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@gmail.com wrote:
2016-01-06 22:37 GMT+08:00 Marc Feeley feeley@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?