7. Declaring relationships between packages

7.1. Syntax of relationship fields

These fields all have a uniform syntax. They are a list of package names separated by commas.

In the Depends, Recommends, Suggests, Pre-Depends, Build-Depends, Build-Depends-Indep and Build-Depends-Arch control fields of the package, which declare dependencies on other packages, the package names listed may also include lists of alternative package names, separated by vertical bar (pipe) symbols |. In such a case, that part of the dependency can be satisfied by any one of the alternative packages. (Alternative dependencies in Build-Depends, Build-Depends-Indep, and Build-Depends-Arch are interpreted specially by Debian autobuilders. See Relationships between source and binary packages for more details.)

All of the fields may restrict their applicability to particular versions of each named package. This is done in parentheses after each individual package name; the parentheses should contain a relation from the list below followed by a version number, in the format described in Version.

The relations allowed are <<, <=, =, >= and >> for strictly earlier, earlier or equal, exactly equal, later or equal and strictly later, respectively. The exception is the Provides field, for which only = is allowed. [1]

Whitespace may appear at any point in the version specification subject to the rules in Syntax of control files, and must appear where it’s necessary to disambiguate; it is not otherwise significant. All of the relationship fields can only be folded in source package template control files. For consistency and in case of future changes to dpkg it is recommended that a single space be used after a version relationship and before a version number; it is also conventional to put a single space after each comma, on either side of each vertical bar, and before each open parenthesis. When opening a continuation line in a relationship field, it is conventional to do so after a comma and before the space following that comma.

For example, a list of dependencies might appear as:

Package: mutt
Version: 1.3.17-1
Depends: libc6 (>= 2.2.1), default-mta | mail-transport-agent

Relationships may be restricted to a certain set of architectures. This is indicated in brackets after each individual package name and the optional version specification. The brackets enclose a non-empty list of Debian architecture names in the format described in Architecture specification strings, separated by whitespace. Exclamation marks may be prepended to each of the names. (It is not permitted for some names to be prepended with exclamation marks while others aren’t.)

For build relationship fields (Build-Depends, Build-Depends-Indep, Build-Depends-Arch, Build-Conflicts, Build-Conflicts-Indep and Build-Conflicts-Arch), if the current Debian host architecture is not in this list and there are no exclamation marks in the list, or it is in the list with a prepended exclamation mark, the package name and the associated version specification are ignored completely for the purposes of defining the relationships.

For example:

Source: glibc
Build-Depends-Indep: texinfo
Build-Depends: kernel-headers-2.2.10 [!hurd-i386],
 hurd-dev [hurd-i386], gnumach-dev [hurd-i386]

requires kernel-headers-2.2.10 on all architectures other than hurd-i386 and requires hurd-dev and gnumach-dev only on hurd-i386. Here is another example showing multiple architectures separated by spaces:

Build-Depends:
 libluajit5.1-dev [i386 amd64 kfreebsd-i386 armel armhf powerpc mips],
 liblua5.1-dev [hurd-i386 ia64 kfreebsd-amd64 s390x sparc],

For binary relationship fields and the Built-Using field, the architecture restriction syntax is only supported in the source package template control file debian/control. When the corresponding binary package control file is generated, the relationship will either be omitted or included without the architecture restriction based on the architecture of the binary package. This means that architecture restrictions must not be used in binary relationship fields for architecture-independent packages (Architecture: all).

For example:

Depends: foo [i386], bar [amd64]

becomes Depends: foo when the package is built on the i386 architecture, Depends: bar when the package is built on the amd64 architecture, and omitted entirely in binary packages built on all other architectures.

If the architecture-restricted dependency is part of a set of alternatives using |, that alternative is ignored completely on architectures that do not match the restriction. For example:

Build-Depends: foo [!i386] | bar [!amd64]

is equivalent to bar on the i386 architecture, to foo on the amd64 architecture, and to foo | bar on all other architectures.

