[Bug c/47143] New: warning about const multidimensional array as function parameter

[Bug c/47143] New: warning about const multidimensional array as function parameter

[rogi at linuxmail dot org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=47143

           Summary: warning about const multidimensional array as function
                    parameter
           Product: gcc
           Version: 4.4.3
            Status: UNCONFIRMED
          Severity: minor
          Priority: P3
         Component: c
        AssignedTo: unassigned@gcc.gnu.org
        ReportedBy: rogi@linuxmail.org


Created attachment 22876
  --> http://gcc.gnu.org/bugzilla/attachment.cgi?id=22876
source which produces warning

Ehllo,

I'm getting some strange warning about this code:

--- snip ---

typedef double mat4[4][4];

void mprod4(mat4 r, const mat4 a, const mat4 b)
{
/* yes, function is empty */
}

int main()
{
    mat4 mr, ma, mb;
    mprod4(mr, ma, mb);
}

------------

gcc output as follows:

--- snip ----

$ gcc -o test test.c
test.c: In function 'main':
test.c:13: warning: passing argument 2 of 'mprod4' from incompatible pointer
type
test.c:4: note: expected 'const double (*)[4]' but argument is of type 'double
(*)[4]'
test.c:13: warning: passing argument 3 of 'mprod4' from incompatible pointer
type
test.c:4: note: expected 'const double (*)[4]' but argument is of type 'double
(*)[4]'

------------

defining the function as:

--- snip ---

void mprod4(mat4 r, mat4 a, mat4 b)
{
}

------------

OR defining matrices at main as:

--- snip ---

mat4 mr;
const mat4 ma;
const mat4 mb;

------------

OR calling teh function in main as:

--- snip ---

mprod4(mr, (const double(*)[4])ma, (const double(*)[4])mb);

------------

OR even defining mat4 as:

--- snip ---

typedef double mat4[16];

------------

make teh warning go away. Wat is happening here? Am I doing something invalid?

I have attached source code.

Thanks for your attention.

[Bug c/47143] warning about const multidimensional array as function parameter

[rogi at linuxmail dot org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=47143

Igor <rogi at linuxmail dot org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
           Severity|minor                       |normal

[Bug c/47143] warning about const multidimensional array as function parameter

[jsm28 at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=47143

Joseph S. Myers <jsm28 at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |RESOLVED
         Resolution|                            |INVALID

--- Comment #1 from Joseph S. Myers <jsm28 at gcc dot gnu.org> 2011-01-01 12:02:28 UTC ---
Not a bug.  The function parameters are of type "pointer to array[4] of const
double" because const on an array type applies to the element type,
recursively, and then the outermost array type, only, of a parameter of array
type decays to a pointer, and the arguments passed are of type "pointer to
array[4] of double" after array-to-pointer decay, and the only case where
qualifiers are permitted to be added in assignment, argument passing etc. is
qualifiers on the immediate pointer target, not those nested more deeply.

[Bug c/47143] warning about const multidimensional array as function parameter

[rogi at linuxmail dot org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=47143

Igor <rogi at linuxmail dot org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|RESOLVED                    |UNCONFIRMED
         Resolution|INVALID                     |

--- Comment #2 from Igor <rogi at linuxmail dot org> 2011-01-01 13:02:09 UTC ---
And is it possible to write function definitions that avoid this while
maintaining my precious a[i][j] accesibility inside functions?

[Bug c/47143] warning about const multidimensional array as function parameter

[rogi at linuxmail dot org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=47143

--- Comment #3 from Igor <rogi at linuxmail dot org> 2011-01-01 13:16:39 UTC ---
(In reply to comment #1)
> Not a bug.  The function parameters are of type "pointer to array[4] of const
> double" because const on an array type applies to the element type,
> recursively, and then the outermost array type, only, of a parameter of array
> type decays to a pointer, and the arguments passed are of type "pointer to
> array[4] of double" after array-to-pointer decay, and the only case where
> qualifiers are permitted to be added in assignment, argument passing etc. is
> qualifiers on the immediate pointer target, not those nested more deeply.

This was somewhat confusing to me. Did you mean that funcion expects:

pointer to array[4] of const doubles

and I am passing

pointer to const array[4] of doubles

instead?

This really seems pretty much teh same to me.

Correct me if I'm wrong please.

[Bug c/47143] warning about const multidimensional array as function parameter

[jsm28 at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=47143

Joseph S. Myers <jsm28 at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |RESOLVED
         Resolution|                            |INVALID

--- Comment #4 from Joseph S. Myers <jsm28 at gcc dot gnu.org> 2011-01-01 15:06:34 UTC ---
If you want to use const here only on the callee side then you will need to use
casts.  I meant what I said: you are passing "pointer to array[4] of double",
with no const in there, to a function expecting "pointer to array[4] of const
double", and this is not a case permitted by C99 6.5.16.1#1.  C does not have a
type "pointer to const array[4] of double".

[Bug c/47143] warning about const multidimensional array as function parameter

[rogi at linuxmail dot org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=47143

Igor <rogi at linuxmail dot org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|RESOLVED                    |UNCONFIRMED
         Resolution|INVALID                     |

--- Comment #5 from Igor <rogi at linuxmail dot org> 2011-01-01 15:42:34 UTC ---
(In reply to comment #4)
> If you want to use const here only on the callee side then you will need to use
> casts.  I meant what I said: you are passing "pointer to array[4] of double",
> with no const in there, to a function expecting "pointer to array[4] of const
> double", and this is not a case permitted by C99 6.5.16.1#1.  C does not have a
> type "pointer to const array[4] of double".

Let me see if I understood.

const double a[4][4]

will become a "pointer to array of consts". So when i call my function passing
a "pointer to array", teh conversion fails since it's only allowed to apply teh
qualifier (const) to teh _pointer type_, which is an array, and not to it's
elements as teh function was expecting.

Correct me if I'm wrong again please and thanks for your patience.

[Bug c/47143] warning about const multidimensional array as function parameter

[jakub at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=47143

Jakub Jelinek <jakub at gcc dot gnu.org> changed:

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

--- Comment #6 from Jakub Jelinek <jakub at gcc dot gnu.org> 2011-01-01 16:36:27 UTC ---
Please stop reopening the PR, there is no bug on the gcc side.

[Bug c/47143] warning about const multidimensional array as function parameter

[muecker at gwdg dot de]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=47143

Martin Uecker <muecker at gwdg dot de> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |muecker at gwdg dot de

--- Comment #7 from Martin Uecker <muecker at gwdg dot de> ---
This is supported as an extension since GCC 5 and will be supported with the
upcoming C23 standard.