From: Logan Capaldo <logancapaldo@...>
Date: 2006-12-29T22:54:34+09:00
Subject: Re: SWIG n00b: irb trapping when accessing C array?

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On 12/28/06, Dan Caugherty <dan.caugherty@gmail.com> wrote:
>
> Hey all --
>
> First, let me set up the situation: I have some machine-generated
> C code that is really nothing more than the declaration and definition
> of various byte arrays.  I'd like to convert it via Ruby and SWIG to
> another format. But something odd happens when I try to use
> the following *.i file in SWIG:
> --
> %module bytearrays
>
> %inline %{
> #include "bytearrays.h" /* just contains typedef for Data */
> extern Data * red;
> extern Data * blue;
> %}
>
> typedef unsigned char   Data;
>
> %{
> static Data data_get(Data * array, unsigned int idx) {
>    if (!array) return 0xFF; /* To do: throw exception */
>    return array[idx];
> }
> %}
>
> Data data_get(Data * array, unsigned int idx);
> --
>
> If I declare the arrays in bytearrays.c like so, I can get irb to
> segfault:
> --
> #include "bytearrays.h"
>
> static Data red[8] = {
>    0x00, 0x01, 0x02, 0x03, 0x04, 0x05, 0x06, 0x07
> };
>
> static Data blue[18] = {
>    0x00, 0x01, 0x02, 0x03, 0x04, 0x05, 0x06, 0x07,
>    0x00, 0x01, 0x02, 0x03, 0x04, 0x05, 0x06, 0x07,
>    0x00, 0x01
> };
> --
> That is, irb will choke on the following expression:
> irb> Bytearrays.data_get(Bytearrays.red, 0)
>
> But everything is fine if I do this:
> --
> #include "bytearrays.h"
>
> static Data red_data[8] = {
>    0x00, 0x01, 0x02, 0x03, 0x04, 0x05, 0x06, 0x07
> };
>
> static Data blue_data[18] = {
>    0x00, 0x01, 0x02, 0x03, 0x04, 0x05, 0x06, 0x07,
>    0x00, 0x01, 0x02, 0x03, 0x04, 0x05, 0x06, 0x07,
>    0x00, 0x01
> };
>
> Data * red = red_data;
> Data * blue = blue_data;
> --
>
> What gives? I'm pretty sure I'm missing something obvious, I just
> don't know what.  For the record, I'm running this on MacOS X
> 10.4.8 (Intel) with Ruby 1.8.5.
>
> Thanks in advance,
> -- Dan C.


I think it's because you have extern Data* red and static Data red[] ...

Make up your mind, are these globals or not?

What is probably happening is that the extern Data* red has some random
(invalid) memory address (or possibly is set to 0). Then you say static
Data* red which the C compiler says "Oh ok, new Data* red, lemme store this
in the static area". Ruby doesn't see the file scoped Data arrays, so it
attempts to deference the global Data arrays, which of course you haven't
assigned anything to.

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