> Is the vtbl mprotected? Is there a chance of making this work?

It works for me on macOS using DMD. Might behave differently on 
different systems with different compilers.

-- 
/Jacob Carlborg