..
   Copyright Spack Project Developers. See COPYRIGHT file for details.

   SPDX-License-Identifier: (Apache-2.0 OR MIT)

.. meta::
   :description lang=en:
      A guide to customizing package settings in Spack using the packages.yaml file, including configuring compilers, specifying external packages, package requirements, and permissions.

.. _packages-config:

Package Settings (packages.yaml)
================================

Spack allows you to customize how your software is built through the ``packages.yaml`` file.
Using it, you can make Spack prefer particular implementations of virtual dependencies (e.g., MPI or BLAS/LAPACK), or you can make it prefer to build with particular compilers.
You can also tell Spack to use *external* software installations already present on your system.

At a high level, the ``packages.yaml`` file is structured like this:

.. code-block:: yaml

   packages:
     package1:
       # settings for package1
     package2:
       # settings for package2
     # ...
     all:
       # settings that apply to all packages.

You can either set build preferences specifically for *one* package, or you can specify that certain settings should apply to *all* packages.
The types of settings you can customize are described in detail below.

Spack's build defaults are in the default ``etc/spack/defaults/packages.yaml`` file.
You can override them in ``~/.spack/packages.yaml`` or ``etc/spack/packages.yaml``.
For more details on how this works, see :ref:`configuration-scopes`.

.. _sec-external-packages:

External packages
-----------------

Spack can be configured to use externally-installed packages rather than building its own packages.
This may be desirable if machines ship with system packages, such as a customized MPI, which should be used instead of Spack building its own MPI.

External packages are configured through the ``packages.yaml`` file.
Here's an example of an external configuration:

.. code-block:: yaml

   packages:
     openmpi:
       externals:
       - spec: "openmpi@1.4.3~debug"
         prefix: /opt/openmpi-1.4.3
       - spec: "openmpi@1.4.3+debug"
         prefix: /opt/openmpi-1.4.3-debug

This example lists two installations of OpenMPI, one with debug information, and one without.
If Spack is asked to build a package that uses one of these MPIs as a dependency, it will use the pre-installed OpenMPI in the given directory.
Note that the specified path is the top-level install prefix, not the ``bin`` subdirectory.

``packages.yaml`` can also be used to specify modules to load instead of the installation prefixes.
The following example says that module ``CMake/3.7.2`` provides CMake version 3.7.2.

.. code-block:: yaml

   cmake:
     externals:
     - spec: cmake@3.7.2
       modules:
       - CMake/3.7.2

Each ``packages.yaml`` begins with a ``packages:`` attribute, followed by a list of package names.
To specify externals, add an ``externals:`` attribute under the package name, which lists externals.
Each external should specify a ``spec:`` string that should be as well-defined as reasonably possible.
If a package lacks a spec component, such as missing a compiler or package version, then Spack will guess the missing component based on its most-favored packages, and it may guess incorrectly.


.. _cmd-spack-external-find:

Automatically find external packages
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

You can run the :ref:`spack external find <spack-external-find>` command to search for system-provided packages and add them to ``packages.yaml``.
After running this command your ``packages.yaml`` may include new entries:

.. code-block:: yaml

   packages:
     cmake:
       externals:
       - spec: cmake@3.17.2
         prefix: /usr

Generally this is useful for detecting a small set of commonly-used packages; for now this is generally limited to finding build-only dependencies.
Specific limitations include:

* Packages are not discoverable by default: For a package to be discoverable with ``spack external find``, it needs to add special logic.
  See :ref:`here <make-package-findable>` for more details.
* The logic does not search through module files, it can only detect packages with executables defined in ``PATH``; you can help Spack locate externals which use module files by loading any associated modules for packages that you want Spack to know about before running ``spack external find``.
* Spack does not overwrite existing entries in the package configuration: If there is an external defined for a spec at any configuration scope, then Spack will not add a new external entry (``spack config blame packages`` can help locate all external entries).

