6.10 What is meant by Pre-Depends?
"Pre-Depends" is a special dependency. In the case of most packages,
dpkg will unpack its archive file (i.e., its .deb
file) independently of whether or not the files on which it depends exist on
the system. Simplistically, unpacking means that dpkg will
extract the files from the archive file that were meant to be installed on your
file system, and put them in place. If those packages depend on the
existence of some other packages on your system, dpkg will refuse
to complete the installation (by executing its "configure" action)
until the other packages are installed.
However, for some packages, dpkg will refuse even to unpack them
until certain dependencies are resolved. Such packages are said to
"Pre-depend" on the presence of some other packages. The Debian
project provided this mechanism to support the safe upgrading of systems from
a.out format to ELF format, where the order
in which packages were unpacked was critical. There are other large upgrade
situations where this method is useful, e.g. the packages with the required
priority and their LibC dependency.
As before, more detailed information about this can be found in the Policy
manual.
