Re: So You Think You Can Const?

On 07/01/2025 23:11, Kaz Kylheku wrote:

On 2025-01-07, Julio Di Egidio <julio@diegidio.name> wrote:

<snipped>

In particular, I am using C90, and compiling with
`gcc ... -ansi -pedantic -Wall -Wextra` (as I have
the requirement to ideally support any device).
>
To the question, I was reading this, but I am not
sure what the quoted passage means:
>
Matt Stancliff, "So You Think You Can Const?",
<https://matt.sh/sytycc>
<< Your compiler, at its discretion, may also choose
     to place any const declarations in read-only storage,
     so if you attempt to hack around the const blocks,
     you could get undefined behavior. >>

 An object defined with a type that is const-qualified
could be put into write-protected storage.

What do you/we mean by "object" in this context?  (Sorry, I do have forgotten, the glossary to begin with.)
Overall, I am surmising this and only this might go write-protected:
   MyStruct_t const T = {...};
While this one allocates a "byte-array", i.e. irrespective of how the pointer we are assigning it is declared:
   MyStruct_t const *pT = malloc(...);
Is my understanding (to that point) correct?

Objects coming from malloc are not defined by a declaration.

I vaguely remember the use of "declaration" vs "definition" I used to find confusing at the time, but some three decades have passed, maybe now I can do better...

ISO C defines the term /effective type/ (// indicates italics)
for the purposes of stating some rules regarding expressions accessing
objects. "The /effective type/ of an object that is not a byte array,
for an access to its stored value, is the declared type of the object"
says the N3301 draft of C23 in section 6.5.1 Expressions/General.
A footnote to this sentence clarifies that "allocated objects have no
declared type", almost certainly meaning dynamically allocated by
the malloc family.

Thank you so much overall, for the explanations and the final assessment: I couldn't hope for a better answer.

How you got the "const MyStruct_t *" pointer is that you first
treated the object as "MyStruct_t *", and filled in its members.
Then you cast the pointer to "const MyStruct_t *".

I was actually going out of my way trying to const-ify everything while trying not to cast anywhere: I totally misremembered what these things actually mean in C, and adding const everywhere possible is one of the habits I have meanwhile acquired from other languages.  But it's coming back...  I *will* do as you say.  :)
Thanks again very much,
Julio