Commit cc32eeae authored by Anakin Skywalker's avatar Anakin Skywalker Committed by Craig Scott

Help: Clarify standard module variable naming

Co-Authored-By: Craig Scott's avatarCraig Scott <craig.scott@crascit.com>
parent 05f4248e
Pipeline #208676 waiting for manual action with stages
in 7 minutes and 43 seconds
......@@ -23,15 +23,14 @@ in turn link to developer guides for CMake itself.
Find Modules
============
A "find module" is a ``Find<PackageName>.cmake`` file to be loaded
by the :command:`find_package` command when invoked for ``<PackageName>``.
A "find module" is a ``Find<PackageName>.cmake`` file to be loaded by the
:command:`find_package` command when invoked for ``<PackageName>``.
The primary task of a find module is to determine whether a package
exists on the system, set the ``<PackageName>_FOUND`` variable to reflect
this and provide any variables, macros and imported targets required to
use the package. A find module is useful in cases where an upstream
library does not provide a
:ref:`config file package <Config File Packages>`.
The primary task of a find module is to determine whether a package is
available, set the ``<PackageName>_FOUND`` variable to reflect this and
provide any variables, macros and imported targets required to use the
package. A find module is useful in cases where an upstream library does
not provide a :ref:`config file package <Config File Packages>`.
The traditional approach is to use variables for everything, including
libraries and executables: see the `Standard Variable Names`_ section
......@@ -91,55 +90,92 @@ Standard Variable Names
For a ``FindXxx.cmake`` module that takes the approach of setting
variables (either instead of or in addition to creating imported
targets), the following variable names should be used to keep things
consistent between find modules. Note that all variables start with
``Xxx_`` to make sure they do not interfere with other find modules; the
same consideration applies to macros, functions and imported targets.
consistent between Find modules. Note that all variables start with
``Xxx_``, which (unless otherwise noted) must match exactly the name
of the ``FindXxx.cmake`` file, including upper/lowercase.
This prefix on the variable names ensures that they do not conflict with
variables of other Find modules. The same pattern should also be followed
for any macros, functions and imported targets defined by the Find module.
``Xxx_INCLUDE_DIRS``
The final set of include directories listed in one variable for use by
client code. This should not be a cache entry.
client code. This should not be a cache entry (note that this also means
this variable should not be used as the result variable of a
:command:`find_path` command - see ``Xxx_INCLUDE_DIR`` below for that).
``Xxx_LIBRARIES``
The libraries to link against to use Xxx. These should include full
paths. This should not be a cache entry.
The libraries to use with the module. These may be CMake targets, full
absolute paths to a library binary or the name of a library that the
linker must find in its search path. This should not be a cache entry
(note that this also means this variable should not be used as the
result variable of a :command:`find_library` command - see
``Xxx_LIBRARY`` below for that).
``Xxx_DEFINITIONS``
Definitions to use when compiling code that uses Xxx. This really
shouldn't include options such as ``-DHAS_JPEG`` that a client
The compile definitions to use when compiling code that uses the module.
This really shouldn't include options such as ``-DHAS_JPEG`` that a client
source-code file uses to decide whether to ``#include <jpeg.h>``
``Xxx_EXECUTABLE``
Where to find the Xxx tool.
``Xxx_Yyy_EXECUTABLE``
Where to find the Yyy tool that comes with Xxx.
The full absolute path to an executable. In this case, ``Xxx`` might not
be the name of the module, it might be the name of the tool (usually
converted to all uppercase), assuming that tool has such a well-known name
that it is unlikely that another tool with the same name exists. It would
be appropriate to use this as the result variable of a
:command:`find_program` command.
``Xxx_YYY_EXECUTABLE``
Similar to ``Xxx_EXECUTABLE`` except here the ``Xxx`` is always the module
name and ``YYY`` is the tool name (again, usually fully uppercase).
Prefer this form if the tool name is not very widely known or has the
potential to clash with another tool. For greater consistency, also
prefer this form if the module provides more than one executable.
``Xxx_LIBRARY_DIRS``
Optionally, the final set of library directories listed in one
variable for use by client code. This should not be a cache entry.
variable for use by client code. This should not be a cache entry.
``Xxx_ROOT_DIR``
Where to find the base directory of Xxx.
``Xxx_VERSION_Yy``
Expect Version Yy if true. Make sure at most one of these is ever true.
``Xxx_WRAP_Yy``
If False, do not try to use the relevant CMake wrapping command.
