CODEWITHSUNDEEP
c
user3029591
user3029591

Reputation:

Understand the code to adjust buffer

Could anyone explain the following code to me?

adjustedbuf = (void *) ((uint64_t) buf & ~(pagesize - 1))

I don't get the idea of this statement.

Upvotes: 1

Views: 46

Answers (2)

Mat
Mat

Reputation: 206689

Assuming pagesize is a power of two, its binary representation will be something like:

  0000 10000000  // assume page size = 2^7 = 128

So pagesize-1 will be:

  0000 01111111  // 127

The negation of that (~) will be a bitmask, with all upper bits set, up to the "page size" bit:

  1111 10000000

If you & that with anything, you end up with a number that is "rounded down" to a multiple of the page size

  1100 10110110
& 1111 10000000
= 1100 10000000

Which is what that statement is doing. It aligns buf to a page size boundary.

(If pagesize isn't a power of two, the whole thing doesn't make much sense.)

Upvotes: 6

bcr
bcr

Reputation: 1328

It's page-aligning the buffer using bit operations. See

How to allocate aligned memory only using the standard library?

for more depth.

Upvotes: 2

Related Questions