[gambit-list] Converting uint32 to scheme object
Marc Feeley
feeley at iro.umontreal.ca
Wed Jan 6 16:28:49 EST 2016
The code I sent previously already handles heap overflows properly (with raising an exception on the Scheme side). When ___err is not ___FIX(___NO_ERR), the operation failed and the glue code that was generated by the FFI will handle this properly by raising an exception.
As I said, a heap overflow could also happen when the heap limit that the user specified with -:hNNN is reached. But that is not a fatal error because the exception can be caught in Scheme.
Marc
> On Jan 6, 2016, at 4:20 PM, Adam <adam.mlmb at gmail.com> wrote:
>
> Marc,
>
> So what we get to then is that that Scheme object allocation could fail. In that case, would the mature way of dealing with it for the code return a special error, and then have a Scheme-side exception handler?
>
> Also just to understand - is the only thin that would allocating objects in the C world to fail, would be that malloc() failed, or are there other more local things that could make the object allocation fail?
>
>
> (define uint32_tag
> (lambda (a)
> (let* ((r ((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 = [ SOME ERROR CODE ]; ?
> }
> } else {
> ___result = ___FAL;
> }
> ") a)))
> (if (eq? a some-error-code) (##raise-heap-overflow-exception) a))
>
>
> 2016-01-06 23:02 GMT+08:00 Marc Feeley <feeley at iro.umontreal.ca>:
> 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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