[gentoo-user] Virtual Packages

[gentoo-user] Virtual Packages

[John]

Have noticed that some packages require virtual packages to installed
as well. For example when installing dev-db/mysql, virtual/mysql is
installed as well. 

What are the purposes of these "virtual" packages??

Thanks 

-- 
John D Maunder
john@articwolf.myzen.co.uk

Re: [gentoo-user] Virtual Packages

[Paul Hartman]

On Wed, May 11, 2011 at 4:43 PM, John <jdm@arcticwolf.myzen.co.uk> wrote:
>
> Have noticed that some packages require virtual packages to installed
> as well. For example when installing dev-db/mysql, virtual/mysql is
> installed as well.
>
> What are the purposes of these "virtual" packages??

I think it is usually when there are alternatives available which can
fit the dependency. For example, you could use MySQL or MariaDB to
satisfy virtual/mysql dependency. That way the alternatives can be
defined once in the virtual package, rather than in every single
package that uses MySQL.

Re: [gentoo-user] Virtual Packages

[Mike Edenfield]

On 5/11/2011 5:43 PM, John wrote:
>
> Have noticed that some packages require virtual packages to installed
> as well. For example when installing dev-db/mysql, virtual/mysql is
> installed as well.
>
> What are the purposes of these "virtual" packages??

They allow other packages to depend on the presence of a 
"service" or "feature" instead of depending on a specific 
implementation.  For example, both sys-libs/pam and 
sys-auth/openpam satisfy "virtual/pam", since most 
applications will work just find against either library. 
There's about a dozen possible packages that provide 
"virtual/bootloader".

It's also used to allow different architectures to have 
different "default" implementations of the same service 
(like virtual/libc).

virtual/mysql, as of v5.1, can mean dev-db/mysql or 
dev-db/mariadb.

--Mike

Re: [gentoo-user] Virtual Packages

[dong l]

I think it is a bit like IoC in OOP programing.
see http://en.wikipedia.org/wiki/Inversion_of_control