Why does right shift in PHP return a negative number?

Question

I am trying to query a bittorrent tracker and am using unpack to get the list of IPs from the response. So, something like this:

$ip = unpack("N", $peers);
$ip_add = ($ip[1]>>24) . "." . (($ip[1]&0x00FF0000)>>16) . "." . (($ip[1]&0x0000FF00)>>8) . "." . ($ip[1]&0x000000FF);

But, for some reason, I am getting the following IP addresses when I print $ip_add:

117.254.136.66
121.219.20.250
-43.7.52.163

Does anyone know what could be going wrong?

score 10 · Accepted Answer · answered Apr 16 '10 at 04:07

10

use long2ip() to transform number back into ip

answered Apr 16 '10 at 04:07

zerkms

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You are a savior! Thanks a lot. Cannot accept as answer within 10 mins of posting :)In any case, could you tell me what was going wrong with my method? – Legend Apr 16 '10 at 04:08
you have to add 2^31 to $ip before calculations ($ip + pow(2, 31)) – zerkms Apr 16 '10 at 04:15

score 5 · Answer 2 · edited May 23 '17 at 10:27

5

As zerkms says, long2ip does what you want. To answer your question, >> is an arithmetic right shift (so named because $i >> $j is equivalent to the arithmetic expression i / 2^j), which propagates the sign bit to preserve the sign of the number. That is, $i < 0 iff ($i >> $j) < 0. PHP doesn't have a logical shift (but you could define your own).

edited May 23 '17 at 10:27

Community

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answered Apr 16 '10 at 04:10

outis

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I see... Understood. Thanks for the explanation. Will go through the info you've provided. +1 Appreciate it. – Legend Apr 16 '10 at 04:12

score 1 · Answer 3 · answered Aug 16 '13 at 22:44

Try this

function decode_ip($str){
    $str = (float)($str);
    $ip = array(
        (int)($str/pow(2,24)),
        (int)($str/pow(2,16) & 0xFF),
        (int)($str/pow(2,8) & 0xFF),
        (int)($str & 0xFF)
    );
    return join(".", $ip);
}

decode_ip("3225422716"); //192.64.11.124

Why does right shift in PHP return a negative number?

3 Answers3