Prevent packages from being built from sources
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

Adding an external spec in ``packages.yaml`` allows Spack to use an external location, but it does not prevent Spack from building packages from sources.
In the above example, Spack might choose for many valid reasons to start building and linking with the latest version of OpenMPI rather than continue using the pre-installed OpenMPI versions.

To prevent this, the ``packages.yaml`` configuration also allows packages to be flagged as non-buildable.
The previous example could be modified to be:

.. code-block:: yaml

   packages:
     openmpi:
       externals:
       - spec: "openmpi@1.4.3~debug"
         prefix: /opt/openmpi-1.4.3
       - spec: "openmpi@1.4.3+debug"
         prefix: /opt/openmpi-1.4.3-debug
       buildable: false

The addition of the ``buildable`` flag tells Spack that it should never build its own version of OpenMPI from sources, and it will instead always rely on a pre-built OpenMPI.

.. note::

   If ``concretizer:reuse`` is on (see :ref:`concretizer-options` for more information on that flag) pre-built specs are taken from: the local store, an upstream store, a registered buildcache and externals in ``packages.yaml``.
   If ``concretizer:reuse`` is off, only external specs in ``packages.yaml`` are included in the list of pre-built specs.

If an external module is specified as not buildable, then Spack will load the external module into the build environment which can be used for linking.

The ``buildable`` attribute does not need to be paired with external packages.
It could also be used alone to forbid packages that may be buggy or otherwise undesirable.

Non-buildable virtual packages
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

Virtual packages in Spack can also be specified as not buildable, and external implementations can be provided.
In the example above, OpenMPI is configured as not buildable, but Spack will often prefer other MPI implementations over the externally available OpenMPI.
Spack can be configured with every MPI provider not buildable individually, but more conveniently:

.. code-block:: yaml

   packages:
     mpi:
       buildable: false
     openmpi:
       externals:
       - spec: "openmpi@1.4.3~debug"
         prefix: /opt/openmpi-1.4.3
       - spec: "openmpi@1.4.3+debug"
         prefix: /opt/openmpi-1.4.3-debug

Spack can then use any of the listed external implementations of MPI to satisfy a dependency, and will choose among them depending on the compiler and architecture.

In cases where the concretizer is configured to reuse specs, and other ``mpi`` providers (available via stores or build caches) are not desirable, Spack can be configured to require specs matching only the available externals:

.. code-block:: yaml

   packages:
     mpi:
       buildable: false
       require:
       - one_of:
         - "openmpi@1.4.3~debug"
         - "openmpi@1.4.3+debug"
     openmpi:
       externals:
       - spec: "openmpi@1.4.3~debug"
         prefix: /opt/openmpi-1.4.3
       - spec: "openmpi@1.4.3+debug"
         prefix: /opt/openmpi-1.4.3-debug

This configuration prevents any spec using MPI and originating from stores or build caches to be reused, unless it matches the requirements under ``packages:mpi:require``.
For more information on requirements see :ref:`package-requirements`.

Specifying dependencies among external packages
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

External packages frequently have dependencies on other software components.
Explicitly modeling these relationships provides Spack with a more complete representation of the software stack.
This ensures that:

- Runtime environments include all necessary components.
- Build-time dependencies are accurately represented.

This comprehensive view, in turn, enables Spack to more reliably build software that depends on these externals.

Spack provides two methods for configuring dependency relationships among externals, each offering different trade-offs between conciseness and explicit control:

- An "inline" spec syntax.
- A structured YAML configuration that is more verbose but also more explicit.

The following sections will detail both approaches.

Dependencies using inline spec syntax
"""""""""""""""""""""""""""""""""""""

Spack allows you to define external package dependencies using the standard spec syntax directly within your package configuration.
This approach is concise and leverages the familiar spec syntax that you already use elsewhere in Spack.

