Casting const void pointer to array of const char pointers properly in C

Question

I have a piece of C code that looks like this:

const char (*foo)[2] = bar();

Now bar() is a function that returns a (const void *). How do I properly cast this const pointer? The code produces this warning from GCC :

"initialization discards qualifiers from pointer target type".   

Here are some of my unsuccessful attempts:

const char (*foo)[2] = (const char *)bar();
const char (*foo)[2] = (const void **)bar();

The original code does work, I just can't get rid of the warnings by properly casting the return value.

EDIT : This has been suggested:

const char (*foo)[2] = (const char (*)[2])bar();

It appears to be correct, but GCC gives this warning :

"cast discards qualifiers from pointer target type"   

which is nearly identical to the original warning.

EDIT 2 : OK, I think I've got it. The real problem here is the ( const void * ) definition of bar(). The const in the definition (const char( * )[2]) refers to the elements of the array, not the pointer to the array. This type definition is essentially an array, which when represented by a void pointer is not const. The real answer is that a ( const void * ) loses its const-ness when cast to (const char ( * )[2]).

Answer 1

Several others have stated the correct cast, but it generates a spurious warning. That warning comes from a possible bug in the C standard, or (depending on your interpretation) a case that GCC should treat specially. I believe the const qualifier can be safely and unambiguously lifted to the array type. You can drop that warning with -Wno-cast-qual but of course that will eliminate warnings for cases that you actually care about.

To elaborate, the type const char (*)[2] means "pointer to array (length 2) of const char". The array is not marked const, just the elements of the array. When compared to the type const void *, the compiler notices that the latter is a pointer to const, where as the former is not, thus generating the warning. The C standard provides no way to mark an array as const, even though a const array would be equivalent to an array of const.

Answer 2

Try:

const char (*foo)[2] = (const char (*)[2])bar();

Edit but if bar returns a pointer to a const array of char pointers as your question title hints, there should be no need for a cast if you assign to a variable of this type:

char* const* foo = bar();