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From: Thiago Adams <thiago.adams@gmail.com>
Newsgroups: comp.lang.c
Subject: Re: When Immutability and Nullability Meet: Exploring Mutable Objects
 in C
Date: Wed, 18 Sep 2024 17:20:46 -0300
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On 17/09/2024 18:27, Thiago Adams wrote:
> 
> I found some use cases where we may want disable const and _not_null (if 
> C had this qualifier) at same time. Kind of "anti-qualifier".
I think



I think my post might not have any replies because it's too long. So 
here's a shorter version.

Consider this sample: It has a error and a warning

#include <stdlib.h>
#include <string.h>

struct person {
   const char* const name;
};

struct person * make(const char* name)
{
   struct person * p = malloc(sizeof *p);
   if (p == 0) return 0;
   p->name = strdup(name);  //error read-only
   return p;
}

void person_delete(struct person* p) {
     free(p->name); //warning
}

int main()
{
   const struct person * p = make("a");
   if (p){
      person_delete(p); //warning
   }
   return 0;
}

My suggestion is remove the error and warning by adding a new qualifier 
mutable

mutable struct person * p = malloc(sizeof *p);

That allows p->name to be changed and person_delete to be called with no 
warnings.

void person_delete(mutable struct person* p) {
     free(p->name); //ok
}

https://godbolt.org/z/5Yo63ab1Y