Comment by simonask
10 hours ago
If they do, that is no longer an implementation of C. It is a dialect of C, and there are many (GNU C being the most popular), but there are real drawbacks to using dialects.
This is in contrast to the other category that exists, which is "implementation-defined".
> If they do, that is no longer an implementation of C.
This is plain wrong. Undefined behaviour, means the C standard specifies no restriction on the behaviour of the program, which is what the implementation chooses to emit. An implementation can very well choose to emit any program it pleases, including programs that encrypt your harddisk, but also programs that stick to well defined rules.
Sure, but the point is that code written against such a compiler is not C and is not portable. It is written in a dialect of C, and that comes with drawbacks.
Writing C (or any language) means adhering to the standard, because that's the definition of the language.
Maybe it’s a generation thing. Languages like ML and Lisp have many implementations, while newer languages like Perl and Python are steered by a single organization. It’s way easier for the latter to have a single source of truth.
The C standard reminds me of Posix. You have a rough guideline if you ever wanted to port a program, but you actually have to learn the new compiler and its actual behavior before doing so.
You can't make any useful software in "Portable C" - or any portable language for that matter.
Side effects matter, and they are always non-portable/implementation defined/dependent on the hardware.
What printf() actaully does is implementation defined - what does "printing mean", does a console even exist? Maybe a user expects it to show graphical ascii/utf8 glyphs on a LCD display? Well, not every computer has that, so now what?
1 reply →
The thing is that the actual compiler behaviour matters more for real-world projects than what the C standard says. E.g. the C standard was always retroactive, it merely tried to reign in wildly different compiler behaviour at the time when the standard was new. It mostly succeeded, but still the most useful C and C++ compiler features are living in non-standard extensions.
Unaligned access being fine in one architecture, but not in others would create separate dialects, regardless of being blessed by ISO C.
Just don't do unaligned access, it's a dialect that doesn't exist currently, and should never exist.