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NAME

       cmake-developer - CMake Developer Reference

INTRODUCTION

       This  manual is intended for reference by developers modifying the CMake source tree itself, and by those
       authoring externally-maintained modules.

ADDING COMPILE FEATURES

       CMake reports an error if a compiler whose features are known does not report support  for  a  particular
       requested  feature.   A  compiler is considered to have known features if it reports support for at least
       one feature.

       When adding a new compile feature to CMake, it is therefore necessary to list support for the feature for
       all CompilerIds which already have one or more feature supported, if the new feature is available for any
       version of the compiler.

       When adding the first supported feature to a particular CompilerId, it is necessary to list  support  for
       all features known to cmake (See CMAKE_C_COMPILE_FEATURES and CMAKE_CXX_COMPILE_FEATURES as appropriate),
       where available for the compiler.  Ensure that the CMAKE_<LANG>_STANDARD_DEFAULT is set to  the  computed
       internal variable CMAKE_<LANG>_STANDARD_COMPUTED_DEFAULT for compiler versions which should be supported.

       It  is  sensible to record the features for the most recent version of a particular CompilerId first, and
       then work backwards.  It is sensible to try to create a continuous range of versions of feature  releases
       of the compiler.  Gaps in the range indicate incorrect features recorded for intermediate releases.

       Generally,  features  are  made  available  for  a  particular  version  if the compiler vendor documents
       availability of the feature with that version.  Note that sometimes partially implemented features appear
       to  be  functional  in  previous  releases  (such  as  cxx_constexpr  in  GNU 4.6, though availability is
       documented in GNU 4.7),  and  sometimes  compiler  vendors  document  availability  of  features,  though
       supporting  infrastructure  is  not  available  (such  as  __has_feature(cxx_generic_lambdas)  indicating
       non-availability in Clang 3.4, though it is documented as available, and fixed in  Clang  3.5).   Similar
       cases for other compilers and versions need to be investigated when extending CMake to support them.

       When a vendor releases a new version of a known compiler which supports a previously unsupported feature,
       and there are already known features for that compiler, the feature should  be  listed  as  supported  in
       CMake for that version of the compiler as soon as reasonably possible.

       Standard-specific/compiler-specific  variables  such  CMAKE_CXX98_COMPILE_FEATURES  are  deliberately not
       documented.  They only exist for the compiler-specific implementation of adding the -std compile flag for
       compilers which need that.

HELP

       The  Help directory contains CMake help manual source files.  They are written using the reStructuredText
       markup syntax and processed by Sphinx to generate the CMake help manuals.

   Markup Constructs
       In addition to using Sphinx to generate the CMake help manuals, we also use  a  C++-implemented  document
       processor  to  print  documents  for  the  --help-*  command-line  help options.  It supports a subset of
       reStructuredText markup.  When authoring or modifying documents, please verify that the command-line help
       looks good in addition to the Sphinx-generated html and man pages.

       The  command-line  help  processor supports the following constructs defined by reStructuredText, Sphinx,
       and a CMake extension to Sphinx.

       CMake Domain directives
              Directives defined in the CMake Domain for defining CMake documentation  objects  are  printed  in
              command-line help output as if the lines were normal paragraph text with interpretation.

       CMake Domain interpreted text roles
              Interpreted  text  roles  defined  in  the  CMake Domain for cross-referencing CMake documentation
              objects are replaced by their link text in command-line help  output.   Other  roles  are  printed
              literally and not processed.

       code-block directive
              Add a literal code block without interpretation.  The command-line help processor prints the block
              content without the leading directive line and with common indentation replaced by one space.

       include directive
              Include another document source  file.   The  command-line  help  processor  prints  the  included
              document inline with the referencing document.

       literal block after ::
              A  paragraph  ending in :: followed by a blank line treats the following indented block as literal
              text without interpretation.  The command-line help processor prints the :: literally  and  prints
              the block content with common indentation replaced by one space.

       note directive
              Call  out  a  side note.  The command-line help processor prints the block content as if the lines
              were normal paragraph text with interpretation.

       parsed-literal directive
              Add a literal block with markup interpretation.  The command-line help processor prints the  block
              content without the leading directive line and with common indentation replaced by one space.

       productionlist directive
              Render context-free grammar productions.  The command-line help processor prints the block content
              as if the lines were normal paragraph text with interpretation.

       replace directive
              Define a |substitution| replacement.  The command-line  help  processor  requires  a  substitution
              replacement to be defined before it is referenced.

       |substitution| reference
              Reference   a   substitution  replacement  previously  defined  by  the  replace  directive.   The
              command-line help processor performs the substitution and replaces all newlines in the replacement
              text with spaces.

       toctree directive
              Include  other  document  sources  in  the Table-of-Contents document tree.  The command-line help
              processor prints the referenced documents inline as part of the referencing document.

