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Re: gsubr.c: C << operator question.

karlheg@inetarena.com (Karl M. Hegbloom) writes:

>  I'm still learning C as well as scheme.  Can someone help me
>  understand this bit of code, please?
> 
> In guile-core/libguile/gsubr.c:84, it goes:
> 
>       long tmp = ((((SCM_CELLPTR)(SCM_CAR(symcell)))-scm_heap_org)<<8);
>       if (SCM_GSUBR_MAX < req + opt + rst) {
> 	fputs("ERROR in scm_make_gsubr: too many args\n", stderr);
> 	exit (1);
>       }
>       if ((tmp>>8) != ((SCM_CELLPTR)(SCM_CAR(symcell))-scm_heap_org))
> 	tmp = 0;
> 
>  ... Why does it first << 8 and then right away do >> 8, recompute and
>  compare like that?  How could the value change?

The value might not fit in the number of bits available (24 for 32 bit
longs).