When configuring an external package with dependencies using the spec syntax, you can include dependency specifications directly in the main ``spec:`` field:

.. code-block:: yaml

   # Specification for the following DAG:
   #
   # o mpileaks@2.3
   # |\
   # | o callpath@1.0
   # |/
   # o mpich@3.0.4
   packages:
     mpileaks:
       externals:
       - spec: "mpileaks@2.3~debug+opt %mpich@3 %callpath"
         prefix: /user/path
     callpath:
       externals:
       - spec: "callpath@1.0 %mpi=mpich"
         prefix: /user/path
     mpich:
       externals:
       - spec: "mpich@3.0.4"
         prefix: /user/path

In this example ``mpileaks`` depends on both ``mpich`` and ``callpath``.
Spack will parse the ``mpileaks`` spec string, and create the appropriate dependency relationships automatically.

Users *need* to ensure that each dependency maps exactly to a single other external package.
In case multiple externals can satisfy the same dependency, or in case no external can satisfy a dependency, Spack will error and point to the configuration line causing the issue.

Whenever no information is given about the dependency type, Spack will infer it from the current package recipe.
For instance, the dependencies in the configuration above are inferred to be of ``build,link`` type from the recipe of ``mpileaks`` and ``callpath``:

.. code-block:: console

   $ spack -m spec --types -l --cover edges mpileaks
   [e]  oelprl6  [    ]  mpileaks@2.3~debug+opt+shared+static build_system=generic platform=linux os=ubuntu20.04 target=icelake
   [e]  jdhzy2t  [bl  ]      ^callpath@1.0 build_system=generic platform=linux os=ubuntu20.04 target=icelake
   [e]  pgem3yp  [bl  ]          ^mpich@3.0.4~debug build_system=generic platform=linux os=ubuntu20.04 target=icelake
   [e]  pgem3yp  [bl  ]      ^mpich@3.0.4~debug build_system=generic platform=linux os=ubuntu20.04 target=icelake

When inferring the dependency types, Spack will also infer virtuals if they are not already specified.

This method's conciseness comes with a strict requirement: each dependency must resolve to a single, unambiguous external package.
This makes the approach suitable for simple or temporary configurations.
In larger, more dynamic environments, however, it can become a maintenance challenge, as adding new external packages over time may require frequent updates to existing specs to preserve their uniqueness.

Dependencies using YAML configuration
"""""""""""""""""""""""""""""""""""""

While the spec syntax offers a concise way to specify dependencies, Spack's YAML-based explicit dependency configuration provides more control and clarity, especially for complex dependency relationships.
This approach uses the ``dependencies:`` field to precisely define each dependency relationship.
The example in the previous section, written using the YAML configuration, becomes:

.. code-block:: yaml

   # Specification for the following DAG:
   #
   # o mpileaks@2.3
   # |\
   # | o callpath@1.0
   # |/
   # o mpich@3.0.4
   packages:
     mpileaks:
       externals:
       - spec: "mpileaks@2.3~debug+opt"
         prefix: /user/path
         dependencies:
         - id: callpath_id
           deptypes: link
         - spec: mpich
           deptypes:
           - "build"
           - "link"
           virtuals: "mpi"
     callpath:
       externals:
       - spec: "callpath@1.0"
         prefix: /user/path
         id: callpath_id
         dependencies:
         - spec: mpich
           deptypes:
           - "build"
           - "link"
           virtuals: "mpi"
     mpich:
       externals:
       - spec: "mpich@3.0.4"
         prefix: /user/path

Each dependency can be specified either by:

- A ``spec:`` that matches an available external package, like in the previous case, or by
- An ``id`` that explicitly references another external package.

Using the ``id`` provides an unambiguous reference to a specific external package, which is essential for differentiating between externals that have similar specs but differ, for example, only by their installation prefix.

The dependency types can be specified in the optional ``deptypes`` field, while virtuals can be specified in the optional ``virtuals`` field.
As before, when the dependency types are not specified, Spack will infer them from the package recipe.

