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digitalmars.D.internals - Modifying vtbl in ClassInfo

reply Jean-Louis Leroy <jl leroy.nyc> writes:
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?
Jun 26 2017
next sibling parent Jacob Carlborg <doob me.com> writes:
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
Jun 27 2017
prev sibling parent reply Walter Bright <newshound2 digitalmars.com> writes:
You could make it work by making a copy of the vtbl[], modifying it as you 
please, and seeing __vptr to it.

If you're trying to implement signals and slots, it's already in the library:

http://dlang.org/phobos/std_signals.html
Jul 21 2017
parent Jean-Louis Leroy <jl leroy.nyc> writes:
On Saturday, 22 July 2017 at 01:18:12 UTC, Walter Bright wrote:

 You could make it work by making a copy of the vtbl[], 
 modifying it as you please, and seeing __vptr to it.

 If you're trying to implement signals and slots, it's already 
 in the library:

 http://dlang.org/phobos/std_signals.html


Thanks for the suggestion. I am working on open methods, not 
signals. So far I've hijacked the 'deallocator' field but now I 
am experimenting with a perfect hash of the vptr. Seems to work 
quite well...
Jul 22 2017