[gambit-list] Converting uint32 to scheme object

Marc Feeley feeley at iro.umontreal.ca
Wed Jan 6 10:02:11 EST 2016


On a 64 bit machine all uint32 values fit in a fixnum so there is no allocation possible.  On a 32 bit machine, converting a large uint32 will cause a heap allocation of a bignum.  However the code is written so that it detects heap overflows and raises a Scheme exception in that case (the Scheme exception can be handled in Scheme because the GC will have prereserved some room in the heap to do further processing in Scheme).

A Scheme heap overflow exception could be raised by a malloc failure or if the Scheme heap size limit is reached (see -:h runtime option).

Try

% gsi -:h1000 -e "(let loop ((x 0)) (loop (list x)))"
*** ERROR IN (string)@1.1 -- Heap overflow

% gsi -:h1000 -e "(with-exception-catcher (lambda (e) (pp (list 'caught e))) (lambda () (let loop ((x 0)) (loop (list x)))))"
(caught #<heap-overflow-exception #2>)

Marc

> On Jan 6, 2016, at 9:48 AM, Adam <adam.mlmb at gmail.com> wrote:
> 
> 
> 
> 2016-01-06 22:37 GMT+08:00 Marc Feeley <feeley at iro.umontreal.ca>:
> Of course one way would be to return a double (which has 53 bits of precision) and use NaN to encode “missing tag”.  I’m not kidding!
> 
> If you don’t like this approach, and I can understand why you wouldn’t, then you have to do the conversion to ___SCMOBJ by directly calling the conversion macros.  Something like this:
> 
> (define uint32_tag
>   (c-lambda
>    (TIFF* ttag_t)
>    scheme-object
>    "
>     ___U32 val;
>     if (TIFFGetField(___arg1, ___arg2, &val) == 1) {
>       if ((___err = ___U32_to_SCMOBJ (___PSTATE, val, &___result, ___RETURN_POS)) == ___FIX(___NO_ERR)) {
>         ___release_scmobj (___result);
>       }
>     } else {
>       ___result = ___FAL;
>     }
>    "))
> 
> Neat! What is the risk involved with this one - I mean, the Scheme object allocation on the Gambit heap could fail under certain circumstances. But would that only be on malloc() failure?




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