       Inline markup constructs not listed above are printed literally in  the  command-line  help  output.   We
       prefer  to  use  inline  markup  constructs that look correct in source form, so avoid use of -escapes in
       favor of inline literals when possible.

       Explicit markup blocks not matching directives listed above are removed from  command-line  help  output.
       Do not use them, except for plain .. comments that are removed by Sphinx too.

       Note that nested indentation of blocks is not recognized by the command-line help processor.  Therefore:

       • Explicit markup blocks are recognized only when not indented inside other blocks.

       • Literal  blocks  after  paragraphs  ending  in  :: but not at the top indentation level may consume all
         indented lines following them.

       Try to avoid these cases in practice.

   CMake Domain
       CMake adds a Sphinx Domain called cmake, also called the “CMake Domain”.   It  defines  several  “object”
       types for CMake documentation:

       command
              A CMake language command.

       generator
              A CMake native build system generator.  See the cmake(1) command-line tool’s -G option.

       manual A CMake manual page, like this cmake-developer(7) manual.

       module A CMake module.  See the cmake-modules(7) manual and the include() command.

       policy A CMake policy.  See the cmake-policies(7) manual and the cmake_policy() command.

       prop_cache, prop_dir, prop_gbl, prop_sf, prop_inst, prop_test, prop_tgt
              A  CMake  cache,  directory,  global,  source  file,  installed  file,  test,  or target property,
              respectively.  See the cmake-properties(7) manual and the set_property() command.

       variable
              A CMake language variable.  See the cmake-variables(7) manual and the set() command.

       Documentation objects in the CMake Domain come from two sources.  First, the CMake  extension  to  Sphinx
       transforms  every  document  named with the form Help/<type>/<file-name>.rst to a domain object with type
       <type>.  The object name is extracted from the document title, which is expected to be of the form:

          <object-name>
          -------------

       and to appear at or near the top of the .rst file before any other lines starting in a letter, digit,  or
       <.   If no such title appears literally in the .rst file, the object name is the <file-name>.  If a title
       does appear, it is expected that <file-name> is equal to  <object-name>  with  any  <  and  >  characters
       removed.

       Second, the CMake Domain provides directives to define objects inside other documents:

          .. command:: <command-name>

           This indented block documents <command-name>.

          .. variable:: <variable-name>

           This indented block documents <variable-name>.

       Object types for which no directive is available must be defined using the first approach above.

   Cross-References
       Sphinx  uses reStructuredText interpreted text roles to provide cross-reference syntax.  The CMake Domain
       provides for each domain object type a role of the same name to cross-reference it.  CMake  Domain  roles
       are inline markup of the forms:

          :type:`name`
          :type:`text <name>`

       where type is the domain object type and name is the domain object name.  In the first form the link text
       will be name (or name() if the type is command) and in the second form the link text will be the explicit
       text.  For example, the code:

          * The :command:`list` command.
          * The :command:`list(APPEND)` sub-command.
          * The :command:`list() command <list>`.
          * The :command:`list(APPEND) sub-command <list>`.
          * The :variable:`CMAKE_VERSION` variable.
          * The :prop_tgt:`OUTPUT_NAME_<CONFIG>` target property.

       produces:

       • The list() command.

       • The list(APPEND) sub-command.

       • The list() command.

       • The list(APPEND) sub-command.

       • The CMAKE_VERSION variable.

       • The OUTPUT_NAME_<CONFIG> target property.

       Note  that  CMake  Domain roles differ from Sphinx and reStructuredText convention in that the form a<b>,
       without a space preceding <, is interpreted as a name instead of link text with an explicit target.  This
       is  necessary  because  we  use <placeholders> frequently in object names like OUTPUT_NAME_<CONFIG>.  The
       form a <b>, with a space preceding <, is still interpreted as a link text with an explicit target.

   Style
   Style: Section Headers
       When marking section titles, make the section decoration line as long as the title text.  Use only a line
       below the title, not above. For example:

          Title Text
          ----------

       Capitalize the first letter of each non-minor word in the title.

