127 lines
4.8 KiB
Markdown
127 lines
4.8 KiB
Markdown
# Useful Modules
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There are a ton of useful modules in CMake's «cmake:modules» collection; but some of them are more useful than others. Here are a few highlights.
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## «module:CMakeDependentOption»
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This adds a command `cmake_dependent_option` that sets an option based on another set of variables being true. It looks like this:
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```cmake
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include(CMakeDependentOption)
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cmake_dependent_option(BUILD_TESTS "Build your tests" ON "VAL1;VAL2" OFF)
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```
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which is just a shortcut for this:
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```cmake
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if(VAL1 AND VAL2)
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set(BUILD_TESTS_DEFAULT ON)
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else()
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set(BUILD_TESTS_DEFAULT OFF)
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endif()
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option(BUILD_TESTS "Build your tests" ${BUILD_TESTS_DEFAULT})
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if(NOT BUILD_TESTS_DEFAULT)
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mark_as_advanced(BUILD_TESTS)
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endif()
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```
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Note that `BUILD_TESTING` is a better way to check for testing being enabled if you use `include(CTest)`, since it is defined for you. This is just an example of `CMakeDependentOption`.
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## «module:CMakePrintHelpers»
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This module has a couple of handy output functions. `cmake_print_properties` lets you easily print properties.
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And `cmake_print_variables` will print the names and values of any variables you give it.
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## «module:CheckCXXCompilerFlag»
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This checks to see if a flag is supported. For example:
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```cmake
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include(CheckCXXCompilerFlag)
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check_cxx_compiler_flag(-someflag OUTPUT_VARIABLE)
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```
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Note that `OUTPUT_VARIABLE` will also appear in the configuration printout, so choose a good name.
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This is just one of many similar modules, such as `CheckIncludeFileCXX`, `CheckStructHasMember`, `TestBigEndian`, and `CheckTypeSize` that allow you
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to check for information about the system (and you can communicate that to your source code).
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## «module:WriteCompilerDetectionHeader»
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This is an amazing module similar to the ones listed above, but special enough to deserve its own section. It allows you
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to look for a list of features that some compilers support, and write out a C++ header file that lets you know whether that
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feature is available. It even can provide compatibility macros for features that have changed names!
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To use:
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```cmake
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write_compiler_detection_header(
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FILE myoutput.h
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PREFIX My
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COMPILERS GNU Clang MSVC Intel
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FEATURES cxx_variadic_templates
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)
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```
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This supports compiler features (defined to 0 or 1), symbols (defined to empty or the symbol), and macros that
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support different names. They will be prefixed with the PREFIX you provide. You can separate compilers into different
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files using `OUTPUT_FILES_DIR`.
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The downside is that you do have to list the compilers you expect to support. If you use the `ALLOW_UNKNOWN_COMPILERS` flag(s),
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you can keep this from erroring on unknown compilers, but it will still leave all features empty.
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## «command:`try_compile`»/«command:`try_run`»
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This is not exactly a module, but is crucial to many of the modules listed above. You can attempt to compile (and possibly run) a bit of code at configure time. This can allow you to get information about the capabilities of your system. The basic syntax is:
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```cmake
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try_compile(
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RESULT_VAR
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bindir
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SOURCES
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source.cpp
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)
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```
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There are lots of options you can add, like `COMPILE_DEFINITIONS`. In CMake 3.8+, this will honor the CMake C/C++/CUDA standard settings. If you use `try_run` instead, it will run the resulting program and give you the output in `RUN_OUTPUT_VARIABLE`.
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## «module:FeatureSummary»
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This is a fairly useful but rather odd module. It allows you to print out a list of packages what were searched for, as well as any options you explicity mark. It's partially but not completely tied into «command:`find_package`». You first include the module, as always:
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```cmake
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include(FeatureSummary)
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```
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Then, for any find packages you have run or will run, you can extend the default information:
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```cmake
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set_package_properties(OpenMP PROPERTIES
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URL "http://www.openmp.org"
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DESCRIPTION "Parallel compiler directives"
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PURPOSE "This is what it does in my package")
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```
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You can also set the `TYPE` of a package to `RUNTIME`, `OPTIONAL`, `RECOMMENDED`, or `REQUIRED`; you can't, however, lower the type of a package; if you have already added a `REQUIRED` package through «command:`find_package`» based on an option, you'll see it listed as `REQUIRED`.
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And, you can mark any options as part of the feature summary. If you choose the same name as a package, the two interact with each other.
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```cmake
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add_feature_info(WITH_OPENMP OpenMP_CXX_FOUND "OpenMP (Thread safe FCNs only)")
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```
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Then, you can print out the summary of features, either to the screen or a log file:
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```cmake
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if(CMAKE_PROJECT_NAME STREQUAL PROJECT_NAME)
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feature_summary(WHAT ENABLED_FEATURES DISABLED_FEATURES PACKAGES_FOUND)
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feature_summary(FILENAME ${CMAKE_CURRENT_BINARY_DIR}/features.log WHAT ALL)
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endif()
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```
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You can build any collection of `WHAT` items that you like, or just use `ALL`.
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