Hi Saurav,
I think it comes down to whether a parameter like "const uint8_t * const"
is C89-compatible. I can't seem to find a reference that gives me a
definitive answer to this question, but it *might* have originally been a
C++ feature that became standard C at some point. Whether that happened
before or after C89 is what I can't seem to find right now.
But there is an easy way to check: write a simple program containing a
function with a "const int * const" parameter, and see if it compiles
without warning with gcc --std=c89 --pedantic. (I would do it myself but I
have only my phone right now.)
If it's C89-compatible, then it should be probably be used to help the
compiler produce better code.
Cheers,
Nathan
On Fri, Jul 12, 2024 at 10:28 AM Saurav Pal <resyfer....@gmail.com> wrote:
> Hi Alan,
>
> Thank you for the information. However, for my case, it's passed through
> function parameters.
>
> For example, let's say it's a helper function to read mem[2]. `mem` in this
> case is a function argument. It isn't reassigned, so the pointer is a
> const, and it isn't used to modify the underlying memory, so that's const
> as well. So:
>
> uint8_t helper(FAR const uint8_t * const mem)
> {
> return mem[2];
> }
>
> `mem` depends on the usage, but is not a compile time constant (like, say,
> we get `mem` from a malloc).
>
> This is an overly simplified case, but I have heard that const helps in
> optimizations.
>
> Regards,
> Saurav Pal.
>
> On Fri, Jul 12, 2024 at 6:28 PM Alan C. Assis <acas...@gmail.com> wrote:
>
> > Hi Saurav,
> >
> > I don't know why it is not much used, maybe Greg or Xiang have some idea.
> >
> > I think the most common use of const for variables that you want to keep
> in
> > flash to avoid keeping it in RAM (to save RAM space for MCU with low RAM
> > memory).
> >
> > The side effect on this case is Flash access is slower.
> >
> > BR,
> >
> > Alan
> >
> > On Thu, Jul 11, 2024 at 3:16 PM Saurav Pal <resyfer....@gmail.com>
> wrote:
> >
> > > Hi all,
> > >
> > > Suppose I have a pointer that I want to pass through function
> parameters.
> > > In the function, neither is the pointer reassigned, nor is the thing it
> > > points to modified in any form.
> > >
> > > So, I would assume its function argument signature to be like FAR const
> > > struct my_struct * const ptr (The second "const" is my focus here).
> > > Similarly for things like const uint8_t my_num, etc. as well.
> > >
> > > However, I have not really seen much use of specifying constants using
> > > const (in the way I have mentioned above). Most of the examples I see
> are
> > > of FAR const struct my_struct * ptr. I have heard the compiler can make
> > > better optimizations for const, and because of this, I try to spam
> const
> > > anywhere and everywhere I can. But I don't see too many such usages
> > across
> > > the codebase, so I was wondering why that is (or I just maybe reading
> the
> > > code wrong, in which case, forgive me 🙏).
> > >
> > > Regards,
> > > Saurav Pal.
> > >
> >
>