       The section header underline character hierarchy is

       • #: Manual group (part) in the master document

       • *: Manual (chapter) title

       • =: Section within a manual

       • -: Subsection or CMake Domain object document title

       • ^: Subsubsection or CMake Domain object document section

       • ": Paragraph or CMake Domain object document subsection

   Style: Whitespace
       Use two spaces for indentation.  Use two spaces between sentences in prose.

   Style: Line Length
       Prefer  to restrict the width of lines to 75-80 columns.  This is not a hard restriction, but writing new
       paragraphs wrapped at 75 columns allows space for adding minor content without significant re-wrapping of
       content.

   Style: Prose
       Use American English spellings in prose.

   Style: Starting Literal Blocks
       Prefer  to mark the start of literal blocks with :: at the end of the preceding paragraph. In cases where
       the following block gets a code-block marker, put a single : at the end of the preceding paragraph.

   Style: CMake Command Signatures
       Command signatures should be marked up as plain literal blocks, not as cmake code-blocks.

       Signatures are separated from preceding content by a section header.  That is, use:

          ... preceding paragraph.

          Normal Libraries
          ^^^^^^^^^^^^^^^^

          ::

            add_library(<lib> ...)

          This signature is used for ...

       Signatures of commands should wrap optional parts with square brackets, and should mark list of  optional
       arguments  with  an  ellipsis (...).  Elements of the signature which are specified by the user should be
       specified with angle brackets, and may be referred to in prose using inline-literal syntax.

   Style: Boolean Constants
       Use  “OFF”  and  “ON”  for  boolean   values   which   can   be   modified   by   the   user,   such   as
       POSITION_INDEPENDENT_CODE.  Such  properties  may be “enabled” and “disabled”. Use “True” and “False” for
       inherent values which can’t be modified after being set, such as the IMPORTED property of a build target.

   Style: Inline Literals
       Mark up references to keywords in signatures, file names, and other technical terms  with  inline-literal
       syntax, for example:

          If ``WIN32`` is used with :command:`add_executable`, the
          :prop_tgt:`WIN32_EXECUTABLE` target property is enabled. That command
          creates the file ``<name>.exe`` on Windows.

   Style: Cross-References
       Mark  up  linkable references as links, including repeats.  An alternative, which is used by wikipedia (‐
       http://en.wikipedia.org/wiki/WP:REPEATLINK), is to link to a reference only once per article. That  style
       is not used in CMake documentation.

   Style: Referencing CMake Concepts
       If  referring to a concept which corresponds to a property, and that concept is described in a high-level
       manual, prefer to link to the manual section instead of the property. For example:

          This command creates an :ref:`Imported Target <Imported Targets>`.

       instead of:

          This command creates an :prop_tgt:`IMPORTED` target.

       The latter should be used only when referring specifically to the property.

       References to manual sections are not automatically created by creating a section, but code such as:

          .. _`Imported Targets`:

       creates a suitable anchor.  Use an anchor name which matches  the  name  of  the  corresponding  section.
       Refer to the anchor using a cross-reference with specified text.

       Imported Targets need the IMPORTED term marked up with care in particular because the term may refer to a
       command keyword (IMPORTED), a target property (IMPORTED), or a concept (Imported Targets).

       Where a property, command or variable is related conceptually to others, by for example, being related to
       the  buildsystem  description,  generator  expressions or Qt, each relevant property, command or variable
       should link to the primary manual, which provides high-level information.   Only  particular  information
       relating to the command should be in the documentation of the command.

   Style: Referencing CMake Domain Objects
       When referring to CMake Domain objects such as properties, variables, commands etc, prefer to link to the
       target object and follow that with the type of object it is.  For example:

          Set the :prop_tgt:`AUTOMOC` target property to ``ON``.

       Instead of

          Set the target property :prop_tgt:`AUTOMOC` to ``ON``.

       The policy directive is an exception, and the type us usually referred to before the link:

          If policy :prop_tgt:`CMP0022` is set to ``NEW`` the behavior is ...

       However, markup self-references with inline-literal syntax.  For  example,  within  the  add_executable()
       command documentation, use

          ``add_executable``

       not

          :command:`add_executable`

       which is used elsewhere.

MODULES

       The Modules directory contains CMake-language .cmake module files.