.. _extra-attributes-for-externals:

Extra attributes for external packages
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

Sometimes external packages require additional attributes to be used effectively.
This information can be defined on a per-package basis and stored in the ``extra_attributes`` section of the external package configuration.
In addition to per-package information, this section can be used to define environment modifications to be performed whenever the package is used.
For example, if an external package is built without ``rpath`` support, it may require ``LD_LIBRARY_PATH`` settings to find its dependencies.
This could be configured as follows:

.. code-block:: yaml

   packages:
     mpich:
       externals:
       - spec: mpich@3.3 +hwloc
         prefix: /path/to/mpich
         extra_attributes:
           environment:
             prepend_path:
               LD_LIBRARY_PATH: /path/to/hwloc/lib64

See :ref:`configuration_environment_variables` for more information on how to configure environment modifications in Spack config files.

.. _configuring-system-compilers-as-external-packages:

Configuring system compilers as external packages
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

In Spack, compilers are treated as packages like any other.
This means that you can also configure system compilers as external packages and use them in Spack.

Spack automatically detects system compilers and configures them in ``packages.yaml`` for you.
You can also run :ref:`spack-compiler-find` to find and configure new system compilers.

When configuring compilers as external packages, you need to set a few :ref:`extra attributes <extra-attributes-for-externals>` for them to work properly.
The ``compilers`` extra attribute field is required to clarify which paths within the compiler prefix are used for which languages:

.. code-block:: yaml

   packages:
     gcc:
       externals:
       - spec: gcc@10.5.0 languages='c,c++,fortran'
         prefix: /usr
         extra_attributes:
           compilers:
             c: /usr/bin/gcc-10
             cxx: /usr/bin/g++-10
             fortran: /usr/bin/gfortran-10

Other fields accepted by compilers under ``extra_attributes`` are ``flags``, ``environment``, ``extra_rpaths``, and ``implicit_rpaths``.

.. code-block:: yaml

   packages:
     gcc:
       externals:
       - spec: gcc@10.5.0 languages='c,c++,fortran'
         prefix: /usr
         extra_attributes:
           compilers:
             c: /usr/bin/gcc-10
             cxx: /usr/bin/g++-10
             fortran: /usr/bin/gfortran-10
           flags:
             cflags: -O3
             fflags: -g -O2
           environment:
             set:
               GCC_ROOT: /usr
             prepend_path:
               PATH: /usr/unusual_path_for_ld/bin
           implicit_rpaths:
           - /usr/lib/gcc
           extra_rpaths:
           - /usr/lib/unusual_gcc_path

The ``flags`` attribute specifies compiler flags to apply to every spec that depends on this compiler.
The accepted flag types are ``cflags``, ``cxxflags``, ``fflags``, ``cppflags``, ``ldflags``, and ``ldlibs``.
In the example above, every spec compiled with this compiler will pass the flags ``-g -O2`` to ``/usr/bin/gfortran-10`` and will pass the flag ``-O3`` to ``/usr/bin/gcc-10``.

The ``environment`` attribute specifies user environment modifications to apply before every time the compiler is invoked.
The available operations are ``set``, ``unset``, ``prepend_path``, ``append_path``, and ``remove_path``.
In the example above, Spack will set ``GCC_ROOT=/usr`` and set ``PATH=/usr/unusual_path_for_ld/bin:$PATH`` before handing control to the build system that will use this compiler.

The ``extra_rpaths`` and ``implicit_rpaths`` fields specify additional paths to pass as rpaths to the linker when using this compiler.
The ``implicit_rpaths`` field is filled in automatically by Spack when detecting compilers, and the ``extra_rpaths`` field is available for users to configure necessary rpaths that have not been detected by Spack.
In addition, paths from ``extra_rpaths`` are added as library search paths for the linker.
In the example above, both ``/usr/lib/gcc`` and ``/usr/lib/unusual_gcc_path`` would be added as rpaths to the linker, and ``-L/usr/lib/unusual_gcc_path`` would be added as well.