Relationships may also be restricted to a certain set of architectures using architecture wildcards in the format described in Architecture wildcards. The syntax for declaring such restrictions is the same as declaring restrictions using a certain set of architectures without architecture wildcards. For example:

Build-Depends: foo [linux-any], bar [any-i386], baz [!linux-any]

is equivalent to foo on architectures using the Linux kernel and any cpu, bar on architectures using any kernel and an i386 cpu, and baz on any architecture using a kernel other than Linux.

Note that the binary package relationship fields such as Depends appear in one of the binary package stanzas of the template control file, whereas the build-time relationships such as Build-Depends appear in the source package stanza of the template control file (which is the first section).

7.2. Binary Dependencies - Depends, Recommends, Suggests, Enhances, Pre-Depends

Packages can declare in their control file that they have certain relationships to other packages - for example, that they cannot be installed at the same time as certain other packages, and/or that they depend on the presence of others.

This is done using the Depends, Pre-Depends, Recommends, Suggests, Enhances, Breaks and Conflicts control fields. Breaks is described in Packages which break other packages - Breaks, and Conflicts is described in Conflicting binary packages - Conflicts. The rest are described below.

These seven fields are used to declare a dependency relationship by one package on another. Except for Enhances and Breaks, they appear in the depending (binary) package’s control file. (Enhances appears in the recommending package’s control file, and Breaks appears in the version of depended-on package which causes the named package to break).

A Depends field takes effect only when a package is to be configured. It does not prevent a package being on the system in an unconfigured state while its dependencies are unsatisfied, and it is possible to replace a package whose dependencies are satisfied and which is properly installed with a different version whose dependencies are not and cannot be satisfied; when this is done the depending package will be left unconfigured (since attempts to configure it will give errors) and will not function properly. If it is necessary, a Pre-Depends field can be used, which has a partial effect even when a package is being unpacked, as explained in detail below. (The other three dependency fields, Recommends, Suggests and Enhances, are only used by the various front-ends to dpkg such as apt-get, aptitude, and dselect.)

Since Depends only places requirements on the order in which packages are configured, packages in an installation run are usually all unpacked first and all configured later. [2]

If there is a circular dependency among packages being installed or removed, installation or removal order honoring the dependency order is impossible, requiring the dependency loop be broken at some point and the dependency requirements violated for at least one package. Packages involved in circular dependencies may not be able to rely on their dependencies being configured before they themselves are configured, depending on which side of the break of the circular dependency loop they happen to be on. If one of the packages in the loop has no postinst script, then the cycle will be broken at that package; this ensures that all postinst scripts are run with their dependencies properly configured if this is possible. Otherwise the breaking point is arbitrary. Packages should therefore avoid circular dependencies where possible, particularly if they have postinst scripts.

The meaning of the five dependency fields is as follows:

Depends

This declares an absolute dependency. A package will not be configured unless all of the packages listed in its Depends field have been correctly configured (unless there is a circular dependency as described above).

The Depends field should be used if the depended-on package is required for the depending package to provide a significant amount of functionality.

The Depends field should also be used if the postinst or prerm scripts require the depended-on package to be unpacked or configured in order to run. In the case of postinst configure, the depended-on packages will be unpacked and configured first. (If both packages are involved in a dependency loop, this might not work as expected; see the explanation a few paragraphs back.) In the case of prerm or other postinst actions, the package dependencies will normally be at least unpacked, but they may be only “Half-Installed” if a previous upgrade of the dependency failed.

Finally, the Depends field should be used if the depended-on package is needed by the postrm script to fully clean up after the package removal. There is no guarantee that package dependencies will be available when postrm is run, but the depended-on package is more likely to be available if the package declares a dependency (particularly in the case of postrm remove). The postrm script must gracefully skip actions that require a dependency if that dependency isn’t available.

Recommends

This declares a strong, but not absolute, dependency.

The Recommends field should list packages that would be found together with this one in all but unusual installations.

