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Oh, forgot to say...Oh, forgot to say...Posted May 24, 2011 18:35 UTC (Tue) by jrn (subscriber, #64214)In reply to: Oh, forgot to say... by khim Parent article: What Every C Programmer Should Know About Undefined Behavior #3/3
Wouldn't a loop to access the internal representation of a float through a pointer to char produce implementation-defined behavior, rather than nasal demons? On the other hand, reading through a pointer to unsigned int is problematic, of course.
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Yes, that's true... Posted May 25, 2011 5:01 UTC (Wed) by khim (subscriber, #9252) [Link] Ah, my bad. Sure, but the temptation is very high to use conversion to int because they are of the same size - and this is impossible to do even if you use two transitions like (int *)(char *)pf. The only way to portably and correctly do that is via memcpy - and with current crop of the compilers it's quite efficient too (both memcpy and pointers will be elided), but this is counter-intuitive if you don't know about undefined behaviors and still think that C is high-level assembler.
Yes, that's true... Posted May 25, 2011 5:19 UTC (Wed) by jrn (subscriber, #64214) [Link]
Even memcpy is not portable to platforms where sizeof(float) != sizeof(int). :)
A rough and incomplete rule of thumb about aliasing rules is that constructs that would be portable given how alignment and size varies from platform to platform are likely to be permitted.
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