   Module Documentation
       To document CMake module Modules/<module-name>.cmake, modify Help/manual/cmake-modules.7.rst to reference
       the module in the toctree directive, in sorted order, as:

          /module/<module-name>

       Then add the module document file Help/module/<module-name>.rst containing just the line:

          .. cmake-module:: ../../Modules/<module-name>.cmake

       The cmake-module directive will scan the module file to  extract  reStructuredText  markup  from  comment
       blocks  that start in .rst:.  At the top of Modules/<module-name>.cmake, begin with the following license
       notice:

          # Distributed under the OSI-approved BSD 3-Clause License.  See accompanying
          # file Copyright.txt or https://cmake.org/licensing for details.

       After this notice, add a BLANK line.  Then, add documentation using a Line Comment block of the form:

          #.rst:
          # <module-name>
          # -------------
          #
          # <reStructuredText documentation of module>

       or a Bracket Comment of the form:

          #[[.rst:
          <module-name>
          -------------

          <reStructuredText documentation of module>
          #]]

       Any number of = may be used in the opening and closing brackets as long as they match.   Content  on  the
       line containing the closing bracket is excluded if and only if the line starts in #.

       Additional such .rst: comments may appear anywhere in the module file.  All such comments must start with
       # in the first column.

       For example, a Modules/Findxxx.cmake module may contain:

          # Distributed under the OSI-approved BSD 3-Clause License.  See accompanying
          # file Copyright.txt or https://cmake.org/licensing for details.

          #.rst:
          # FindXxx
          # -------
          #
          # This is a cool module.
          # This module does really cool stuff.
          # It can do even more than you think.
          #
          # It even needs two paragraphs to tell you about it.
          # And it defines the following variables:
          #
          # * VAR_COOL: this is great isn't it?
          # * VAR_REALLY_COOL: cool right?

          <code>

          #[========================================[.rst:
          .. command:: xxx_do_something

           This command does something for Xxx::

            xxx_do_something(some arguments)
          #]========================================]
          macro(xxx_do_something)
            <code>
          endmacro()

       Test the documentation formatting by running cmake --help-module <module-name>, and also by enabling  the
       SPHINX_HTML  and  SPHINX_MAN  options  to  build  the  documentation.   Edit the comments until generated
       documentation looks satisfactory.  To have a .cmake file in this directory NOT show  up  in  the  modules
       documentation,     simply     leave     out    the    Help/module/<module-name>.rst    file    and    the
       Help/manual/cmake-modules.7.rst toctree entry.

   Find Modules
       A “find module” is a Modules/Find<package>.cmake file to be loaded by  the  find_package()  command  when
       invoked for <package>.

       The  primary  task  of  a  find  module  is  to determine whether a package exists on the system, set the
       <package>_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 config
       file package.

       The traditional approach is to use variables for everything, including libraries and executables: see the
       Standard  Variable Names section below.  This is what most of the existing find modules provided by CMake
       do.

       The more modern approach is to behave as much like config file packages files as possible,  by  providing
       imported target.  This has the advantage of propagating Target Usage Requirements to consumers.

       In  either case (or even when providing both variables and imported targets), find modules should provide
       backwards compatibility with old versions that had the same name.

       A FindFoo.cmake module will typically be loaded by the command:

          find_package(Foo [major[.minor[.patch[.tweak]]]]
                       [EXACT] [QUIET] [REQUIRED]
                       [[COMPONENTS] [components...]]
                       [OPTIONAL_COMPONENTS components...]
                       [NO_POLICY_SCOPE])

       See the find_package() documentation for details on what variables are set for the find module.  Most  of
       these are dealt with by using FindPackageHandleStandardArgs.

       Briefly,  the module should only locate versions of the package compatible with the requested version, as
       described by the Foo_FIND_VERSION family of variables.  If Foo_FIND_QUIETLY is set  to  true,  it  should
       avoid  printing  messages,  including  anything  complaining  about  the  package  not  being  found.  If
       Foo_FIND_REQUIRED is set to true, the module should issue a FATAL_ERROR if the package cannot  be  found.
       If neither are set to true, it should print a non-fatal message if it cannot find the package.

       Packages  that  find  multiple  semi-independent  parts (like bundles of libraries) should search for the
       components listed in Foo_FIND_COMPONENTS if it is set , and only  set  Foo_FOUND  to  true  if  for  each
       searched-for  component  <c>  that  was  not  found,  Foo_FIND_REQUIRED_<c>  is  not  set  to  true.  The
       HANDLE_COMPONENTS argument of find_package_handle_standard_args() can be used to implement this.

       If Foo_FIND_COMPONENTS is not set, which modules are searched for and required is up to the find  module,
       but should be documented.

       For  internal  implementation,  it  is  a  generally  accepted  convention  that  variables starting with
       underscore are for temporary use only.