Suggests

This is used to declare that one package may be more useful with one or more others. Using this field tells the packaging system and the user that the listed packages are related to this one and can perhaps enhance its usefulness, but that installing this one without them is perfectly reasonable.

Enhances

This field is similar to Suggests but works in the opposite direction. It is used to declare that a package can enhance the functionality of another package.

Pre-Depends

This field is like Depends, except that it also forces dpkg to complete installation of the packages named before even starting the installation of the package which declares the pre-dependency, as follows:

When a package declaring a pre-dependency is about to be unpacked the pre-dependency can be satisfied if the depended-on package is either fully configured, or even if the depended-on package(s) are only in the “Unpacked” or the “Half-Configured” state, provided that they have been configured correctly at some point in the past (and not removed or partially removed since). In this case, both the previously-configured and currently “Unpacked” or “Half-Configured” versions must satisfy any version clause in the Pre-Depends field.

When the package declaring a pre-dependency is about to be configured, the pre-dependency will be treated as a normal Depends. It will be considered satisfied only if the depended-on package has been correctly configured. However, unlike with Depends, Pre-Depends does not permit circular dependencies to be broken. If a circular dependency is encountered while attempting to honor Pre-Depends, the installation will be aborted.

Pre-Depends are also required if the preinst script depends on the named package. It is best to avoid this situation if possible.

Pre-Depends should be used sparingly, preferably only by packages whose premature upgrade or installation would hamper the ability of the system to continue with any upgrade that might be in progress.

You should not specify a Pre-Depends entry for a package before this has been discussed on the debian-devel mailing list and a consensus about doing that has been reached. See Dependencies.

When selecting which level of dependency to use you should consider how important the depended-on package is to the functionality of the one declaring the dependency. Some packages are composed of components of varying degrees of importance. Such a package should list using Depends the package(s) which are required by the more important components. The other components’ requirements may be mentioned as Suggestions or Recommendations, as appropriate to the components’ relative importance.

7.3. Packages which break other packages - Breaks

When one binary package declares that it breaks another, dpkg will refuse to allow the package which declares Breaks to be unpacked unless the broken package is deconfigured first, and it will refuse to allow the broken package to be reconfigured.

A package will not be regarded as causing breakage merely because its configuration files are still installed; it must be at least “Half-Installed”.

A special exception is made for packages which declare that they break their own package name or a virtual package which they provide (see below): this does not count as a real breakage.

Normally a Breaks entry will have an “earlier than” version clause; such a Breaks is introduced in the version of an (implicit or explicit) dependency which violates an assumption or reveals a bug in earlier versions of the broken package, or which takes over a file from earlier versions of the package named in Breaks. This use of Breaks will inform higher-level package management tools that the broken package must be upgraded before the new one.

If the breaking package also overwrites some files from the older package, it should use Replaces to ensure this goes smoothly. See Overwriting files and replacing packages - Replaces for a full discussion of taking over files from other packages, including how to use Breaks in those cases.

Many of the cases where Breaks should be used were previously handled with Conflicts because Breaks did not yet exist. Many Conflicts fields should now be Breaks. See Conflicting binary packages - Conflicts for more information about the differences.

7.4. Conflicting binary packages - Conflicts

When one binary package declares a conflict with another using a Conflicts field, dpkg will refuse to allow them to be unpacked on the system at the same time. This is a stronger restriction than Breaks, which prevents the broken package from being configured while the breaking package is in the “Unpacked” state but allows both packages to be unpacked at the same time.

If one package is to be unpacked, the other must be removed first. If the package being unpacked is marked as replacing (see Overwriting files and replacing packages - Replaces, but note that Breaks should normally be used in this case) the one on the system, or the one on the system is marked as deselected, or both packages are marked Essential, then dpkg will automatically remove the package which is causing the conflict. Otherwise, it will halt the installation of the new package with an error. This mechanism is specifically designed to produce an error when the installed package is Essential, but the new package is not.

