1ECM-DEVELOPER(7)              Extra CMake Modules             ECM-DEVELOPER(7)
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NAME

6       ecm-developer - ECM Developer Reference
7

WRITING MODULES

9       The  CMake 3 documentation (and cmake-developer(7) in particular) has a
10       lot of useful information about  writing  CMake  modules,  including  a
11       large  section  devoted to find modules. This guide will only highlight
12       things that are particular to the Extra CMake Modules project.
13
14       Most of these are stylistic points. For example, the license header for
15       a module in ECM should look like:
16
17          # SPDX-FileCopyrightText: 20XX Your Name <your.email@example.com>
18          #
19          # SPDX-License-Identifier: BSD-3-Clause
20
21       Documentation  is  written  in reStructuredText format and put inside a
22       bracket comment with a .rst: id after the opening bracket:
23
24          #[=======================================================================[.rst:
25          The docs
26          #]=======================================================================]
27
28       (docs/sphinx/ext/ecm.py has code to extract the rst text from a comment
29       with such wrapping)
30
31       Functions  should be used instead of macros unless there is a good rea‐
32       son not to (and that reason should be noted in a comment),  and  lower‐
33       case should be used for macros, functions and commands.
34
35       4  spaces  is the generally-recommended indent, although there are sev‐
36       eral files that use 2 spaces; consistency within a file is more  impor‐
37       tant than consistency across files.
38
39       If in doubt, look at how other modules in Extra CMake Modules are writ‐
40       ten, and follow the same pattern.
41
42   Find Modules
43       A good template for find module documentation is:
44
45          #[=======================================================================[.rst:
46          FindFoo
47          -------
48
49          Finds the Foo library.
50
51          This will define the following variables:
52
53          ``Foo_FOUND``
54              True if (the requested version of) Foo is available
55          ``Foo_VERSION``
56              The version of Foo, if it is found
57          ``Foo_LIBRARIES``
58              This can be passed to target_link_libraries() instead of the ``Foo::Foo``
59              target
60          ``Foo_INCLUDE_DIRS``
61              This should be passed to target_include_directories() if the target is not
62              used for linking
63          ``Foo_DEFINITIONS``
64              This should be passed to target_compile_options() if the target is not
65              used for linking
66
67          If ``Foo_FOUND`` is TRUE, it will also define the following imported target:
68
69          ``Foo::Foo``
70              The Foo library
71
72          In general we recommend using the imported target, as it is easier to use.
73          Bear in mind, however, that if the target is in the link interface of an
74          exported library, it must be made available by the package config file.
75          #]=======================================================================]
76
77       Note the use of definition lists for the variables.
78
79       Because of the ECMUseFindModules module, projects may easily make local
80       copies  of  find  modules,  and may install those copies with their own
81       CMake project config files. For this reason, find  modules  should  in‐
82       clude the full BSD 3-clause license:
83
84          #=============================================================================
85          # Copyright 20XX Your Name <your.email@example.com>
86          #
87          # Redistribution and use in source and binary forms, with or without
88          # modification, are permitted provided that the following conditions
89          # are met:
90          #
91          # 1. Redistributions of source code must retain the copyright
92          #    notice, this list of conditions and the following disclaimer.
93          # 2. Redistributions in binary form must reproduce the copyright
94          #    notice, this list of conditions and the following disclaimer in the
95          #    documentation and/or other materials provided with the distribution.
96          # 3. The name of the author may not be used to endorse or promote products
97          #    derived from this software without specific prior written permission.
98          #
99          # THIS SOFTWARE IS PROVIDED BY THE AUTHOR ``AS IS'' AND ANY EXPRESS OR
100          # IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES
101          # OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED.
102          # IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR ANY DIRECT, INDIRECT,
103          # INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT
104          # NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
105          # DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
