I have defined a type for objects we use. The details of the type should not be relevant for the users of the type, so I am "hiding" it behind a typedef. As it happens, this type is currently implemented as an array. This is problematic when we want to pass around pointers to variables of this type.
Example:
#define OBJECT_SIZE 5 typedef char object_t[OBJECT_SIZE]; // A type that happens to be an array int main( void ) { object_t my_object; object_t * p_object; p_object = my_object; // Compiler warning - pointer to different objects // (Correct warning - array decays to pointer to char. ) p_object = &my_object; // Compiler warning - pointer to different objects // Why this warning? // According to knowledgeable people, this should be // OK with respect to the C spec return 0; }
So, the question is: how can I obtain the address of the variable in a way that will give me a pointer to the given type? That is, I need a pointer to array[OBJECT_SIZE] of char, not a plain pointer to char.
With kind regards Asbj.S.
Not only does the programming language have such a thing as "pointer to array"; it even works exactly the same as it would for every other "pointer to something".
That is my understanding too. But how do I get the pointer to the array? As seen from my original example (relevant part of it included below) C51 objects when I try to take the address of the array and assign it to a pointer to the type of the array, giving a warning about "pointers to different objects".
typedef char object_t[OBJECT_SIZE]; // A type that happens to be an array int main( void ) { object_t my_object; // An object of the given type object_t * p_object; // And a pointer to an object of the same type p_object = &my_object; // Compiler warning - pointer to different objects }
Try type casting. Let the compiler know exactly what you want to do...
eg: p_object = (object_t*)(&my_object);
-Dave
But how do I get the pointer to the array?
The same way you would do it for any other variable: by using the '&' operator.
C51 objects when I try to take the address of the array and assign it to a pointer to the type of the array
I'm convinced the compiler is quite incorrect there.
Although the language definition doesn't restrict what a compiler may emit diagnostic messages about, this one is pointless and factually wrong.
Try type casting.
I would advise against that. Type casts of pointers are never the right solution, no matter what the problem seems to be.
Actually, so am I now. I have read up on the spec (well not exactly, the K&R2 appendix). And while an array often is converted into a pointer to its base type, pointing to the first element of the array, this conversion does explicitely not take place when the array is the operand of the & operator. Which means that "&array" should give the correct result.
I am discussing the issue with Keil.
Asbj.S.