Where to find the base directory of the module.
``Xxx_VERSION_VV``
Variables of this form specify whether the ``Xxx`` module being provided
is version ``VV`` of the module. There should not be more than one
variable of this form set to true for a given module. For example, a
module ``Barry`` might have evolved over many years and gone through a
number of different major versions. Version 3 of the ``Barry`` module
might set the variable ``Barry_VERSION_3`` to true, whereas an older
version of the module might set ``Barry_VERSION_2`` to true instead.
It would be an error for both ``Barry_VERSION_3`` and ``Barry_VERSION_2``
to both be set to true.
``Xxx_WRAP_YY``
When a variable of this form is set to false, it indicates that the
relevant wrapping command should not be used. The wrapping command
depends on the module, it may be implied by the module name or it might
be specified by the ``YY`` part of the variable.
``Xxx_Yy_FOUND``
If False, optional Yy part of Xxx system is not available.
For variables of this form, ``Yy`` is the name of a component for the
module. It should match exactly one of the valid component names that
may be passed to the :command:`find_package` command for the module.
If a variable of this form is set to false, it means that the ``Yy``
component of module ``Xxx`` was not found or is not available.
Variables of this form would typically be used for optional components
so that the caller can check whether an optional component is available.
``Xxx_FOUND``
Set to false, or undefined, if we haven't found, or don't want to use
Xxx.
When the :command:`find_package` command returns to the caller, this
variable will be set to true if the module was deemed to have been found
successfully.
``Xxx_NOT_FOUND_MESSAGE``
Should be set by config-files in the case that it has set
``Xxx_FOUND`` to FALSE. The contained message will be printed by the
:command:`find_package` command and by
``find_package_handle_standard_args()`` to inform the user about the
problem.
:command:`find_package_handle_standard_args` to inform the user about the
problem. Use this instead of calling :command:`message` directly to
report a reason for failing to find the module or package.
``Xxx_RUNTIME_LIBRARY_DIRS``
Optionally, the runtime library search path for use when running an
......@@ -160,23 +196,36 @@ same consideration applies to macros, functions and imported targets.
``Xxx_VERSION_PATCH``
The patch version of the package found, if any.
The following names should not usually be used in CMakeLists.txt files, but
are typically cache variables for users to edit and control the
behaviour of find modules (like entering the path to a library manually)
The following names should not usually be used in ``CMakeLists.txt`` files.
They are intended for use by Find modules to specify and cache the locations
of specific files or directories. Users are typically able to set and edit
these variables to control the behavior of Find modules (like entering the
path to a library manually):
``Xxx_LIBRARY``
The path of the Xxx library (as used with :command:`find_library`, for
example).
The path of the library. Use this form only when the module provides a
single library. It is appropriate to use this as the result variable
in a :command:`find_library` command.
``Xxx_Yy_LIBRARY``
The path of the Yy library that is part of the Xxx system. It may or
may not be required to use Xxx.
The path of library ``Yy`` provided by the module ``Xxx``. Use this form
when the module provides more than one library or where other modules may
also provide a library of the same name. It is also appropriate to use
this form as the result variable in a :command:`find_library` command.
``Xxx_INCLUDE_DIR``
Where to find headers for using the Xxx library.
When the module provides only a single library, this variable can be used
to specify where to find headers for using the library (or more accurately,
the path that consumers of the library should add to their header search
path). It would be appropriate to use this as the result variable in a
:command:`find_path` command.
``Xxx_Yy_INCLUDE_DIR``
Where to find headers for using the Yy library of the Xxx system.
If the module provides more than one library or where other modules may
also provide a library of the same name, this form is recommended for
specifying where to find headers for using library ``Yy`` provided by
the module. Again, it would be appropriate to use this as the result
variable in a :command:`find_path` command.
To prevent users being overwhelmed with settings to configure, try to
keep as many options as possible out of the cache, leaving at least one
......@@ -185,7 +234,8 @@ not-found library (e.g. ``Xxx_ROOT_DIR``). For the same reason, mark
most cache options as advanced. For packages which provide both debug
and release binaries, it is common to create cache variables with a
``_LIBRARY_<CONFIG>`` suffix, such as ``Foo_LIBRARY_RELEASE`` and
``Foo_LIBRARY_DEBUG``.
``Foo_LIBRARY_DEBUG``. The :module:`SelectLibraryConfigurations` module
can be helpful for such cases.
While these are the standard variable names, you should provide
backwards compatibility for any old names that were actually in use.
......
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