Gentoo Development Guide
Slotting
Packages can support having multiple versions installed simultaneously. This is
useful for libraries which may have changed interfaces between versions
—
for
example, the gtk+ package can install both versions 2.24 and 3.6 in
parallel. This feature is called slotting.
Most packages have no need for slotting. These packages specify SLOT="0" in
the ebuilds. This is not the same as specifying an empty slot
—
an empty
slot means "disable slotting entirely", and should not be used.
Portage permits at most one instance of a package installation per SLOT
value. For example, say we have the following:
foo-1.1withSLOT="1"foo-1.2withSLOT="1"foo-2.0withSLOT="2"foo-2.1withSLOT="2"
Then the user could have, say, foo-1.2 and foo-2.0 installed in
parallel, but not foo-1.1 and foo-1.2. Note that it is entirely
possible that the user may have foo-2.0 installed and no foo-1.x at all.
To DEPEND upon a package in a specific slot, refer to
SLOT Dependencies.
Sub-Slots
Sometimes a package installs a library that changes interfaces between versions,
but it's undesirable or inconvenient to allow some of these versions to be installed
simultaneously. In EAPI=5 and higher, this situation can be handled by
using sub-slots, which are delimited from the regular slot by a / character,
as in SLOT="slot/subslot". Packages can
request to be
automatically rebuilt when the subslot of a runtime dependency changes.
For example, suppose package foo installs a library whose soname is
different for different versions. It would be reasonable to use the soname version
as the sub-slot name:
foo-1.1installslibfoo.so.5—SLOT="1/5"foo-1.2installslibfoo.so.6—SLOT="1/6"foo-2.0installslibfoo-2.so.0—SLOT="2/0"foo-2.1installslibfoo-2.so.1—SLOT="2/1"
Other ebuilds that install binaries which link to libfoo-2 (or libfoo)
can then request to be automatically rebuilt when the installed version of
foo:2 or foo:1 changes sub-slots
—
for example, when the user
upgrades from foo-2.0 to foo-2.1.
If an ebuild does not explicitly declare a sub-slot, the regular slot is used as the value of the sub-slot by default.
Note
Care must be taken when using sub-slots in a library ebuild for the first time.
Adding sub-slots will trigger rebuilds for all the packages that already use sub-slot
dependencies (e.g. Switching from SLOT="0" to SLOT="0/14" in media-libs/libpng and
package foo depends on libpng:0=).
Therefore, it's best if you start using sub-slots in the library when the existing library
interface changes.
Slot Names
Current versions of portage accept slot and sub-slot names that begin with an
alphanumeric character or '_', and contain alphanumerics and '_',
'-', '.', and '+' characters.
