How can I revert a package to a specific version on Gentoo?

Question

I'm a Debian user pretty much new to Gentoo. I wanted to install a specific version of a package namely the 1.39.0 version of dev-libs/boost, http://packages.gentoo.org/package/dev-libs/boost. But I already did "emerge dev-libs/boost" which installed me version 1.46.1-r1.

machine ~ # emerge --search boost
(...)
*  dev-libs/boost
      Latest version available: 1.46.1-r1
      Latest version installed: 1.46.1-r1
      Size of files: 41,016 kB
      Homepage:      http://www.boost.org/
      Description:   Boost Libraries for C++
      License:       Boost-1.0

How can I revert this package back to version 1.39.0?

Thank you in advance.

Answer 1

The boost packages are slotted, so you can actually have more than one version installed.

To emerge that version, simply issue:

emerge -a =dev-libs/boost-1.39.0

If you want to remove the newer version (quite dangerous, you could have a lot of stuff dependent on it), you could:

emerge --unmerge =dev-libs/boost-1.46.1-r1

and run a revdep-rebuild afterwards.

To switch your environment from one version to the other (if you kept both), use eselect boost list/set.