       Like all  modules,  find  modules  should  be  properly  documented.   To  add  a  module  to  the  CMake
       documentation, follow the steps in the Module Documentation section above.

   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.

       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.

       Xxx_LIBRARIES
              The  libraries  to link against to use Xxx. These should include full paths.  This should not be a
              cache entry.

       Xxx_DEFINITIONS
              Definitions to use when compiling code that uses Xxx. 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.

       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.

       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.

       Xxx_Yy_FOUND
              If False, optional Yy part of Xxx system is not available.

       Xxx_FOUND
              Set to false, or undefined, if we haven’t found, or don’t want to use Xxx.

       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 find_package() command and by find_package_handle_standard_args()
              to inform the user about the problem.

       Xxx_RUNTIME_LIBRARY_DIRS
              Optionally, the runtime library search path for use when running an executable  linked  to  shared
              libraries.   The list should be used by user code to create the PATH on windows or LD_LIBRARY_PATH
              on UNIX.  This should not be a cache entry.

       Xxx_VERSION
              The full version string of the package found, if any.  Note that  many  existing  modules  provide
              Xxx_VERSION_STRING instead.

       Xxx_VERSION_MAJOR
              The major version of the package found, if any.

       Xxx_VERSION_MINOR
              The minor version of the package found, if any.

       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)

       Xxx_LIBRARY
              The path of the Xxx library (as used with find_library(), for example).

       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.

       Xxx_INCLUDE_DIR
              Where to find headers for using the Xxx library.

       Xxx_Yy_INCLUDE_DIR
              Where to find headers for using the Yy library of the Xxx system.

       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 option which can be used to disable use of the module, or locate a
       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.

       While these are the standard variable names, you should provide backwards compatibility for any old names
       that were actually in use.  Make sure you comment them as deprecated, so that no-one starts using them.

   A Sample Find Module
       We will describe how to create a simple find module for a library Foo.

       The first thing that is needed is a license notice.

          # Distributed under the OSI-approved BSD 3-Clause License.  See accompanying
          # file Copyright.txt or https://cmake.org/licensing for details.

       Next  we  need  module  documentation.   CMake’s  documentation system requires you to follow the license
       notice with a blank line and then with a documentation marker and the name of  the  module.   You  should
       follow this with a simple statement of what the module does.

          #.rst:
          # FindFoo
          # -------
          #
          # Finds the Foo library
          #

       More  description  may be required for some packages.  If there are caveats or other details users of the
       module should be aware of, you can add further paragraphs below this.  Then you  need  to  document  what
       variables and imported targets are set by the module, such as

          # This will define the following variables::
          #
          #   Foo_FOUND    - True if the system has the Foo library
          #   Foo_VERSION  - The version of the Foo library which was found
          #
          # and the following imported targets::
          #
          #   Foo::Foo   - The Foo library

       If  the  package  provides  any  macros, they should be listed here, but can be documented where they are
       defined.  See the Module Documentation section above for more details.

       Now the actual libraries and so on have to be found.  The code here will obviously vary  from  module  to
       module  (dealing  with  that,  after  all,  is the point of find modules), but there tends to be a common
       pattern for libraries.

       First, we try to use pkg-config to find the library.  Note that we cannot rely on this, as it may not  be
       available, but it provides a good starting point.

          find_package(PkgConfig)
          pkg_check_modules(PC_Foo QUIET Foo)

       This should define some variables starting PC_Foo_ that contain the information from the Foo.pc file.

       Now  we  need  to find the libraries and include files; we use the information from pkg-config to provide
       hints to CMake about where to look.

          find_path(Foo_INCLUDE_DIR
            NAMES foo.h
            PATHS ${PC_Foo_INCLUDE_DIRS}
            PATH_SUFFIXES Foo
          )
          find_library(Foo_LIBRARY
            NAMES foo
            PATHS ${PC_Foo_LIBRARY_DIRS}
          )

       If you have a good way of getting the version (from a  header  file,  for  example),  you  can  use  that
       information   to   set   Foo_VERSION   (although   note   that   find  modules  have  traditionally  used
       Foo_VERSION_STRING, so you may want to set  both).   Otherwise,  attempt  to  use  the  information  from
       pkg-config

          set(Foo_VERSION ${PC_Foo_VERSION})

       Now we can use FindPackageHandleStandardArgs to do most of the rest of the work for us

          include(FindPackageHandleStandardArgs)
          find_package_handle_standard_args(Foo
            FOUND_VAR Foo_FOUND
            REQUIRED_VARS
              Foo_LIBRARY
              Foo_INCLUDE_DIR
            VERSION_VAR Foo_VERSION
          )

       This  will  check  that the REQUIRED_VARS contain values (that do not end in -NOTFOUND) and set Foo_FOUND
       appropriately.  It will also cache those values.  If Foo_VERSION is  set,  and  a  required  version  was
       passed  to  find_package(),  it will check the requested version against the one in Foo_VERSION.  It will
       also print messages as appropriate; note that if the package was found, it will print the contents of the
       first required variable to indicate where it was found.

