Gentoo Development Guide

Dependencies

Automatic dependency resolution is one of the most useful features provided by emerge.

Build Dependencies

The DEPEND ebuild variable should specify any dependencies which are required to unpack, patch, compile or install the package (but see Implicit System Dependency for exemptions).

Runtime Dependencies

The RDEPEND ebuild variable should specify any dependencies which are required at runtime. This includes libraries (when dynamically linked), any data packages and (for interpreted languages) the relevant interpreter. In EAPI=3 or older, if this variable is not specified it defaults to the value of DEPEND, however the implicit usage is frowned upon. In EAPI=4, the implicit behaviour was removed and the assignment is always explicit.

Note that when installing from a binary package, only RDEPEND will be checked. It is therefore necessary to include items even if they are also listed in DEPEND.

Items which are in RDEPEND but not DEPEND could in theory be merged after the target package. Portage does not currently do this.

Post-Merge Dependencies

The PDEPEND variable specifies dependencies which must be merged after the package. This is sometimes used for plugins which have a dependency upon the package being merged. Generally PDEPEND should be avoided in favour of RDEPEND except where this will create circular dependency chains.

Implicit System Dependency

All packages have an implicit compile-time and runtime dependency upon the entire system target. It is therefore not necessary, nor advisable, to specify dependencies upon toolchain packages like gcc, libc and so on, except where specific versions or packages (for example, glibc over uclibc) are required. Note that this rule also needs consideration for packages like flex, zlib and libtool, which aren't in the system target for every profile. For example, the embedded profile doesn't have zlib in system target, the libtool ABI might change and break building order and flex might get removed from the system target in future.

However, packages which are included in the system target, or are dependencies of system target packages, should generally include a complete dependency list (excluding bootstrap packages). This makes emerge -e system possible when installing from a stage 1 or stage 2 tarball.

Basic Dependency Syntax

A basic DEPEND specification might look like the following:

DEPEND="dev-lang/ruby
    dev-ruby/ruby-gtk2
    dev-ruby/mysql-ruby"

Each atom is the full category and name of a package. Atoms are separated by arbitrary whitespace — convention is to specify one atom per line for readability purposes. When specifying names, the category part should be treated as mandatory.

Version Dependencies

Sometimes a particular version of a package is needed. Where this is known, it should be specified. A simple example:

DEPEND=">=dev-libs/openssl-0.9.7d"

This states that at least version 0.9.7d of openssl is required.

Version Specifiers

Available version specifiers are:

Specifier Meaning
>=app-misc/foo-1.23 Version 1.23 or later is required.
>app-misc/foo-1.23 A version strictly later than 1.23 is required.
~app-misc/foo-1.23 Version 1.23 (or any 1.23-r*) is required.
=app-misc/foo-1.23 Exactly version 1.23 is required. If at all possible, use the ~ form to simplify revision bumps.
<=app-misc/foo-1.23 Version 1.23 or older is required.
<app-misc/foo-1.23 A version strictly before 1.23 is required.

Ranged Dependencies

To specify "version 2.x (not 1.x or 3.x)" of a package, it is necessary to use the asterisk postfix. This is most commonly seen in situations like:

DEPEND="gtk? ( =x11-libs/gtk+-1.2* )"

Note that the equals sign is mandatory, and that there is no dot before the asterisk. Also note that when selecting all versions in a specific SLOT, SLOT dependencies should be used (see below).

Blockers

Sometimes two packages cannot be installed in parallel. This is handled by blockers. A softblocker is specified as follows:

RDEPEND="!app-misc/foo"

Portage will try to resolve this conflict automatically if possible. Sometimes we need to use a hardblocker to ensure correct emerge order. Those cannot be resolved by Portage and must be taken care of by the user. A hardblocker is specified as follows:

RDEPEND="!!app-misc/foo"

Hardblockers always take precedence over softblockers and need at least EAPI="2". Also note that blockers are usually runtime rather than buildtime.

Specific versions can also be blocked:

RDEPEND="!<app-misc/foo-1.3"

Blockers can be optional based upon USE flags as per normal dependencies.

Blockers added to older ebuilds should not be expected to be retroactive. If the user already has the ebuild installed, any changes to the ebuild should not be expected to make any difference. This means that you should add the blockers to whichever ebuild is the newest (even if it means that logically it would seem backwards). For example, certain versions of portage don't like some versions of bash, but the blocker was put into bash because that was the newer package that caused the issues.

SLOT Dependencies

In order to depend on a package in a specific SLOT you must specify at least EAPI="1".

