Thread: Store pointee to an int variable

Hi all,

I am trying to assign a pointee into an integer and subtract a constant from that. Is this possible?

I have the following code:

Code:

#define OFFSET 0xC0000000
uint32_t *Pointer_A = (uint32_t*)0xDF3F1E00;
uint32_t Variable_A = Pointer_A - OFFSET;
printk("Variable_A is: 0x%X \n", Variable_A);

This gives 0xDF3F1E00 instead of the desirable 0x1F3F1E00.

Any hints?

Originally Posted by quzah

You aren't dereferencing Pointer_A. You are just subtracting from its address, not its value. See also: ptrdiff

Thanks for your reply. I don't want to dereference Pointer_A, I just want to subtract an OFFSET from the address Pointer_A points to and store the result into a variable (Variable_A).

Thanks.

You generally want to avoid converting between pointers and integers. However, you appear to be doing something non-portable anyway, so I'll assume these conversions make sense.

You appear to want to subtract a particular number of bytes from a pointer. The problem is that when you add to/subtract from a pointer, the compiler scales the value in terms of the size of the pointed-to object. So a uint32_t* points to a 4-byte object (pedants: it's probably OK to assume 8-bit bytes here), which means when you add 1 to a uint32_t*, you're actually adding 4 bytes (this way each time you increment a uint32_t*, it points to the next uint32_t).

The easiest way, of course, would be to subtract 0xC0000000 from 0xDF3F1E00, as an integer. So, given your code, you can try:

Code:

uint32_t Variable_A = (uint32_t)Pointer_A;
Variable_A -= OFFSET;

You can also use the fact that an unsigned char is one byte to get the pointer arithmetic to scale by bytes:

Code:

uint32_t Variable_A = (uint32_t)((unsigned char *)Pointer_A - OFFSET);

There's no guarantee that the result will be aligned properly, but I presume that's something you're aware of.

Thank you all for your replies (specially cas for the detail explanation given!). What tabstop suggested in the first place worked like a charm. Thanks again.

printk is printk() if I'm not wrong.
Not sure what he's doing according to his posts.