       At  this point, we have to provide a way for users of the find module to link to the library or libraries
       that were found.  There are two approaches,  as  discussed  in  the  Find  Modules  section  above.   The
       traditional variable approach looks like

          if(Foo_FOUND)
            set(Foo_LIBRARIES ${Foo_LIBRARY})
            set(Foo_INCLUDE_DIRS ${Foo_INCLUDE_DIR})
            set(Foo_DEFINITIONS ${PC_Foo_CFLAGS_OTHER})
          endif()

       If  more  than one library was found, all of them should be included in these variables (see the Standard
       Variable Names section for more information).

       When providing imported targets, these  should  be  namespaced  (hence  the  Foo::  prefix);  CMake  will
       recognize  that values passed to target_link_libraries() that contain :: in their name are supposed to be
       imported targets (rather than just library names), and will produce appropriate  diagnostic  messages  if
       that target does not exist (see policy CMP0028).

          if(Foo_FOUND AND NOT TARGET Foo::Foo)
            add_library(Foo::Foo UNKNOWN IMPORTED)
            set_target_properties(Foo::Foo PROPERTIES
              IMPORTED_LOCATION "${Foo_LIBRARY}"
              INTERFACE_COMPILE_OPTIONS "${PC_Foo_CFLAGS_OTHER}"
              INTERFACE_INCLUDE_DIRECTORIES "${Foo_INCLUDE_DIR}"
            )
          endif()

       One thing to note about this is that the INTERFACE_INCLUDE_DIRECTORIES and similar properties should only
       contain information  about  the  target  itself,  and  not  any  of  its  dependencies.   Instead,  those
       dependencies  should also be targets, and CMake should be told that they are dependencies of this target.
       CMake will then combine all the necessary information automatically.

       The type of the IMPORTED target created in the add_library() command can always be specified  as  UNKNOWN
       type.   This  simplifies  the  code in cases where static or shared variants may be found, and CMake will
       determine the type by inspecting the files.

       If the library is available with multiple configurations,  the  IMPORTED_CONFIGURATIONS  target  property
       should also be populated:

          if(Foo_FOUND)
            if (NOT TARGET Foo::Foo)
              add_library(Foo::Foo UNKNOWN IMPORTED)
            endif()
            if (Foo_LIBRARY_RELEASE)
              set_property(TARGET Foo::Foo APPEND PROPERTY
                IMPORTED_CONFIGURATIONS RELEASE
              )
              set_target_properties(Foo::Foo PROPERTIES
                IMPORTED_LOCATION_RELEASE "${Foo_LIBRARY_RELEASE}"
              )
            endif()
            if (Foo_LIBRARY_DEBUG)
              set_property(TARGET Foo::Foo APPEND PROPERTY
                IMPORTED_CONFIGURATIONS DEBUG
              )
              set_target_properties(Foo::Foo PROPERTIES
                IMPORTED_LOCATION_DEBUG "${Foo_LIBRARY_DEBUG}"
              )
            endif()
            set_target_properties(Foo::Foo PROPERTIES
              INTERFACE_COMPILE_OPTIONS "${PC_Foo_CFLAGS_OTHER}"
              INTERFACE_INCLUDE_DIRECTORIES "${Foo_INCLUDE_DIR}"
            )
          endif()

       The  RELEASE  variant  should  be listed first in the property so that that variant is chosen if the user
       uses a configuration which is not an exact match for any listed IMPORTED_CONFIGURATIONS.

       Most of the cache variables should be hidden in the ccmake interface unless the user explicitly  asks  to
       edit them.

          mark_as_advanced(
            Foo_INCLUDE_DIR
            Foo_LIBRARY
          )

       If  this  module  replaces  an  older  version, you should set compatibility variables to cause the least
       disruption possible.

          # compatibility variables
          set(Foo_VERSION_STRING ${Foo_VERSION})

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