Packing executables

One of the biggest challenges with embedding platforms is the limitation related to file sizes. Here is the hint how to make executables smaller - strip them and pack them:

  • strip is a tool from GNU binutils, it discards symbols. Usually the platform toolchain has one.
  • upx is an excellent executable packer. Can be downloaded as binary or sources from the UPX sf.net site.

As an example, I'll show you the packing of Python 2.6.7 binary:

$ ls -s --block-size=KB python
6493kB python
$ strip -s python
$ ls -s --block-size=KB python
1696kB python
$ upx --best python
                       Ultimate Packer for eXecutables
                          Copyright (C) 1996 - 2010
UPX 3.05        Markus Oberhumer, Laszlo Molnar & John Reiser   Apr 27th 2010

        File size         Ratio      Format      Name
   --------------------   ------   -----------   -----------
   1692400 ->    608896   35.98%  linux/ElfAMD   python

Packed 1 file.
$ ls -s --block-size=KB python
611kB python

So, we've got 10:1 ratio, very impressive! :-)

Stripping is a very useful method, it saves a lot of the file size in our case. However, sometimes it doesn't help at all (depending of the executable and the way it was built). So try it before use; and let me show an example of packing without stripping:

$ ls -s --block-size=KB python
6493kB python
$ upx --best python
                       Ultimate Packer for eXecutables
                          Copyright (C) 1996 - 2010
UPX 3.05        Markus Oberhumer, Laszlo Molnar & John Reiser   Apr 27th 2010

        File size         Ratio      Format      Name
   --------------------   ------   -----------   -----------
   6489682 ->   2029592   31.27%  linux/ElfAMD   python

Packed 1 file.
$ ls -s --block-size=KB python
2032kB python

The ratio is not so good, and it was really slower. So please consider various ways to pack your files. Also don't forget to check the Wikipedia page "Executable compression".

P.S. Please note that packing executables can break its functionality. For example (from my experience), upx sometimes breaks applications which use frozen Python modules.

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