Access violations, reloaded

[mike]

Hi!

First off I wanted to say how great the new features are - D is getting  
more and more impressive. On a side note it's funny that by reading your  
discussions about Ruby iterators I got interested in Ruby, had a look into  
it and I was very impressed too. It seems that Ruby and D are both quite  
on the cutting edge of programming languages these days.

Anyway, here's the problem:

(I posted about that some time ago in the D.learn ng, I hope it's ok to  
follow up on it here. Code is pseudo-code typed from memory.)

I have a function that gets a C string from a DLL (it's a byte *, but the  
plugin works with char * internally). Host provides the buffer, plugin  
fills it:

' char[] getDisplayValue()
' {
'      byte[2000] temp;
'      pluginInstance.dispatch(GET_VALUE, &temp[0]);
'      return myConvertFunc(temp);
' }

myConvertFunc looks like that:

' char[] myConvertFunc(byte *buffer)
' {
'      return strip(toString(cast(char *)buffer)); // When I .dup here it  
crashes even faster!
' }

Then I have the GUI render code:

' [...]
' surface = SDL_RenderText(toStringz(getDisplayValue));
' SDL_BlitSurface(surface, position);
' SDL_FreeSurface(surface);
' [...]

Somewhere in this lurks an Access Violation which occurs randomly EITHER  
in the convert function, when getDisplayValue wants to return the value,  
in the SDL_RenderText OR when blitting the surface onto the screen. It is  
NOT reproducable exactly where it happens, but it happens roughly after  
the same time (about a minute or two, never actually timed it) each run.  
Memory goes up constantly while it's running. My best guess is that  
there's a problem with local variables, .dup-ing them and the GC which  
collects at the wrong time.

I now rewrote it like that:

' void getDisplayValueZ(byte *buffer)
' {
'     pluginInstance.dispatch(GET_VALUE, buffer);
' }

And in the render code:

' [...]
' byte[2000] buffer;
' getDisplayValueZ(&buffer[0]);
' surface = SDL_RenderText(cast(char *)&buffer[0]);
' SDL_BlitSurface(surface, position);
' SDL_FreeSurface(surface);
' [...]

That works. No Access Violation, no increase in memory usage.

Anyway, I'm still curious about it. I understand why there's no increase  
in memory usage in the second version, but I don't understand why there's  
an Access Violation in the first version to begin with. There must be some  
fundamental issue that I didn't quite get yet, those Access Violations  
bite me every now and then. Any hint would be great.

Regards,
Mike

-- 
Erstellt mit Operas revolutionärem E-Mail-Modul: http://www.opera.com/mail/