.. _package-requirements:

Package Requirements
--------------------

Spack can be configured to always use certain compilers, package versions, and variants during concretization through package requirements.

Package requirements are useful when you find yourself repeatedly specifying the same constraints on the command line, and wish that Spack respects these constraints whether you mention them explicitly or not.
Another use case is specifying constraints that should apply to all root specs in an environment, without having to repeat the constraint everywhere.

Apart from that, requirements config is more flexible than constraints on the command line, because it can specify constraints on packages *when they occur* as a dependency.
In contrast, on the command line it is not possible to specify constraints on dependencies while also keeping those dependencies optional.

.. seealso::

   FAQ: :ref:`Why does Spack pick particular versions and variants? <faq-concretizer-precedence>`


Requirements syntax
^^^^^^^^^^^^^^^^^^^

The package requirements configuration is specified in ``packages.yaml``, keyed by package name and expressed using the Spec syntax.
In the simplest case you can specify attributes that you always want the package to have by providing a single spec string to ``require``:

.. code-block:: yaml

   packages:
     libfabric:
       require: "@1.13.2"

In the above example, ``libfabric`` will always build with version 1.13.2.
If you need to compose multiple configuration scopes ``require`` accepts a list of strings:

.. code-block:: yaml

   packages:
     libfabric:
       require:
       - "@1.13.2"
       - "%gcc"

In this case ``libfabric`` will always build with version 1.13.2 **and** using GCC as a compiler.

For more complex use cases, require accepts also a list of objects.
These objects must have either a ``any_of`` or a ``one_of`` field, containing a list of spec strings, and they can optionally have a ``when`` and a ``message`` attribute:

.. code-block:: yaml

   packages:
     openmpi:
       require:
       - any_of: ["@4.1.5", "%c,cxx,fortran=gcc"]
         message: "in this example only 4.1.5 can build with other compilers"

``any_of`` is a list of specs.
One of those specs must be satisfied and it is also allowed for the concretized spec to match more than one.
In the above example, that means you could build ``openmpi@4.1.5%gcc``, ``openmpi@4.1.5%clang`` or ``openmpi@3.9%gcc``, but not ``openmpi@3.9%clang``.

If a custom message is provided, and the requirement is not satisfiable, Spack will print the custom error message:

.. code-block:: spec

   $ spack spec openmpi@3.9%clang
   ==> Error: in this example only 4.1.5 can build with other compilers

We could express a similar requirement using the ``when`` attribute:

.. code-block:: yaml

   packages:
     openmpi:
       require:
       - any_of: ["%c,cxx,fortran=gcc"]
         when: "@:4.1.4"
         message: "in this example only 4.1.5 can build with other compilers"

In the example above, if the version turns out to be 4.1.4 or less, we require the compiler to be GCC.
For readability, Spack also allows a ``spec`` key accepting a string when there is only a single constraint:

.. code-block:: yaml

   packages:
     openmpi:
       require:
       - spec: "%c,cxx,fortran=gcc"
         when: "@:4.1.4"
         message: "in this example only 4.1.5 can build with other compilers"

This code snippet and the one before it are semantically equivalent.

Finally, instead of ``any_of`` you can use ``one_of`` which also takes a list of specs.
The final concretized spec must match one and only one of them:

.. code-block:: yaml

   packages:
     mpich:
       require:
       - one_of: ["+cuda", "+rocm"]

In the example above, that means you could build ``mpich+cuda`` or ``mpich+rocm`` but not ``mpich+cuda+rocm``.

.. note::

   For ``any_of`` and ``one_of``, the order of specs indicates a preference: items that appear earlier in the list are preferred (note that these preferences can be ignored in favor of others).

.. note::

   When using a conditional requirement, Spack is allowed to actively avoid the triggering condition (the ``when=...`` spec) if that leads to a concrete spec with better scores in the optimization criteria.
   To check the current optimization criteria and their priorities you can run ``spack solve zlib``.

