# Possible Zlib Bug

**URL:** <https://www.ruby-forum.com/t/possible-zlib-bug/114219>\
**Category:** Ruby\
**Created:** [September 10, 2007, 2:28am UTC](https://www.ruby-forum.com/t/possible-zlib-bug/114219 "2007-09-10T02:28:32Z")\
**Posts on this page:** 3\
**Page:** 1

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**Author:** ![meekish](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/m/e95f7d/32.png) [@meekish](https://www.ruby-forum.com/u/meekish)\
**Post date:** [September 10, 2007, 2:28am UTC](https://www.ruby-forum.com/t/possible-zlib-bug/114219/1 "2007-09-10T02:28:32Z")

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# I originally posted this message to core as I assumed that it would be the mailing list with the most knowledge of Ruby’s Zlib library, but received no reply, so I’m posting it again here.

I’m porting SWX PHP ( [swxformat.org](http://swxformat.org) ) to Ruby (  
[http://rubyforge.org/projects/rswx](http://rubyforge.org/projects/rswx) ). SWX is a data exchange format—a  
replacement for Flash Remoting, SOAP, et al. The idea is to assemble  
Flash SWF files by converting Ruby objects (hashes, array, strings,  
integers, etc.) to SWF bytecode.

SWF files support compression using ZLib, and so I’m implementing  
compression using  
Ruby’s ZLib library.

As I’m porting SWX PHP to Ruby, I am able to generate a SWF file using  
SWX PHP to see what I need to generate in rSWX ( my port ).

There seems to be a bug in Ruby’s ZLib implementation that creates a  
different header and trailer than PHP’s gzcompress() (the function  
used to compress in SWX PHP).

Here is the bytecode of two SWF files containing the same data (one  
generated with SWX PHP and the other generated with rSWX):  
[http://pastie.caboo.se/95038](http://pastie.caboo.se/95038)

I’ve highlighted the discrepant parts of the two files (Please note  
that the first 8 bits of a compressed SWF file are left uncompressed,  
thus the header of the compressed section of the file starts on the  
second line of the pastied result).

And, needless to say, the compressed SWF file generated with SWX PHP  
works, and the one generated with rSWX does not.

I know very little about compression or Zlib so I’m quite lost at this  
point. I’ve tried to make this post as thorough as possible so that  
someone with a greater knowledge of hex bytecode could help me fix the  
issue without much effort. If I could post anything else to make it  
easier to find the issue, please let me know and I’ll be happy to do  
it.

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<div class="post-metadata">

**Author:** ![meekish](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/m/e95f7d/32.png) [@meekish](https://www.ruby-forum.com/u/meekish)\
**Post date:** [September 10, 2007, 4:19am UTC](https://www.ruby-forum.com/t/possible-zlib-bug/114219/2 "2007-09-10T04:19:44Z")

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Well, I’m thoroughly embarrassed. Maybe that’s why no one would take  
me seriously on the core mailing list. Case closed.

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<div class="post-metadata">

**Author:** ![meekish](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/m/e95f7d/32.png) [@meekish](https://www.ruby-forum.com/u/meekish)\
**Post date:** [September 10, 2007, 3:56am UTC](https://www.ruby-forum.com/t/possible-zlib-bug/114219/3 "2007-09-10T03:56:42Z")

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On Sep 9, 7:27 pm, “Jed H.” [removed\_email\_address@domain.invalid](mailto:removed_email_address@domain.invalid) wrote:

> I know very little about compression or Zlib so I’m quite lost at this  
> point. I’ve tried to make this post as thorough as possible so that  
> someone with a greater knowledge of hex bytecode could help me fix the  
> issue without much effort. If I could post anything else to make it  
> easier to find the issue, please let me know and I’ll be happy to do  
> it.

I don’t know much about zlib, but I did notice one thing in your  
pastie example. This:

swx\_file.slice!(0…8)

removes 9 bytes from the string. Maybe that should be:

swx\_file.slice!(0,8), or  
swx\_file.slice!(0…7)

?

Jeremy