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| __TOC__
| | {{CMake/Template/Moved}} |
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| This page documents some of the changes and new features available in CMake 2.6. | | This page has moved [https://gitlab.kitware.com/cmake/community/wikis/doc/cmake/notes/2.6 here]. |
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| =Exporting and Importing Targets=
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| CMake 2.6 introduces support for exporting targets from one project and importing them into another.
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| The main feature allowing this functionality is the notion of an <code>IMPORTED</code> target.
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| Here we present imported targets and then show how CMake files may be generated by a project to export its targets for use by other projects.
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| ==Importing Targets==
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| Imported targtes are used to convert files outside the project on disk into logical targets inside a CMake project.
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| They are created using the <code>IMPORTED</code> option to <code>add_executable</code> and <code>add_library</code> commands.
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| No build files are generated for imported targets.
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| They are useful simply for convenient, flexible reference to outside executables and libraries.
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| Consider the following example which creates and uses an IMPORTED executable target:
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| add_executable(generator IMPORTED) # 1
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| set_property(TARGET generator PROPERTY IMPORTED_LOCATION "/path/to/some_generator") # 2
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|
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| set(GENERATED_SRC ${CMAKE_CURRENT_BINARY_DIR}/generated.c)
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| add_custom_command(
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| OUTPUT ${GENERATED_SRC}
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| COMMAND generator ${GENERATED_SRC} # 3
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| )
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|
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| add_executable(myexe src1.c src2.c ${GENERATED_SRC})
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| Line #1 creates a new CMake target called "<code>generator</code>".
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| Line #2 tells CMake the location of the file on disk to import.
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| Line #3 references the target in a custom command. The generated build system will contain a command line such as
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| /path/to/some_generator /project/binary/dir/generated.c
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| in the rule to generate the source file.
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| Libraries may also be used through imported targets:
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| add_library(foo IMPORTED)
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| set_property(TARGET foo PROPERTY IMPORTED_LOCATION /path/to/libfoo.a)
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| add_executable(myexe src1.c src2.c)
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| target_link_libraries(myexe foo)
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| The generated build system will contain a command line such as
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| ... -o myexe /path/to/libfoo.a ...
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| in the rule to link <code>myexe</code>.
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| On Windows a .dll and its .lib import library may be imported together:
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| add_library(bar IMPORTED)
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| set_property(TARGET bar PROPERTY IMPORTED_LOCATION c:/path/to/bar.dll)
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| set_property(TARGET bar PROPERTY IMPORTED_IMPLIB c:/path/to/bar.lib)
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| add_executable(myexe src1.c src2.c)
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| target_link_libraries(myexe bar)
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| The generated build system will contain a command line such as
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| ... -o myexe.exe c:/path/to/bar.lib ...
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| in the rule to link <code>myexe</code>.
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| A library with multiple configurations may be imported with a single target:
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| add_library(foo IMPORTED)
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| set_property(TARGET foo PROPERTY IMPORTED_LOCATION_RELEASE c:/path/to/foo.lib)
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| set_property(TARGET foo PROPERTY IMPORTED_LOCATION_DEBUG c:/path/to/foo_d.lib)
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| add_executable(myexe src1.c src2.c)
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| target_link_libraries(myexe foo)
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| The generated build system will link <code>myexe</code> to <code>foo.lib</code> when it is built in the release configuration and <code>foo_d.lib</code> when built in the debug configuration.
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| ==Exporting Targets==
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| Imported targets on their own are useful, but they still require the project that imports targets to know the locations of the target files on disk.
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| The real power of this feature is when the project providing the target files also provides a file to help import them.
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| ===Exporting from and Installation Tree===
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| The <code>install(TARGETS)</code> and <code>install(EXPORT)</code> commands work together to install a target and a file to help import it.
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| For example, the code
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| add_executable(generator generator.c)
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| install(TARGETS generator DESTINATION lib/myproj/generators EXPORT myproj-targets)
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| install(EXPORT myproj-targets DESTINATION lib/myproj)
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| installs the files
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| <prefix>/lib/myproj/generators/generator
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| <prefix>/lib/myproj/myproj-targets.cmake
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| The <code>myproj-targets.cmake</code> file contains code such as
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| add_executable(generator IMPORTED)
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| set_property(
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| TARGET generator
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| PROPERTY IMPORTED_LOCATION ${PREFIX}/lib/myproj/generators/generator
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| )
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| (in practice the <code>${PREFIX}</code> is computed relative to the file location automatically).
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| An outside project may now contain code such as
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| include(${PREFIX}/lib/myproj/myproj-targets.cmake) # 1
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| set(GENERATED_SRC ${CMAKE_CURRENT_BINARY_DIR}/generated.c)
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| add_custom_command(
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| OUTPUT ${GENERATED_SRC}
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| COMMAND generator ${GENERATED_SRC} # 2
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| )
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| add_executable(myexe src1.c src2.c ${GENERATED_SRC})
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| Line #1 loads the target import script. The script may import any number of targets. Their locations are computed relative to the script location so the install tree may be moved around.
