[fpc-pascal] catch segfaults

[Martin]

On 14/03/2013 09:47, Jonas Maebe wrote:
>
> On 14 Mar 2013, at 01:48, Xiangrong Fang wrote:
>
>> The document said it is "deprecated"? Also, I want this to be cross
>> platform, not for unix only.  The use case is:
>>
>> try
>>  buf := GetMemory(1024);
>>  size := 10240;
>>  stream.Read(buf^, size);
>> except
>>  ??
>> end;
>
> That use case is a textbook example of why you should never try to 
> catch and handle access violations, except possibly if you then warn 
> the user that anything could happen if he continues (including erasing 
> all files on his hard drive, if he's very unlucky).
>
> An access violation often indicates that memory has been corrupted. 
> Catching the access violation does not undo the memory corruption, it 
> just informs you about this fact. If you continue after catching the 
> exception in the above program, you will be working with corrupted 
> memory, which means that the behaviour of your program becomes 
> completely unpredictable afterwards.
>
@Xiangrong

In fact (after I saw the screenshot, that indeed the alloc mem is less 
than the read), there is a possibility that the above good does NOT even 
crash. Yet it corrupts memory and the app will crash at a random later 
point.

If the 1024 bytes happen to be allocated in such way that they are 
followed by other already alocated memory, then they can be accessed 
without SigSegV.
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