Setting default requirements
^^^^^^^^^^^^^^^^^^^^^^^^^^^^

You can also set default requirements for all packages under ``all`` like this:

.. code-block:: yaml

   packages:
     all:
       require: "%[when=%c]c=clang %[when=%cxx]cxx=clang"

which means every spec will be required to use ``clang`` as the compiler for C and C++ code.

.. warning::

   The simpler config ``require: %clang`` will fail to build any package that does not include compiled code, because those packages cannot depend on ``clang`` (alias for ``llvm+clang``).
   In most contexts, default requirements must use either conditional dependencies or a :ref:`toolchain <toolchains>` that combines conditional dependencies.

Requirements on variants for all packages are possible too, but note that they are only enforced for those packages that define these variants, otherwise they are disregarded.
For example:

.. code-block:: yaml

   packages:
     all:
       require:
       - "+shared"
       - "+cuda"

will just enforce ``+shared`` on ``zlib``, which has a boolean ``shared`` variant but no ``cuda`` variant.

Constraints in a single spec literal are always considered as a whole, so in a case like:

.. code-block:: yaml

   packages:
     all:
       require: "+shared +cuda"

the default requirement will be enforced only if a package has both a ``cuda`` and a ``shared`` variant, and will never be partially enforced.

Finally, ``all`` represents a *default set of requirements* - if there are specific package requirements, then the default requirements under ``all`` are disregarded.
For example, with a configuration like this:

.. code-block:: yaml

   packages:
     all:
       require:
       - "build_type=Debug"
       - "%[when=%c]c=clang %[when=%cxx]cxx=clang"
     cmake:
       require:
       - "build_type=Debug"
       - "%c,cxx=gcc"

Spack requires ``cmake`` to use ``gcc`` and all other nodes (including ``cmake`` dependencies) to use ``clang``.
If enforcing ``build_type=Debug`` is needed also on ``cmake``, it must be repeated in the specific ``cmake`` requirements.

.. _setting-requirements-on-virtual-specs:

Setting requirements on virtual specs
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

A requirement on a virtual spec applies whenever that virtual is present in the DAG.
This can be useful for fixing which virtual provider you want to use:

.. code-block:: yaml

   packages:
     mpi:
       require: "mvapich2 %c,cxx,fortran=gcc"

With the configuration above the only allowed ``mpi`` provider is ``mvapich2`` built with ``gcc``/``g++``/``gfortran``.

Requirements on the virtual spec and on the specific provider are both applied, if present.
For instance with a configuration like:

.. code-block:: yaml

   packages:
     mpi:
       require: "mvapich2 %c,cxx,fortran=gcc"
     mvapich2:
       require: "~cuda"

you will use ``mvapich2~cuda %c,cxx,fortran=gcc`` as an ``mpi`` provider.

.. _package-strong-preferences:

Conflicts and strong preferences
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

If the semantic of requirements is too strong, you can also express "strong preferences" and "conflicts" from configuration files:

.. code-block:: yaml

   packages:
     all:
       prefer:
       - "%c,cxx=clang"
       conflict:
       - "+shared"

The ``prefer`` and ``conflict`` sections can be used whenever a ``require`` section is allowed.
The argument is always a list of constraints, and each constraint can be either a simple string, or a more complex object:

.. code-block:: yaml

   packages:
     all:
       conflict:
       - spec: "%c,cxx=clang"
         when: "target=x86_64_v3"
         message: "reason why clang cannot be used"

The ``spec`` attribute is mandatory, while both ``when`` and ``message`` are optional.

.. note::

   Requirements allow for expressing both "strong preferences" and "conflicts".
   The syntax for doing so, though, may not be immediately clear.
   For instance, if we want to prevent any package from using ``%clang``, we can set:

   .. code-block:: yaml

      packages:
        all:
          require:
          - one_of: ["%clang", "@:"]

   Since only one of the requirements must hold, and ``@:`` is always true, the rule above is equivalent to a conflict.
   For "strong preferences" the same construction works, with the ``any_of`` policy instead of the ``one_of`` policy.

