Pointers Are More Abstract Than You Might Expect in C

Re:Pointers Are More Abstract Than You Might Expect in C by jcranmer on Monday July 02, 2018 @09:57AM (#17441628)

What changes are you thinking of? The only change with respect to undefined behavior I can think of off the top of my head is punning via unions, which C99 changed from undefined behavior to well-defined behavior.

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Re:Pointers Are More Abstract Than You Might Expect in C by mpweiher on Monday July 02, 2018 @03:18PM (#17444585)

According to

http://www.complang.tuwien.ac.at/kps2015/proceedings/KPS_201...

new undefined behaviors were added, and for significant bits.

I think the bigger issue is that compiler writers have taken "undefined" to mean complete liberty to do whatever the hell they want, even fairly far away from the actual location of the undefined behavior.

And it turns out that the C89 standard had a range of "permissible" actions on undefined behavior. To me "permissible" is restrictive, as in you are not allowed to do anything outside that range.

Newer standards kept that section (3.4.3) the same, word for word, except for changing "permissible" to "possible". To me, "possible" is not restrictive but illustrative.

Now you might think this is reading tea-leaves on my part, and you might be correct, but on the other hand that change of interpretation is exactly what happened. Compilers used to not do these things, and nowadays compilers do do these things, and compiler writers cite the standard as giving them permission. Also consider to what extent wording of standards is finessed and fought over, and that exactly this one word was changed, in a section that otherwise remained the same word for word.

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