[Bug c/49748] New: char * const * cannot be assigned to char const * const *

[Bug c/49748] New: char * const * cannot be assigned to char const * const *

[gcc at misc dot lka.org.lu]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=49748

           Summary: char * const * cannot be assigned to char const *
                    const *
           Product: gcc
           Version: 4.4.3
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c
        AssignedTo: unassigned@gcc.gnu.org
        ReportedBy: gcc@misc.lka.org.lu


Trying to assign a pointer value A with type "char * const *" into a variable B
declared as "char const * const * " triggers a warning, although the
const-yness of the type is _increased_ not decreased.

Such a thing may be needed when trying to call a function that displays a list
of strings:

void displayList(char const * const * list)
{
   /* display list */
}

The char const * declaration is needed if somewhere in the program, this
function is called with an array of static strings:

   char const  *list1[] = { "hello", "world", NULL };
   displayList(list1);


However, if elsewhere in the program, we want to call the function with a list
of _dynamically_ allocated strings, we get a "initialization from incompatible
pointer type" warning:
   char **list2;
   for(i=0; i< N; i++)
      list2[i] = strdup( some expression );

   ...
   displayList(list2);

   for(i=0; i<N; i++)
      free(list2[i]);

Because of the free(), we cannot declare list2[] as char const ** , but then
this restriction on _adding_ the const qualifier during assignment or parameter
passing is causing trouble.


There is a very similar case, where such a restriction on adding a const
qualifier makes sense:

  assiging  char **  to  char const **

(i.e. the case where in the target, only the string _contents_ are const, but
not the string pointers themselves).

In this case, the assignment warning would be appropriate, in order to prevent
the following:

char *launderConst(char const *in)
{
    char * source[1];
    char const ** target = source;

    target[0] = in;
    return source[0];
}


However if the target list itself is const (rather than just its string
contents), we would never be able to do this:
    char const *const* target = source;
    target[0] = in;

So why the warning about the first line, if the second one is forbidden anyways
(assigning a value to a const)?


Or is there another way of declaring the displayList() function without
triggering a warning in any of both use-cases?

[Bug c/49748] char * const * cannot be assigned to char const * const *

[redi at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=49748

--- Comment #1 from Jonathan Wakely <redi at gcc dot gnu.org> 2011-07-14 16:44:08 UTC ---
the bug reporting guidelines clearly ask for a complete and self-contained test
case, not several snippets of code which don't form a complete program
http://gcc.gnu.org/bugs/

the warning is correct, see http://c-faq.com/ansi/constmismatch.html

[Bug c/49748] char * const * cannot be assigned to char const * const *

[gcc at misc dot lka.org.lu]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=49748

--- Comment #2 from Alain Knaff <gcc at misc dot lka.org.lu> 2011-07-14 17:38:59 UTC ---
Created attachment 24756
  --> http://gcc.gnu.org/bugzilla/attachment.cgi?id=24756
Test program for const warning bug

> the warning is correct, see http://c-faq.com/ansi/constmismatch.html

Thanks for this link. This does indeed confirm what I said, in much less words
than I needed. Moreover, it points out that the issue doesn't exist in C++.

However, it doesn't explain why the issue still present in C.

Any insights into this question?

If this is just to keep some paper-pushers happy, maybe we could keep the
current behavior as the default, but have a -W flag to make gcc mimic g++'s
behavior?

[Bug c/49748] char * const * cannot be assigned to char const * const *

[tim.ruehsen at gmx dot de]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=49748

Tim Ruehsen <tim.ruehsen at gmx dot de> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |tim.ruehsen at gmx dot de

--- Comment #3 from Tim Ruehsen <tim.ruehsen at gmx dot de> 2012-02-23 14:12:00 UTC ---
Just to clarify it and to add a complete program.

#### x.c ####
void f(const char *const *args) {}

int main(int argc, char **argv) {
        f(argv);
        return 0;
}
####

Compiling with g++ (4.6.2) will do without warning.
Compiling with gcc (4.6.2) gives:

x.c: In function 'main':
x.c:4:2: warning: passing argument 1 of 'f' from incompatible pointer type
[enabled by default]
x.c:1:6: note: expected 'const char * const*' but argument is of type 'char **'


This is somewhat annoying when trying to harden older C sources with
-Wwrite-strings.
One has to insert (very) many casts to avoid the above warning.
This is much work that could be avoided by an apropriate -W option.

[Bug c/49748] char * const * cannot be assigned to char const * const *

[mpolacek at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=49748

Marek Polacek <mpolacek at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |RESOLVED
                 CC|                            |mpolacek at gcc dot gnu.org
         Resolution|---                         |FIXED

--- Comment #4 from Marek Polacek <mpolacek at gcc dot gnu.org> ---
(In reply to Tim Ruehsen from comment #3)
> This is much work that could be avoided by an apropriate -W option.

GCC 5 has now the -Wincompatible-pointer-types option that can be used to quiet
the warning.