Why subtraction is used to compare in nginx_rbtree.c?
mdounin at mdounin.ru
Tue Mar 24 15:42:34 UTC 2015
On Tue, Mar 24, 2015 at 03:39:23PM +0800, 张奕普 wrote:
> When I study the rbtree of nginx I can't understand the comments. Can
> someone give me a concrete example?Thanks :)
> * Timer values
> * 1) are spread in small range, usually several minutes,
> * 2) and overflow each 49 days, if milliseconds are stored in 32 bits.
> * The comparison takes into account that overflow.
> /* node->key < temp->key */
> p = ((ngx_rbtree_key_int_t) (node->key - temp->key) < 0)
> ? &temp->left : &temp->right;
Consider (assuming 32-bit keys):
node->key = 4294967295
temp->key = 0
The 0 value is bigger, because it's (4294967295 + 1). And that's
what the code checks. The
(ngx_rbtree_key_int_t) (node->key - temp->key)
(ngx_rbtree_key_int_t) (4294967295 - 0)
and becomes negative after casting to signed (note: this
conversion result is actually implementation-defined, but this is
how it works in practice).
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