Re: So You Think You Can Const?

On 01/07/25 11:32 AM, Julio Di Egidio wrote:

 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. >>

Strictly speaking, the passage is misleading. It dues not matter whether the compiler decided to place const data into read-only storage. If you "hack around" data constness (i.e. if you attempt to modify const data), you _always_ get undefined behavior, regardless of where the data is actually stored.

I do not understand if just declaring that a pointer
is to constant data may incur in that problem

It can't. The original quoted passage is obviously meant to be about constant data, i.e. about _top-level_ constness.
Meanwhile, pointer declared as "pointer to constant data" is simply a constant _access path_ to some data. Constness of an access path does not imply constness of the actual data that path leads to. Forcefully removing constness from the access path and subsequently modifying the pointed data is perfectly legal (if inelegant) and causes no undefined behavior. Of course, this is only valid when the data itself is not const.

E.g. consider this little internal helper of mine
(which implements an interface that is public to
do an internal thing...), where I am casting to
pointer to non-constant data in order to free the
pointed data (i.e. without warning):
 ```c
static int MyStruct_free_(MyStruct_t const *pT) {
     assert(pT);
      free((MyStruct_t *)pT);
      return 0;
}
```
 Assuming, as said, that the data was originally
allocated with malloc, is that code safe or
something can go wrong even in that case?

It is perfectly safe. One can even argue that standard declaration if `free` as `void free(void *)` is defective. It should have been `void free(const void *)` from the very beginning.
--
Best regards,
Andrey