Thread: __attribute__((aligned(x))) - Declare that a struct should be aligned to x

You shouldn't need any special declarations. Just make sure that when you allocate space for the page table that the space is page aligned.

Print the address (in hex), and make sure the right-most 12 bits are zero

It should be 0x?????000

BTW, a struct with only one element (even an array) will be packed.

Originally Posted by User Name:

> You shouldn't need any special declarations. Just make sure that when you allocate space for the page table that the space is page aligned.
That's the point of trying to align it to 4kb. I don't plan on ever enabling PAE, so I should be fine with 4kb.

I'm not sure how you expect alignment attributes to cause the structure itself to be 4k aligned. Alignment affects the members of the struct. If you want the entire struct to be aligned on a 4k boundary then you simply need to allocate it that way.

Suppose you used malloc() to allocate one of these structures (you probably aren't, since I have a good idea what you are doing here, but for instance). malloc() will align allocations to 8 bytes on its own, but it has no idea that the thing you are trying to allocate has a stricter alignment requirement than that. It's your own responsibility to make sure it aligns and the compiler can't really help you with it.

> Alignment affects the members of the struct. If you want the entire struct to be aligned on a 4k boundary then you simply need to allocate it that way.
If declared at compile time, it affects the structure alignment as well. See

Code:

typedef struct page_struct{
	unsigned int entry[1024];
}__attribute__((aligned(4096))) page_dir_t, page_table_t; /* athough their uses are different, they have the same structure */

int main()
{
	page_table_t t;
	printf("%p", &t);
    return 0;
}

Always prints 0x*000.

BTW, I don't even have a malloc yet. I'll probably ask you quite a few questions when I start on it. Such as: Does malloc return addresses from a common pool of memory(system wide) or process specific pool?

Originally Posted by User Name:

If declared at compile time, it affects the structure alignment as well.

Static alignment requirements are usually handled by declaring the structure in it's own .c file and then using linker directives to make sure the linker places it at the appropriate location. As you get deeper into this you'll probably want to control not only the alignment but the exact address where it goes, and ONLY the linker can do that.