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Re: Incompatible pointer types with GCC

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August Karlstrom <fusionfile@gmail.com> writes:

> My question is whether this is a valid ANSI C (C89) program or not:
>
> 	typedef int Row[10];
>
> 	void P(const Row A[])

The [] might confuse some people.  This is simply

        void P(const Row *A)
> 	{
> 	}
>
> 	int main(void)
> 	{
> 	    Row A[10];
>
> 	    P(A);
> 	    return 0;
> 	}
>
> It compiles cleanly with Clang 3.5.0 and the options -pedantic
> -std=c89 -Wall.
>
> GCC 4.9.2, however, is not happy:
>
> $ gcc -c -o test.o -g -pedantic -std=c89 -Wall test.c
> test.c: In function ‘main’:
> test.c:12:4: warning: passing argument 1 of ‘P’ from incompatible
> pointer type
>   P(A);
>     ^
> test.c:3:6: note: expected ‘const int (*)[10]’ but argument is of type
> ‘int (*)[10]’
>  void P(const Row A[])
>       ^
>
> Any clues?

I think gcc is right to complain.

Whilst the types are not compatible, that's not really the problem here.
For example, you can pass an int * to a function that takes const int *
and those two types are not compatible.  Argument passing is done "as if
by assignment", and in an assignment the pointed-to type on the left can
have extra qualifiers.  But here, the const qualifies the element type,
not the pointed-to type, which is an array type.  (C has not qualified
array types at all).

The call is safe, of course, but C chose to use a simple rule rather
than trying to identify only those calls (and assignments) that are not
safe.  C++ has a more comprehensive rule.

Cases like this (and passing to a const T ** parameter) are some of the
rare times when you really need a cast.

-- 
Ben.

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Thread

Incompatible pointer types with GCC August Karlstrom <fusionfile@gmail.com> - 2016-12-16 11:41 +0100
  Re: Incompatible pointer types with GCC BartC <bc@freeuk.com> - 2016-12-16 11:06 +0000
  Re: Incompatible pointer types with GCC Ben Bacarisse <ben.usenet@bsb.me.uk> - 2016-12-16 17:07 +0000
    Re: Incompatible pointer types with GCC August Karlstrom <fusionfile@gmail.com> - 2016-12-16 23:02 +0100
      Re: Incompatible pointer types with GCC Ben Bacarisse <ben.usenet@bsb.me.uk> - 2016-12-17 00:48 +0000
      Re: Incompatible pointer types with GCC martin.uecker@med.uni-goettingen.de - 2016-12-18 08:06 -0800
        Re: Incompatible pointer types with GCC Ben Bacarisse <ben.usenet@bsb.me.uk> - 2016-12-18 20:11 +0000
          Re: Incompatible pointer types with GCC supercat@casperkitty.com - 2016-12-18 14:43 -0800
            Re: Incompatible pointer types with GCC Tim Rentsch <txr@alumni.caltech.edu> - 2016-12-29 13:33 -0800

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