Re: Common Traps for C extensions

Robert Klemme wrote in post #1073887:
> On Thu, Aug 30, 2012 at 9:34 AM, Bernhard Brodowsky
> <[e-mail]> wrote:
>
>> Hi, thanks for your answer. Somehow it didn't compile as C++ code, if I
>> do the conversions,
>
> Well, as long as we do not see the code and / or the error we can't
> really tell why it did not compile.  I created a test this morning
> which worked but threw it away.  You do need to compile the catching
> and conversion code with extern "C" with a C++ compiler though.
>

I defined a function with two VALUE args and a VALUE return type and I 
wanted this to be a method with one argument (plus the self Argument) 
but I always got this error:

error: invalid conversion from 'VALUE (*)(VALUE, VALUE) {aka long 
unsigned int (*)(long unsigned int, long unsigned int)}' to 'VALUE 
(*)(...) {aka long unsigned int (*)(...)}' [-fpermissive]


>> Exactly, that is what I am talking about, checking for non-null does not
>> suffice, because Ruby calls my alloc function first, so the pointer is
>> actually valid, but it points to an uninitialized object.
>
> Yeah, but your allocation function could place one or more NULL
> pointers in the structure which get filled later, couldn't it?
>

Yes, I solved it exactly this way.

Henry Maddocks wrote in post #1073985:
> On 30/08/2012, at 7:34 PM, Bernhard Brodowsky wrote:
>
>>
>> But now I am thinking of possible traps I might run into. For example,
>> the Ruby interpreter forces me to differentiate between allocation and
>> initialization and in my current implementation, the User could redefine
>> the initialize() method of my class and then call another method, which
>> results in undefined behaviour or possibly segfault.
>
>
> Someone might sub-class and not call super.
>
> If it's possible to seg fault your extension then you are doing it
> wrong. After calling the alloc function, your class might not be
> 'valid', but it must be 'safe'. This is the reason Ruby added the alloc
> hook.

Yes, that is what I am trying to achieve, but it seems to be more 
difficult than I thought in the first moment, but it is safe for any 
scenario I can think of right now.

Cheers,
Bernhard

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