align macro kernel
bitwise-operators, c, linux, linux-kernel, operators
Solution
Say you have a number: `0x1006`
For some reasons you want to align it to a `4` bytes boundary.
With a 4-byte boundary, you know aligned values are `0x1000`, `0x1004`, `0x1008`, etc. You then also know the aligned value of `0x1006` is `0x1008`.
How would you get `0x1008`? The alignment mask for alignment value `4` is `(4 - 1) = 0x03`
Now `0x1006 + 0x03 = 0x1009` and `0x1009 & ~0x03 = 0x1008`
This operation is the `__ALIGN_MASK` macro.
If you want to pass the value `4` (the alignment) instead of directly `0x03` (the alignment mask), you have the `ALIGN` macro
Problem
I am unable to understand what this macro does. These are defined in `linux-kernel` but my doubt is independent of that. I am unable to understand what does `(((x)+(mask))&~(mask))` line does. ``` #define ALIGN(x,a) __ALIGN_MASK(x,(typeof(x))(a)-1) #define __ALIGN_MASK(x,mask) (((x)+(mask))&~(mask)) ``` Any help appreciated.