Void* arithmetic

[NewProgrammer]

Hello everyone,

For a school project I am working on, I must use an array of unspecified objects:


void* voidarray;


In addition to this pointer, I need another one that points to a specific location beyond that address:


void* voidarray2 = voidarray + offset; // offset is an integer


However, I just learned that void* arithmetic is not allowed in C. How can I get voidarray2 to point to the specific location without using void* arithmetic?



NewProgrammer

[kbw]

You almost certainly shouldn't be using void* for this. void* is a pointer to anything. You clearly need a pointer to a some kind of object.

You should define an object hierarchy and use a pointer to that.

You'd only need a pointer to one past the end if you intend to iterate over a list of these objects, so there's a lot more going on than you're saying. You reall need to be clear in your mind what you want to do in order to instruct the computer.

[Catfish]

However, I just learned that void* arithmetic is not allowed in C.

You shouldn't do it, but it is supported... and a void pointer behaves a lot like a char pointer.

How can I get voidarray2 to point to the specific location without using void* arithmetic?

Do the pointer arithmetic on non-void pointers (cast them), and use the void pointer to store the result, I guess?

[NewProgrammer]

My project is to give my own implementation of malloc and free (called "my_malloc()" and "my_free") based on the buddy system.

I am using void pointers because I cannot predict what object a user of my program will want to allocate. Hence, "my_malloc()" returns a void* array, which the user can cast into what they want.



Here is where I run into a challenge: For those familiar with the buddy system, we know that if a user requests an 8k block, and the smallest block available is a 32k block, we must split the 32k block in half twice.

The split() function is the challenge. Before splitting anything, there is already a pointer that points to the head of the 32k block. Since I am splitting it, I want a pointer to point midway through the block so that it will point to the head of the right buddy. Doing that without doing void* arithmetic is my challenge at the moment.

[kbw]

You allocator seems to be working with blocks of chars (or unsigned chars). So use that internally. But you cast the pointer you return to a void*. You certainly don't use void* internally because you're working with blocks of bytes.

[NewProgrammer]

Char pointers are a great idea, since each char is a single byte. Thank you, kbw.

NewProgrammer