106          # THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
107          # (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF
108          # THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
109          #=============================================================================
110
111       Find  modules should always provide imported targets in addition to the
112       traditional variables (like Foo_LIBRARIES, etc).
113
114       Unlike find modules shipped with CMake, if the module requires  a  spe‐
115       cific  CMake version it is not enough to warn when the minimum required
116       version is not high enough: you should also produce an error  when  the
117       actual  CMake  version  being used is not high enough. This can be done
118       with:
119
120          if(CMAKE_VERSION VERSION_LESS 3.16.0)
121              message(FATAL_ERROR "CMake 3.16.0 is required by FindFoo.cmake")
122          endif()
123          if(CMAKE_MINIMUM_REQUIRED_VERSION VERSION_LESS 3.16.0)
124              message(AUTHOR_WARNING "Your project should require at least CMake 3.16.0 to use FindFoo.cmake")
125          endif()
126
127       The  ECMFindModuleHelpers  module  has  several  useful  functions  and
128       macros.  For  example, it allows you to replace the above version check
129       with:
130
131          ecm_find_package_version_check(Foo)
132
133   Components
134       Using ECMFindModuleHelpers, creating a find module for a  library  with
135       several inter-dependent components is reasonably straightforward. After
136       the documentation, you need to include the module and do the usual ver‐
137       sion check:
138
139          include(ECMFindModuleHelpers)
140          ecm_find_package_version_check(Foo)
141
142       The   important   macros   are   ecm_find_package_parse_components  and
143       ecm_find_package_handle_library_components.  These take a list of  com‐
144       ponents, and query other variables you provide to find out the informa‐
145       tion they require.  The documentation for ECMFindModuleHelpers provides
146       more information, but a simple setup might look like:
147
148          set(Foo_known_components Bar Baz)
149          set(Foo_Bar_pkg_config "foo-bar")
150          set(Foo_Bar_lib "bar")
151          set(Foo_Bar_header "foo/bar.h")
152          set(Foo_Bar_pkg_config "foo-baz")
153          set(Foo_Baz_lib "baz")
154          set(Foo_Baz_header "foo/baz.h")
155
156       If Baz depends on Bar, for example, you can specify this with
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158          set(Foo_Baz_component_deps "Bar")
159
160       Then call the macros:
161
162          ecm_find_package_parse_components(Foo
163              RESULT_VAR Foo_components
164              KNOWN_COMPONENTS ${Foo_known_components}
165          )
166          ecm_find_package_handle_library_components(Foo
167              COMPONENTS ${Foo_components}
168          )
169
170       Of  course,  if  your components need unusual handling, you may want to
171       replace ecm_find_package_handle_library_components with, for example, a
172       foreach  loop  over  the components (the body of which should implement
173       most of what a normal find module does, including  setting  Foo_<compo‐
174       nent>_FOUND).
175
176       At  this  point,  you should set Foo_VERSION using whatever information
177       you have available (such as from  parsing  header  files).   Note  that
178       ecm_find_package_handle_library_components  will  set it to the version
179       reported by pkg-config of the first component found, but  this  depends
180       on the presence of pkg-config files, and the version of a component may
181       not be the same as the version of the whole package.  After that,  fin‐
182       ish off with
183
184          include(FindPackageHandleStandardArgs)
185          find_package_handle_standard_args(Foo
186              FOUND_VAR
187                  Foo_FOUND
188              REQUIRED_VARS
189                  Foo_LIBRARIES
190              VERSION_VAR
191                  Foo_VERSION
192              HANDLE_COMPONENTS
193          )
194
195          include(FeatureSummary)
196          set_package_properties(Foo PROPERTIES
197              URL "https://www.foo.example.com/"
198              DESCRIPTION "A library for doing useful things")
199

SUBMITTING MODULES

201       Proposed new modules should be submitted using the KDE Review Board in‐
202       stance, and  be  assigned  to  the  buildsystem  and  extracmakemodules
203       groups.  You should be able to point to two separate projects that will
204       make use of the module.
205
206       The       mailing       list       can        be        found        at
207       https://mail.kde.org/mailman/listinfo/kde-buildsystem.
208
210       KDE Developers
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2155.102                            Jan 14, 2023                 ECM-DEVELOPER(7)
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