To depend on a specific SLOT, :SLOT should be appended to the package name, where 'SLOT' is the SLOT of the package wanted:

DEPEND="qt3? ( x11-libs/qt:3 )
    gtk? ( x11-libs/gtk+:2 )

To depend on a specific version or version-range within a SLOT we use:

DEPEND="qt3? ( ~x11-libs/qt-3.3.8:3 )
    gtk? ( >=x11-libs/gtk+-2.24.9:2 )

Slot Operators

In EAPI="5" and higher, you can use slot operators appended to the package name to declare whether or not your package should be rebuilt after the versions satisfying its runtime dependencies are updated to versions with a different slot or sub-slot:

  • := means that any slot is acceptable, and that your package should be rebuilt if the version best matching the runtime dependency is updated to a version with a different slot or subslot;
  • :* means that any slot is acceptable, and explicitly declares that changes in the slot or sub-slot can be ignored;
  • :SLOT= means that only the 'SLOT' slot is acceptable, and that your package should be rebuilt if the version matching the runtime dependency is updated to another version with this slot but with a different subslot;
  • :SLOT means that only the 'SLOT' slot is acceptable, and that changes in the sub-slot can be ignored (like in previous EAPIs).
  • :SLOT/SUBSLOT means a dependency on a specific slot and sub-slot pair, which can be useful for packages installing pre-built binaries that require a library with a particular soname version corresponding to the sub-slot.

For example:

RDEPEND="media-libs/cogl:1.0=
    gnutls? ( >=net-libs/gnutls-2.8:= )"

USE-Conditional Dependencies

To depend upon a certain package if and only if a given USE flag is set:

DEPEND="perl? ( dev-lang/perl )
    ruby? ( >=dev-lang/ruby-1.8 )
    python? ( dev-lang/python )"

It is also possible to depend upon a certain package if a given USE flag is not set:

RDEPEND="!crypt? ( net-misc/netkit-rsh )"

This should not be used for disabling a certain USE flag on a given architecture. In order to do this, the architecture team should add the USE flag to their use.mask file in the profiles/arch directory of the Portage tree.

This can be nested:

DEPEND="!build? (
    gcj? (
        gtk? (
            x11-libs/libXt
            x11-libs/libX11
            x11-libs/libXtst
            x11-proto/xproto
            x11-proto/xextproto
            >=x11-libs/gtk+-2.2
            x11-libs/pango
        )
        >=media-libs/libart_lgpl-2.1
    )
    >=sys-libs/ncurses-5.2-r2
    nls? ( sys-devel/gettext )
)"

Any of Many Dependencies

To depend on either foo or bar:

DEPEND="|| ( app-misc/foo app-misc/bar )"

To depend on either foo or bar if the baz USE flag is set:

DEPEND="baz? ( || ( app-misc/foo app-misc/bar ) )"

Any of Many Versus USE

Say fnord can be built against either foo or bar. Then a USE flag is not necessary if and only if all of the following hold:

  • fnord is merged on a system which has foo and not bar installed. foo is then unmerged, and bar is installed. fnord must continue to work correctly.
  • A binary package of fnord made on a system with foo and not bar can be taken and installed on a system with bar and not foo.

Built with USE Dependencies

In order to use built with use dependencies you must specify EAPI="2".

Available specifiers are:

Specifier Meaning
app-misc/foo[bar] foo must have bar enabled.
app-misc/foo[bar,baz] foo must have both bar and baz enabled.
app-misc/foo[-bar,baz] foo must have bar disabled and baz enabled.

There are also shortcuts for conditional situations:

Compact form Equivalent expanded form
app-misc/foo[bar?] bar? ( app-misc/foo[bar] ) !bar? ( app-misc/foo )
app-misc/foo[!bar?] bar? ( app-misc/foo ) !bar? ( app-misc/foo[-bar] )
app-misc/foo[bar=] bar? ( app-misc/foo[bar] ) !bar? ( app-misc/foo[-bar] )
app-misc/foo[!bar=] bar? ( app-misc/foo[-bar] ) !bar? ( app-misc/foo[bar] )

Use dependency defaults

If a dependency is introducing or removing a USE flag in new versions, a use dependency default may be used. Appending a (+) or (-) suffix will indicate whether the absence of the flag from a particular version should indicate its presence or absence.

>=dev-libs/boost-1.48[threads(+)] will treat all versions without threads as having it set.

Legacy Inverse USE-Conditional Dependency Syntax

Once upon a time the : conditional operator was allowed in *DEPEND:

DEPEND="use-flag? ( app-misc/foo ) : ( app-misc/bar )"

This syntax is no longer permitted. It is exactly equivalent to the following, which should be used instead:

DEPEND="use-flag?  ( app-misc/foo )
    !use-flag? ( app-misc/bar )"

It is useful to recognise the legacy syntax and to know that it is no longer valid.