A package will not cause a conflict merely because its configuration files are still installed; it must be at least “Half-Installed”.

A special exception is made for packages which declare a conflict with their own package name, or with a virtual package which they provide (see below): this does not prevent their installation, and allows a package to conflict with others providing a replacement for it. You use this feature when you want the package in question to be the only package providing some feature.

Normally, Breaks should be used instead of Conflicts since Conflicts imposes a stronger restriction on the ordering of package installation or upgrade and can make it more difficult for the package manager to find a correct solution to an upgrade or installation problem. Breaks should be used

• when moving a file from one package to another (see Overwriting files and replacing packages - Replaces),

• when splitting a package (a special case of the previous one), or

• when the breaking package exposes a bug in or interacts badly with particular versions of the broken package.

Conflicts should be used

• when two packages provide the same file and will continue to do so,

• in conjunction with Provides when only one package providing a given virtual facility can be unpacked at a time (see Virtual packages - Provides),

• in other cases where one must prevent simultaneous installation of two packages for reasons that are ongoing (not fixed in a later version of one of the packages) or that must prevent both packages from being unpacked at the same time, not just configured.

Be aware that adding Conflicts is normally not the best solution when two packages provide the same files. Depending on the reason for that conflict, using alternatives or renaming the files is often a better approach. See, for example, Binaries.

Neither Breaks nor Conflicts should be used unless two packages cannot be installed at the same time or installing them both causes one of them to be broken or unusable. Having similar functionality or performing the same tasks as another package is not sufficient reason to declare Breaks or Conflicts with that package.

A Conflicts entry may have an “earlier than” version clause if the reason for the conflict is corrected in a later version of one of the packages. However, normally the presence of an “earlier than” version clause is a sign that Breaks should have been used instead. An “earlier than” version clause in Conflicts prevents dpkg from upgrading or installing the package which declares such a conflict until the upgrade or removal of the conflicted-with package has been completed, which is a strong restriction.

7.5. Virtual packages - Provides

As well as the names of actual (“concrete”) packages, the package relationship fields Depends, Recommends, Suggests, Enhances, Pre-Depends, Breaks, Conflicts, Build-Depends, Build-Depends-Indep, Build-Depends-Arch, Build-Conflicts, Build-Conflicts-Indep and Build-Conflicts-Arch may mention “virtual packages”.

A virtual package is one which appears in the Provides control field of another package. The effect is as if the package(s) which provide a particular virtual package name had been listed by name everywhere the virtual package name appears. (See also Virtual packages)

If there are both concrete and virtual packages of the same name, then the dependency may be satisfied (or the conflict caused) by either the concrete package with the name in question or any other concrete package which provides the virtual package with the name in question. This is so that, for example, supposing we have

Package: foo
Depends: bar

and someone else releases an enhanced version of the bar package they can say:

Package: bar-plus
Provides: bar

and the bar-plus package will now also satisfy the dependency for the foo package.

A Provides field may contain version numbers, and such a version number will be considered when considering a dependency on or conflict with the virtual package name. For example, given the following packages:

Package: foo
Depends: bar (>= 1.0)

Package: bar
Version: 0.9

Package: bar-plus
Provides: bar (= 1.0)

the bar-plus package will satisfy the dependency for the foo package with the virtual package name, as above. If the Provides field does not specify a version number, it will not satisfy versioned dependencies or violate versioned Conflicts or Breaks. For example, given the following packages:

Package: foo
Depends: bar (>= 1.0)

Package: bar
Version: 0.9

Package: bar-plus
Provides: bar (= 1.0)

Package: bar-clone
Provides: bar

the bar-plus package will satisfy the dependency for the foo package, but the bar-clone package will not.

To specify which of a set of real packages should be the default to satisfy a particular dependency on a virtual package, list the real package as an alternative before the virtual one.

