Easy GTK binary compatibility

If you compile ROX-Filer on a system with GTK 2.4, it will still work on a system with GTK 2.8. Compile it on a 2.8 system, and it will fail to run with 2.4. This problem isn't specific to ROX-Filer (or to GTK); recent versions of ROX-CLib and programs using it stopped working because they gained a phantom dependency on libglitz. Here's how to avoid these problems...

Introduction

One solution is to keep a complete Linux system around with the old version of GTK installed (either on a separate machine or in a chooted jail) and compile everything on that. This is how the ROX-Filer binary releases are normally built. There are some problems with this approach:

• A complete Linux distribution with GTK takes up a lot of disk space and takes a while to download.
• Not all programs need GTK 2.4. If you want to compile something for 2.2, you need yet another Linux installation.
• It's more hassle to build this way.
• People producing binaries for other platforms (PPC, x86_64, etc) need to set up a similar system.
• If you use a function from a newer GTK, you don't find out about the problem until release time.

Another approach, favoured by autopackage, is to use header versioning. Here, you add code to the GTK header files so that they can emulate older versions. The program sets some pre-processor macros to indicate the version it wants, and the headers emulate that version. This also has some problems:

• Annotating all the header files with this emulation code is hard work, and you have to keep doing it for every new version.
• The headers become more and more cluttered over time as more versions must be supported.
• If the library author makes a mistake, they probably won't notice. The developer of a program using the library won't notice. The packager creating a binary release won't notice. Only the end-user user notices, when the binary they download doesn't run. The fix requires going all the way back to the library author.

So, this method is a lot of work, is easy to get wrong, and makes the code more complicated. Not ideal.

A third solution is to install various different versions of the headers out-of-prefix (e.g., under /opt/gtk+-2.x) and set environment variables to make the compiler use one of them rather than the system default version. Luckily, downloading and installing out-of-prefix and setting environment variables are the Zero Install injector's speciality!

Using the injector to compile your program

To test this approach, I made a set of GTK, GLib and Pango headers available through Zero Install. I created a source interface file for ROX-Filer, called ROX-Filer/ROX-Filer-src.xml, which uses them:

<interface xmlns="http://zero-install.sourceforge.net/2004/injector/interface">
  <name>ROX-Filer-src</name>
  <summary>source code for ROX-Filer</summary>
  <description>
    Executing this program will compile a ROX-Filer binary.
  </description>
 
  <group>
    <requires interface="http://0install.net/2006/interfaces/GTK-dev">
      <version not-before='2.4' before='2.6'/>
      <environment insert="lib/pkgconfig" name="PKG_CONFIG_PATH"/>
      <environment insert="" name="GTKHEADERS"/>
    </requires>
 
    <requires interface="http://0install.net/2006/interfaces/GLib-dev">
      <version not-before='2.4' before='2.6'/>
      <environment insert="lib/pkgconfig" name="PKG_CONFIG_PATH"/>
      <environment insert="" name="GLIBHEADERS"/>
    </requires>
 
    <requires interface="http://0install.net/2006/interfaces/Pango-dev">
      <version before='1.5'/>
      <environment insert="lib/pkgconfig" name="PKG_CONFIG_PATH"/>
      <environment insert="" name="PANGOHEADERS"/>
    </requires>
 
    <implementation main='src/build' id="." version="2.4.1.0" released='Snapshot'/>
  </group>
</interface>

When executed, this runs ROX-Filer/src/build with PKG_CONFIG_PATH pointing to the .pc (pkgconfig) files for GTK 2.4.14, GLib 2.4.8 and Pango 1.4.1. The build script just runs make:

#!/bin/sh
cd `dirname "$0"`
make "$@"

I modified ROX-Filer's Makefile.in to set the gtkprefix and glibprefix pkgconfig variables to whereever Zero Install has placed the header files (normally the variable is simply called prefix, but we want to avoid affecting anything else here so I renamed them in the .pc files). The new Makefile has these lines:

PKG_CONFIG_FLAGS=--define-variable=gtkprefix="${GTKHEADERS}" \
                 --define-variable=glibprefix="${GLIBHEADERS}" \
                 --define-variable=pangoprefix="${PANGOHEADERS}"

CFLAGS = [...] `${PKG_CONFIG} ${PKG_CONFIG_FLAGS} --cflags gtk+-2.0 libxml-2.0`
LDFLAGS = [...] `${PKG_CONFIG} ${PKG_CONFIG_FLAGS} --libs gtk+-2.0 libxml-2.0`

Now, when I build ROX-Filer (by executing 0launch ROX-Filer-src.xml) it opens a dialog box prompting me to choose a version of the GTK and GLib headers:

When I select GTK 2.4.14, GLib 2.4.8 and Pango 1.4.1 (currently the only versions I've made available) it compiles a binary against those headers. This binary works with both my current GTK (2.8.16), and on my real (chrooted) GTK 2.4.14 installation.

Of course, an extra benefit here is that users who want to compile from source don't need to have the GTK headers already installed; Zero Install will get them automatically. Also, because compiling this way all the time is easy, I should notice immediately if I use a function that isn't in GTK 2.4.

Dependencies

For comparison, here are the dependencies listed in the resulting object files (objdump -p ROX-Filer|grep NEEDED):

Compiled with 0launch	Compiled normally

libgtk-x11-2.0.so.0	libgtk-x11-2.0.so.0
libgdk-x11-2.0.so.0     libgdk-x11-2.0.so.0
libatk-1.0.so.0         libXrandr.so.2
libgdk_pixbuf-2.0.so.0  libXi.so.6
libpangox-1.0.so.0      libXinerama.so.1
libpango-1.0.so.0       libXext.so.6
libgobject-2.0.so.0     libatk-1.0.so.0
libgmodule-2.0.so.0     libgdk_pixbuf-2.0.so.0
libdl.so.2              libpangocairo-1.0.so.0
libglib-2.0.so.0        libfontconfig.so.1
libxml2.so.2            libXcursor.so.1
libz.so.1               libpango-1.0.so.0
libm.so.6               libcairo.so.2
libX11.so.6             libXrender.so.1
libICE.so.6             libX11.so.6
libSM.so.6              libgobject-2.0.so.0
libc.so.6               libgmodule-2.0.so.0
                        libdl.so.2
                        libglib-2.0.so.0
                        libxml2.so.2
                        libz.so.1
                        libm.so.6
                        libICE.so.6
                        libSM.so.6
                        libc.so.6

As you can see, the older version has considerably fewer requirements, making it much easier for people to install.

Future work

I've tested this with ROX-Filer, but not with ROX-CLib's libglitz problem (because my version of GTK doesn't insert a dependency on libglitz). Hopefully it will work there too.

GTK 2.4 is the earliest version of GTK that ROX supports. However, other projects will want to compile against different versions (2.0, 2.2, 2.6, etc). Zero Install's requires element needs a way to set the default version (otherwise you have to select different versions depending on which program you are compiling).

Only GTK, GLib and Pango have been done. This approach could also be used for ATK, etc.

See also: Discussion on rox-devel