.. _package-preferences:

Package Preferences
-------------------

In some cases package requirements can be too strong, and package preferences are the better option.
Package preferences do not impose constraints on packages for particular versions or variants values, they rather only set defaults.
The concretizer is free to change them if it must, due to other constraints, and also prefers reusing installed packages over building new ones that are a better match for preferences.

.. seealso::

   FAQ: :ref:`Why does Spack pick particular versions and variants? <faq-concretizer-precedence>`


The ``target`` and ``providers`` preferences can only be set globally under the ``all`` section of ``packages.yaml``:

.. code-block:: yaml

   packages:
     all:
       target: [x86_64_v3]
       providers:
         mpi: [mvapich2, mpich, openmpi]

These preferences override Spack's default and effectively reorder priorities when looking for the best compiler, target or virtual package provider.
Each preference takes an ordered list of spec constraints, with earlier entries in the list being preferred over later entries.

In the example above all packages prefer to target the ``x86_64_v3`` microarchitecture and to use ``mvapich2`` if they depend on ``mpi``.

The ``variants`` and ``version`` preferences can be set under package specific sections of the ``packages.yaml`` file:

.. code-block:: yaml

   packages:
     opencv:
       variants: +debug
     gperftools:
       version: [2.2, 2.4, 2.3]

In this case, the preference for ``opencv`` is to build with debug options, while ``gperftools`` prefers version 2.2 over 2.4.

Any preference can be overwritten on the command line if explicitly requested.

Preferences cannot overcome explicit constraints, as they only set a preferred ordering among homogeneous attribute values.
Going back to the example, if ``gperftools@2.3:`` was requested, then Spack will install version 2.4 since the most preferred version 2.2 is prohibited by the version constraint.

.. _package_permissions:

Package Permissions
-------------------

Spack can be configured to assign permissions to the files installed by a package.

In the ``packages.yaml`` file under ``permissions``, the attributes ``read``, ``write``, and ``group`` control the package permissions.
These attributes can be set per-package, or for all packages under ``all``.
If permissions are set under ``all`` and for a specific package, the package-specific settings take precedence.

The ``read`` and ``write`` attributes take one of ``user``, ``group``, and ``world``.

.. code-block:: yaml

   packages:
     all:
       permissions:
         write: group
         group: spack
     my_app:
       permissions:
         read: group
         group: my_team

The permissions settings describe the broadest level of access to installations of the specified packages.
The execute permissions of the file are set to the same level as read permissions for those files that are executable.
The default setting for ``read`` is ``world``, and for ``write`` is ``user``.
In the example above, installations of ``my_app`` will be installed with user and group permissions but no world permissions, and owned by the group ``my_team``.
All other packages will be installed with user and group write privileges, and world read privileges.
Those packages will be owned by the group ``spack``.

The ``group`` attribute assigns a Unix-style group to a package.
All files installed by the package will be owned by the assigned group, and the sticky group bit will be set on the install prefix and all directories inside the install prefix.
This will ensure that even manually placed files within the install prefix are owned by the assigned group.
If no group is assigned, Spack will allow the OS default behavior to go as expected.

.. _assigning-package-attributes:

Assigning Package Attributes
----------------------------

You can assign class-level attributes in the configuration:

.. code-block:: yaml

   packages:
     mpileaks:
       package_attributes:
         # Override existing attributes
         url: http://www.somewhereelse.com/mpileaks-1.0.tar.gz
         # ... or add new ones
         x: 1

Attributes set this way will be accessible to any method executed in the package.py file (e.g. the ``install()`` method).
Values for these attributes may be any value parseable by yaml.

These can only be applied to specific packages, not "all" or virtual packages.