If the virtual package represents a facility that can only be provided by one real package at a time, such as the mail-transport-agent virtual package that requires installation of a binary that would conflict with all other providers of that virtual package (see Mail transport, delivery and user agents), all packages providing that virtual package should also declare a conflict with it using Conflicts. This will ensure that at most one provider of that virtual package is unpacked or installed at a time.

7.6. Overwriting files and replacing packages - Replaces

Packages can declare in their control file that they should overwrite files in certain other packages, or completely replace other packages. The Replaces control field has these two distinct purposes.

7.6.1. Overwriting files in other packages

It is usually an error for a package to contain files which are on the system in another package. However, if the overwriting package declares that it Replaces the one containing the file being overwritten, then dpkg will replace the file from the old package with that from the new. The file will no longer be listed as “owned” by the old package and will be taken over by the new package. Normally, Breaks should be used in conjunction with Replaces. [3]

For example, if a package foo is split into foo and foo-data starting at version 1.2-3, foo-data would have the fields

Replaces: foo (<< 1.2-3)
Breaks: foo (<< 1.2-3)

in its control file. The new version of the package foo would normally have the field

Depends: foo-data (>= 1.2-3)

(or possibly Recommends or even Suggests if the files moved into foo-data are not required for normal operation).

If a package is completely replaced in this way, so that dpkg does not know of any files it still contains, it is considered to have “disappeared”. It will be marked as not wanted on the system (selected for removal) and “Not-Installed”. Any conffiles details noted for the package will be ignored, as they will have been taken over by the overwriting package. The package’s postrm script will be run with a special argument to allow the package to do any final cleanup required. See Summary of ways maintainer scripts are called. [4]

For this usage of Replaces, virtual packages (see Virtual packages - Provides) are not considered when looking at a Replaces field. The packages declared as being replaced must be mentioned by their real names.

This usage of Replaces only takes effect when both packages are at least partially on the system at once. It is not relevant if the packages conflict unless the conflict has been overridden.

7.6.2. Replacing whole packages, forcing their removal

Second, Replaces allows the packaging system to resolve which package should be removed when there is a conflict (see Conflicting binary packages - Conflicts). This usage only takes effect when the two packages do conflict, so that the two usages of this field do not interfere with each other.

In this situation, the package declared as being replaced can be a virtual package, so for example, all mail transport agents (MTAs) would have the following fields in their control files:

Provides: mail-transport-agent
Conflicts: mail-transport-agent
Replaces: mail-transport-agent

ensuring that only one MTA can be unpacked at any one time. See Virtual packages - Provides for more information about this example.

7.7. Relationships between source and binary packages - Build-Depends, Build-Depends-Indep, Build-Depends-Arch, Build-Conflicts, Build-Conflicts-Indep, Build-Conflicts-Arch

Source packages that require certain binary packages to be installed or absent at the time of building the package may declare relationships to those binary packages.

This is done using the Build-Depends, Build-Depends-Indep, Build-Depends-Arch, Build-Conflicts, Build-Conflicts-Indep and Build-Conflicts-Arch control fields.

Build-dependencies on “build-essential” binary packages can be omitted. Please see Package relationships for more information.

The dependencies and conflicts they define must be satisfied (as defined earlier for binary packages) in order to invoke the targets in debian/rules, as follows:

clean

Only the Build-Depends and Build-Conflicts fields must be satisfied when this target is invoked.

build-arch, and binary-arch

The Build-Depends, Build-Conflicts, Build-Depends-Arch, and Build-Conflicts-Arch fields must be satisfied when these targets are invoked.

build-indep, and binary-indep

The Build-Depends, Build-Conflicts, Build-Depends-Indep, and Build-Conflicts-Indep fields must be satisfied when these targets are invoked.

build and binary

The Build-Depends, Build-Conflicts, Build-Depends-Indep, Build-Conflicts-Indep, Build-Depends-Arch, and Build-Conflicts-Arch fields must be satisfied when these targets are invoked.

