Question:

Can I have two pointers of different types (uint32_t * and char *) pointing to the very same address?

Here is why I want to have this:

I want to convert UTF-8 to UTF-32 and vice versa in C.

Lets say, I have a variable of type uint32_t that contains one UTF-32 encoded unicode character. And I already know that it needs 4 byte when encoded in UTF-8. It's binary representation is this:

00000000000aaabbbbbbccccccdddddd

a, b, c and d are 4 different ranges where each bit can be 0 or 1. With clever bitwise &, | and << operations I can rearrange these bits so that at the end there is this new distribution:

00000aaa00bbbbbb00cccccc00dddddd

And then I can flip some bits (using | again), to get this

11110aaa10bbbbbb10cccccc10dddddd

When I split this into 4 subsequent char variables in an array I have this:

11110aaa  10bbbbbb  10cccccc  10dddddd

which is exactly the UTF-8 encoding of the same unicode character.

So, the very same 4 byte in memory shall be one single uint32_t variable and at the same time an array of 4 char variables:

So, I want to have this:

uint32_t *utf32;
char utf8[4];

• *utf32 is a pointer that points to a single 4 bytes long uint32_t variable.
• utf8 is a pointer to an array of 4 char elements, each 1 byte long.

And I want that both pointers point to the very same address. So I can write a utf32 encoded character into the variable utf32, transform it in place, and then read the result form the array utf32. Is this possible? If so: How can I do it?

(I used this technique very often when I was coding in COBOL in the previous millennium, because in COBOL it's easy to overload the same region in the memory with many different definitions. But I don't know how to do it in C.)

I have found a lot of questions dealing with 2 pointers pointing to the same address, but in these questions the pointers have always the same type. And some other questions are about why you get an error if a pointer defined with a certain type points to an address that was defined with another type. But I didn't find anything about two pointers of different types sharing the same address.

CodePudding user response：

Can I have two pointers of different types (uint32_t * and char *) pointing to the very same address?

Yes, you can.

union U {
  uint32_t ui32;
  char c[4];
};

union U u;
u.ui32 = ...

uint32_t *pi = &u.ui32;
char *cp = u.c;

assert(pi == cp);

There are some C language rules which you'll violate IF you use the resulting char* to do something other than copying the data in or out, but the "two diffierent pointer types pointing to the same address" is not a problem in itself.

CodePudding user response：

Yes, two pointers of different types can point to the same address.

Let's say that somewhere in your memory is this utf32 and you know where that is so I will refer to this as address.

So if you'd want to treat these 4 bytes like a uint32 you could do this:

uint32_t* utf32 = address;

And you can just as easily treat is as a char array:

char* utf8 = address;

If you then want to access a char you just do:

utf8[index]