Author: Joe Barr
That’s where ldconfig comes in, and why it pays to know a little about what it does and how to use it.Dynamically linked libraries are located and loaded at run time, either by ld.so or ld-linux.so, depending on how it was compiled: in a.out or ELF format. Each of these programs looks through a list of places to find libraries when they are needed. One place they both look in is a file named /etc/ld.so.cache, which contains an ordered list of libraries to be searched for the needed module.
To get an idea how this works, let’s use ldd to print the dependencies for a well-known program, like the GIMP. On my system, that command looks like this:
ldd /opt/gnome/bin/gimp
ldd then politely shows each dependency, including a pointer to the actual library location where it was found, and an offset
and results in this:
linux-gate.so.1 => (0xffffe000)
libgimpwidgets-2.0.so.0 => /opt/gnome/lib/libgimpwidgets-2.0.so.0 (0x4002d000)
libgimpmodule-2.0.so.0 => /opt/gnome/lib/libgimpmodule-2.0.so.0 (0x4010c000)
libgimpcolor-2.0.so.0 => /opt/gnome/lib/libgimpcolor-2.0.so.0 (0x40111000)
libgimpthumb-2.0.so.0 => /opt/gnome/lib/libgimpthumb-2.0.so.0 (0x4011a000)
libgimpmath-2.0.so.0 => /opt/gnome/lib/libgimpmath-2.0.so.0 (0x40122000)
libgimpbase-2.0.so.0 => /opt/gnome/lib/libgimpbase-2.0.so.0 (0x40127000)
libgtk-x11-2.0.so.0 => /opt/gnome/lib/libgtk-x11-2.0.so.0 (0x40135000)
libgdk-x11-2.0.so.0 => /opt/gnome/lib/libgdk-x11-2.0.so.0 (0x403fd000)
libatk-1.0.so.0 => /opt/gnome/lib/libatk-1.0.so.0 (0x40478000)
libgdk_pixbuf-2.0.so.0 => /opt/gnome/lib/libgdk_pixbuf-2.0.so.0 (0x40491000)
libm.so.6 => /lib/tls/libm.so.6 (0x404a5000)
libpangoxft-1.0.so.0 => /opt/gnome/lib/libpangoxft-1.0.so.0 (0x404c8000)
libpangox-1.0.so.0 => /opt/gnome/lib/libpangox-1.0.so.0 (0x404d0000)
libart_lgpl_2.so.2 => /usr/lib/libart_lgpl_2.so.2 (0x404db000)
libpangoft2-1.0.so.0 => /opt/gnome/lib/libpangoft2-1.0.so.0 (0x404f1000)
libpango-1.0.so.0 => /opt/gnome/lib/libpango-1.0.so.0 (0x40517000)
libgobject-2.0.so.0 => /opt/gnome/lib/libgobject-2.0.so.0 (0x4054f000)
libgmodule-2.0.so.0 => /opt/gnome/lib/libgmodule-2.0.so.0 (0x40583000)
libdl.so.2 => /lib/libdl.so.2 (0x40588000)
libglib-2.0.so.0 => /opt/gnome/lib/libglib-2.0.so.0 (0x4058c000)
libfontconfig.so.1 => /usr/lib/libfontconfig.so.1 (0x4060c000)
libfreetype.so.6 => /usr/lib/libfreetype.so.6 (0x4063a000)
libc.so.6 => /lib/tls/libc.so.6 (0x406a9000)
libX11.so.6 => /usr/X11R6/lib/libX11.so.6 (0x407c2000)
libXrandr.so.2 => /usr/X11R6/lib/libXrandr.so.2 (0x408bf000)
libXi.so.6 => /usr/X11R6/lib/libXi.so.6 (0x408c3000)
libXinerama.so.1 => /usr/X11R6/lib/libXinerama.so.1 (0x408cb000)
libXft.so.2 => /usr/X11R6/lib/libXft.so.2 (0x408ce000)
libXfixes.so.3 => /usr/X11R6/lib/libXfixes.so.3 (0x408e1000)
libXcursor.so.1 => /usr/X11R6/lib/libXcursor.so.1 (0x408e6000)
libXrender.so.1 => /usr/X11R6/lib/libXrender.so.1 (0x408f0000)
libXext.so.6 => /usr/X11R6/lib/libXext.so.6 (0x408f8000)
/lib/ld-linux.so.2 (0x40000000)
libexpat.so.0 => /usr/lib/libexpat.so.0 (0x40906000)
Now let’s see what happens if we rename one of those dependencies, so it can’t be found at run time. We’ll rename to /usr/X11R6/lib/libXrender.so.1 to /usr/X11R6/lib/libXrender.backup.so.1 with this mv command:
mv /usr/X11R6/lib/libXrender.so.1 /usr/X11R6/lib/libXrender.backup.so.1
Now, when we try to run the GIMP from the command line, we get:
warthawg@linux:~> gimp gimp: error while loading shared libraries: libXrender.so.1: cannot open shared object file: No such file or directory
Important Note: Do this right now, before you forget. Use the mv command to restore the library back to its original name, like this:
mv /usr/X11R6/lib/libXrender.backup.so.1 /usr/X11R6/lib/libXrender.so.1
Failure to do so will cause your X session to crash.
Where is there?
Remember that file mentioned right up at the top, /etc/ld.so.cache? It contains a complete list of all the shared object libraries that ld.so or ld-linux.so will search in machine readable format. It is created by ldconfig, based on the contents of a human readable text file named /etc/ld.so.conf. Here’s a dump of my /etc/ld.so.conf file:
/usr/X11R6/lib/Xaw95 /usr/X11R6/lib/Xaw3d /usr/X11R6/lib /usr/i486-linux/lib /usr/i486-linux-libc5/lib=libc5 /usr/i486-linux-libc6/lib=libc6 /usr/i486-linuxaout/lib /usr/i386-suse-linux/lib /usr/local/lib /usr/openwin/lib /opt/kde/lib /opt/kde2/lib /opt/kde3/lib /opt/gnome/lib /opt/gnome2/lib include /etc/ld.so.conf.d/*.conf
Now let’s say that I’ve added a new application to my system. Following the project documentation, I’ve also added libXYZ.so.2, and when I did the library ended up in the /usr/local/XYZ/lib directory. Since my /etc/ld.so.conf file doesn’t contain a reference to that directory, the library will never get into the cache used by ld.so and ld-linux.so at run time, and I won’t be able to run the new application.
Fixing that problem is exactly as easy as it sounds. Simply add the directory to your /etc/ld.so.conf file, and then — as root — run the ldconfig program so it rebuilds /etc/ld.so.cache.
Note that even if that directory had been in your /etc/ld.so.conf file previously, the cache would not contain an entry for the new library. You would still need to run the ldconfig program to update the cache.
There you have it. A little deeper than we usually go in CLI Magic, but very much worth the effort when you just gotta have that app.