Alternative dependencies are allowed in the Build-Depends, Build-Depends-Indep, and Build-Depends-Arch fields, but Debian’s autobuilders normally discard the dependencies after the first. This is done to give alternative dependencies a consistent interpretation that reduces the risk of inconsistencies between repeated builds. If, for example, the first-listed dependency would normally be available but is temporarily not installable, the autobuilder fails rather than install a subsequent dependency that may significantly change the behavior of the package.

More specifically, Debian autobuilders perform the following transformation on alternative dependencies in the Build-Depends, Build-Depends-Indep, and Build-Depends-Arch fields:

1. Discard any alternatives that are restricted to architectures that do not match the host architecture.

2. Discard any alternatives specifying different package names than the now-first alternative. (Alternatives specifying the same package name are kept to permit relationships such as foo (<= x) | foo (>= y).)

For example, an autobuilder for the amd64 architecture would treat the following dependency:

foo-special [armhf] | foo (<= 4) | foo (>= 4.2) | bar

as if it were:

foo (<= 4) | foo (>= 4.2)

The normal effect is to use only the first alternative that is valid on the relevant architecture and fail if that alternative is not installable.

While this rule for build dependencies may limit the usefulness of alternatives, they can still be used to provide flexibility when building the package outside of Debian’s autobuilders.

The autobuilders for the Debian backports and experimental suites do not perform this transformation and instead use the same dependency resolution rules as normal package installations to choose which alternative dependency to install.

7.8. Additional source packages used to build the binary - Built-Using

Some binary packages incorporate parts of other packages when built but do not have to depend on those packages. Examples include linking with static libraries or incorporating source code from another package during the build. In this case, the source packages of those other packages are part of the complete source (the binary package is not reproducible without them).

When the license of either the incorporated parts or the incorporating binary package requires that the full source code of the incorporating binary package be made available, the Built-Using field must list the corresponding source package for any affected binary package incorporated during the build, [5] including an “exactly equal” (“=”) version relation on the version that was used to build that version of the incorporating binary package. [6]

This causes the Debian archive to retain the versions of the source packages that were actually incorporated. In particular, if the versions of the incorporated parts are updated but the incorporating binary package is not rebuilt, the older versions of the incorporated parts will remain in the archive in order to satisfy the license.

A package using the source code from the gcc-4.6-source binary package built from the gcc-4.6 source package would have this field in its control file:

Built-Using: gcc-4.6 (= 4.6.0-11)

A package including binaries from grub2 and loadlin would have this field in its control file:

Built-Using: grub2 (= 1.99-9), loadlin (= 1.6e-1)

This field should be used only when there are license or DFSG requirements to retain the referenced source packages. It should not be added solely as a way to locate packages that need to be rebuilt against newer versions of their build dependencies.

[1]

The relations < and > were previously allowed, but they were confusingly defined to mean earlier/later or equal rather than strictly earlier/later. dpkg still supports them with a warning, but they are no longer allowed by Debian Policy.

[2]

This approach makes dependency resolution easier. If two packages A and B are being upgraded, the installed package A depends on exactly the installed package B, and the new package A depends on exactly the new package B (a common situation when upgrading shared libraries and their corresponding development packages), satisfying the dependencies at every stage of the upgrade would be impossible. This relaxed restriction means that both new packages can be unpacked together and then configured in their dependency order.

[3]

To see why Breaks is normally needed in addition to Replaces, consider the case of a file in the package foo being taken over by the package foo-data. Replaces will allow foo-data to be installed and take over that file. However, without Breaks, nothing requires foo to be upgraded to a newer version that knows it does not include that file and instead depends on foo-data. Nothing would prevent the new foo-data package from being installed and then removed, removing the file that it took over from foo. After that operation, the package manager would think the system was in a consistent state, but the foo package would be missing one of its files.

[4]

Replaces is a one way relationship. You have to install the replacing package after the replaced package.

[5]

Build-Depends in the source package is not adequate since it (rightfully) does not document the exact version used in the build.

[6]

The archive software might reject packages that refer to non-existent sources.