Voodoo

I hate now knowing how things work. Especially things that I
rely on heavily as a hacker. But I love it when I get a chance
to finally figure out how the damn thing works and it turns out
to be beautifully simple.

This week I found that I had to debug a small part of libtool’s
behaviour. It had always worked for me before and I had assumed
it worked a certain way, but in this case it just wasn’t
working properly. I could feel myself breaking out in a cold
sweat. Don’t make me debug libtool! Please God, noooo…

But the clouds parted. And it was simple.

When you build an executable, and it links against a library
in the same module, you need some way to be able to run that
uninstalled executable with the uninstalled library. So,
libtool puts a script where you think your uninstalled
executable should be and here’s what it does

  • When you run the script for the first time, it re-links
    your executable with --rpath $path-to-lib/.libs and
    places that executable in .libs/lt-myexec.
  • That causes the linker to to put a DT_RPATH ELF
    attribute in the executable which tells the dynamic loader
    where to looks for libraries. (See the ld.so manpage)
  • And finally, the script just runs the modified executable.

For example:

  $> ls -l .libs/*gdk-pixbuf-csource
  -rwxrwxr-x    1 markmc   markmc      51262 Feb 26 11:28 .libs/gdk-pixbuf-csource
  $> ./gdk-pixbuf-csource
  Usage: gdk-pixbuf-csource [options] [image]
  $> ls -l .libs/*gdk-pixbuf-csource
  -rwxrwxr-x    1 markmc   markmc      51262 Feb 26 11:28 .libs/gdk-pixbuf-csource
  -rwxrwxr-x    1 markmc   markmc      51294 Feb 28 15:48 .libs/lt-gdk-pixbuf-csource
  $> objdump -p .libs/gdk-pixbuf-csource | grep RPATH
  $> objdump -p .libs/lt-gdk-pixbuf-csource | grep RPATH
    RPATH       /gnome/head/cvs/gtk+/gdk-pixbuf/.libs:/gnome/head/INSTALL/lib
  $> ldd .libs/gdk-pixbuf-csource | grep gdk_pixbuf
          libgdk_pixbuf-2.0.so.0 => /gnome/head/INSTALL/lib/libgdk_pixbuf-2.0.so.0 (0x0087d000)
  $> ldd .libs/lt-gdk-pixbuf-csource | grep gdk_pixbuf
          libgdk_pixbuf-2.0.so.0 => /gnome/head/cvs/gtk+/gdk-pixbuf/.libs/libgdk_pixbuf-2.0.so.0 (0x004a0000)

Not exactly, earth shattering. But nice to know.

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