# Variable corruption when using ioctl?

**URL:** <https://www.ruby-forum.com/t/variable-corruption-when-using-ioctl/140994>\
**Category:** Ruby\
**Created:** [June 20, 2008, 4:20pm UTC](https://www.ruby-forum.com/t/variable-corruption-when-using-ioctl/140994 "2008-06-20T16:20:27Z")\
**Posts on this page:** 3\
**Page:** 1

<div class="post-metadata">

**Author:** ![A\_Sussex](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/a/f14d63/32.png) [@A\_Sussex](https://www.ruby-forum.com/u/A_Sussex)\
**Post date:** [June 20, 2008, 4:20pm UTC](https://www.ruby-forum.com/t/variable-corruption-when-using-ioctl/140994/1 "2008-06-20T16:20:27Z")

</div>

I’m attempting to read the address assigned to an interface. After  
calling ioctl(SIOCGIFADDR) - which is 0x8915 on my system - I find  
that the destination buffer is corrupted and contains random garbage.  
Here I do this twice, with different results:

irb(main):008:0\> buf = [‘eth0’,’’].pack(‘a16h16’)  
=\>  
“eth0\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000”  
irb(main):009:0\> s.ioctl(0x8915, buf)  
=\> 0  
irb(main):010:0\> buf  
=\>  
“eth0\000\000\000\000\000\000\000\000\000\000\000\000\002\000\000\000\300\250\n\020\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\211\000\000\000\230n\b\b\370D\352\267Keiju  
ISHITSUKA(removed\_email\_address@domain.invalid)\n\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\021”  
irb(main):011:0\>

…

# irb(main):025:0\> buf = [‘eth0’,’’].pack(‘a16h16’) =\> “eth0\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000” irb(main):026:0\> s.ioctl(0x8915, buf) =\> 0 irb(main):027:0\> buf =\> "eth0\000\000\000\000\000\000\000\000\000\000\000\000\002\000\000\000\300\250\n\020\000\000\000\000\000\000\000\000\230\025\016\b\370D\352\267 :op\n\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\251\002\000\000\300\035\n\b\370D\352\267ex\_state

EXPR\_BEG\n\000\*rest)\n\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\021"  
irb(main):028:0\>

Is this a bug?

I’m using irb / Ruby 1.8.2-1 on Debian Etch.

---

<div class="post-metadata">

**Author:** ![A\_Sussex](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/a/f14d63/32.png) [@A\_Sussex](https://www.ruby-forum.com/u/A_Sussex)\
**Post date:** [June 20, 2008, 5:44pm UTC](https://www.ruby-forum.com/t/variable-corruption-when-using-ioctl/140994/2 "2008-06-20T17:44:06Z")

</div>

A Sussex wrote:

> I’m attempting to read the address assigned to an interface. After  
> calling ioctl(SIOCGIFADDR) - which is 0x8915 on my system - I find  
> that the destination buffer is corrupted and contains random garbage.

Well, on linux it can’t know the size of the argument given to  
ioctl(2) and for this reason it give it to ioctl() an argument with  
a default length of (256 + 1). This is why you see that buf has a  
size of 257 after the call to #ioctl.

You must use only the valid part of buf after the call.

Guy Decoux

---

<div class="post-metadata">

**Author:** ![A\_Sussex](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/a/f14d63/32.png) [@A\_Sussex](https://www.ruby-forum.com/u/A_Sussex)\
**Post date:** [June 23, 2008, 1:14pm UTC](https://www.ruby-forum.com/t/variable-corruption-when-using-ioctl/140994/3 "2008-06-23T13:14:10Z")

</div>

Guy Decoux wrote:

> You must use only the valid part of buf after the call.  
> So I guess I’m seeing bits of irb’s memory because the extended part  
> of buf isn’t initialised before use. (Please correct me if I’m wrong.)

Thanks for your help.