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From: Keith Thompson <Keith.S.Thompson+u@gmail.com>
Newsgroups: comp.lang.c
Subject: Re: So You Think You Can Const?
Date: Sat, 11 Jan 2025 20:03:12 -0800
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Kaz Kylheku <643-408-1753@kylheku.com> writes:
> On 2025-01-11, Andrey Tarasevich <andreytarasevich@hotmail.com> wrote:
[...]
>> In C and C++ (as well in virtually all higher level languages) 
>> "constness" is not a physical concept.
>
> It is for defined objects. A const object of static duration can be
> put into ROM, or virtual memory configured read-only. Either of these
> can generate a trap if written.

True, but a conforming implementation could put all objects into
read/write memory.  Attempting to modify a const-qualified object
still has undefined behavior, but is likely to "work", unless the
compiler performs optimizations based on the assumption that such
an object won't be modified.

For that matter, a compiler can put a non-const object into ROM if
it can determine that the program never attempts to modify it.
(As far as I know that's not a common optimization.)

All the standard really says about "const" is that directly modifying
a const-qualified object is a constraint violation, and indirectly
modifying it has undefined behavior.  This gives compilers some
additional freedom in how to treat such objects (storing them in
ROM, for example) *and* provides some information that can be used
in optimization (e.g., not reloading a value from memory because
it can't have changed).

-- 
Keith Thompson (The_Other_Keith) Keith.S.Thompson+u@gmail.com
void Void(void) { Void(); } /* The recursive call of the void */