From: Shashank Date <shanko_date@...>
Date: 2005-01-27T05:26:49+09:00
Subject: Re: Debugging C Extensions and [A bit OT] general C Questions

Hi Brian,

--- Brian Schr�der <ruby@brian-schroeder.de> wrote:

> Hello Group,
> 
> I'm right now learning to write C extensions and it
> works quite good. The only problem is, that my C was
> never very profound and I seem to have forgotten all
> of it in the meantime. So obviously I write a lot of
> bugs. The question is, how do I debug them.
> 
> Right now I'm using printf to output things to the
> console,

That is how I debug all my C code :-)

>  but I'd rather like to debug the process in
> somethink like ddd. Is this possible?

Yes, I have done that too, although not with
ruby-extensions.

> I tried to connect to the running process, but it
> said to me that the process was "no program".
> 
> And now the OT question:
> 
> anybody knows of a compact C manual on the web,
> where I can for example learn how in this code:
> 
> /* Structure containing the volume data */
> typedef struct {
> 	unsigned long width, height, depth, length;
> 	double *data;
> } SRVolume;
> 
> /* Create a new SRVolume Datastructure, initialized
> to size 0x0x0 
>  * Is this allocation method correct, or am I
> copying the data onto the
>  * stack and back?
>  */
> SRVolume rvAlloc() {
> 	SRVolume *result = calloc(1, sizeof(SRVolume));
> 	result->data = calloc(0, sizeof(double));
> 	return(*result);
> }
> 
> the result data data is returned? 

Yes. I'm in the habit of type casting the calloc, like
so:
 
  SRVolume *result = (SRVolume *)calloc(1,
sizeof(SRVolume));

which, if nothing else makes my compiler happy ;-)

> I'd think that
> there is no copying of large amounts of data
> involved, 

Correct. Only the pointer is copied. I would check to
make sure that it did not return a NULL.

> but I'm not 100% shure because I haven't
> found a good manual explaining this yet.

Have you tried this:

http://www.eskimo.com/%7escs/cclass/krnotes/top.html

> Additionally I get under special circumstances
> (Creating a volume, setting a value and using
> p(volume)) the message that it tries to free a wrong
> pointer. 

What is p(volume) ? Is it in Ruby or C?

> If i change p(volume) to puts volume.inspect 

... looks like it is in Ruby. 

> it works. This is shurely not enough
> description, but maybe you could point me to some
> more information on the GC and other things that can
> bring forth this error message.

Can you show us some more code surrounding it?

HTH, 

> Best regards,
> 
> Brian

-- shanko

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