[fpc-pascal] USB Human Interface Devices

[James Richters]

I am compiling with the Text IDE, and I set it up with the same settings I was using with the 32bit version…  and yes I did turn on Range checking, so that explains why we didn’t notice it before.

 

James

 

From: fpc-pascal <fpc-pascal-bounces at lists.freepascal.org> On Behalf Of Jean SUZINEAU
Sent: Friday, August 9, 2019 12:34 AM
To: James Richters <james at productionautomation.net>; 'FPC-Pascal users discussions' <fpc-pascal at lists.freepascal.org>
Subject: Re: [fpc-pascal] USB Human Interface Devices

 

Hi James,

 

Le 09/08/2019 à 02:19, James Richters a écrit :

so this was strange... it was working before
 
line 81            For J := 1 to Length(PortPath)-1 do
line 82              Write('->',PortPath[I]);
 
Line 82 is using variable I but it's in a loop that uses J, I didn't think that made a lot of sense.. so without really analyzing how any of this was supposed to work.. I just changed the I to a J and then I got this:

I think it's a typo. 

I think we didn't saw it before because Range type checking was disabled ( {$RANGECHECKS OFF} or {$R-} ).  

It seems it's the default value for the compiler: https://www.freepascal.org/docs-html/prog/progsu65.html . Very often in Windows system functions you have to deal with records with declared with a length of 1 and the real length stored a few bytes before in the record. This could not work with {$R+} 

But it's seems that for some reason it's enabled for your compiler (may be a different default value when your recompiled with fpcupdeluxe, or more likely a different default value in your settings for fp.exe ?).

I'm not an expert in memory allocation, but I think that while a certain amount of memory is allocated after the memory pointed to by PortPath and you're in {$R-} mode, you can happily read several bytes after the actual end of PortPath without any segmentation fault or memory access exception.

 

 
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