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| The importing project need only find the import script (see the packaging section of this document for how this might be done).
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| Line #2 references the generator executable in a custom command. The resulting build system will run the executable from its installed location.
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| Libraries may also be exported and imported:
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| add_library(foo STATIC foo1.c)
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| install(TARGETS foo DESTINATION lib EXPORTS myproj-targets)
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| install(EXPORT myproj-targets DESTINATION lib/myproj)
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| This installs the library and an import file referencing it. Outside projects may simply write
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| include(${PREFIX}/lib/myproj/myproj-targets.cmake)
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| add_executable(myexe src1.c)
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| target_link_libraries(myexe foo)
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| and the executable will be linked to the library <code>foo</code> exported and installed by the original project.
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| Any number of target installations may be associated with the same export name.
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| The export names are considered global so any directory may contribute a target installation.
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| Only one call to the <code>install(EXPORT)</code> command is needed to install an import file that references all targets.
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| Both of the examples above may be combined into a single export, even if they are in different subdirectories of the project:
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| # A/CMakeLists.txt
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| add_executable(generator generator.c)
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| install(TARGETS generator DESTINATION lib/myproj/generators EXPORT myproj-targets)
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|
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| # B/CMakeLists.txt
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| add_library(foo STATIC foo1.c)
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| install(TARGETS foo DESTINATION lib EXPORTS myproj-targets)
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|
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| # Top CMakeLists.txt
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| add_subdirectory(A)
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| add_subdirectory(B)
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| install(EXPORT myproj-targets DESTINATION lib/myproj)
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| ===Exporting from a Build Tree===
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| Typically projects are built and installed before being used by an outside project.
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| However in some cases it is desirable to export targets directly from a build tree.
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| The targets may then be used by an outside project that references the build tree with no installation involved.
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| The <code>export</code> command is used to generate a file exporting targets from a project build tree.
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| For example, the code
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| add_executable(generator generator.c)
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| export(TARGETS generator FILE myproj-exports.cmake)
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| will create a file in the project build tree called <code>myproj-exports.cmake</code> that contains code such as
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| add_executable(generator IMPORTED)
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| set_property(TARGET generator PROPERTY IMPORTED_LOCATION "/path/to/build/tree/generator")
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| This file may be loaded by an outside project that is aware of the project build tree in order to use the executable to generate a source file.
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| An example application of this feature is for building a generator executable on a host platform when cross compiling.
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| The project containing the generator executable may be built on the host platform and then the project that is being cross-compiled for another platform may load it.
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| =Packages=
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| This section documents creation and use of packages that help projects locate each other.
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| These features are distinct from CPack which is meant for creating source and binary distributions and installers.
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| The <code>find_package</code> command has been enhanced with features to help find packages without the use of "find" modules (FindXXX.cmake files).
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| Projects that are aware of CMake may provide a "package configuration file" inside their installation trees.
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| Naming the file correctly and installing it in a suitable location will allow the <code>find_package</code> command to find it easily.
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| ==Package Configuration Files==
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| Consider a project "Foo" that installs the following files:
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| <prefix>/include/foo-1.2/foo.h
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| <prefix>/lib/foo-1.2/libfoo.a
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| It may also provide a CMake package configuration file
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| <prefix>/lib/foo-1.2/foo-config.cmake
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| with content such as
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| # ...
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| # (compute PREFIX relative to file location)
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| # ...
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| set(foo_INCLUDE_DIRS ${PREFIX}/include/foo-1.2)
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| set(foo_LIBRARY ${PREFIX}/lib/foo-1.2/libfoo.a)
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| If another project wishes to use Foo it need only to locate the <code>foo-config.cmake</code> file and load it to get all the information it needs about package content locations.
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| Since the package configuration file is provided by the package installation it already knows all the file locations.
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| The <code>find_package</code> command may be used to search for the configuration file:
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| find_package(Foo)
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| This command (assuming no <code>FindFoo.cmake</code> module exists) constructs a set of installation prefixes and searches under each prefix in several locations.
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| Given the name "Foo", it looks for a file called "<code>FooConfig.cmake</code>" or "<code>foo-config.cmake</code>".
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| The full set of locations is specified in the <code>find_package</code> command documentation, but one place it looks is
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| <prefix>/lib/Foo*/
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| where "<code>Foo*</code>" is a case-insensitive globbing expression.
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| In our example the globbing expression will match "<code><prefix>/lib/foo-1.2</code>" and the configuration file will be found.
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| Once found, a package configuration file is immediately loaded. It contains all the information the project needs to use the package.
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| ===Packaging and Exporting===
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| Package configuration files may also work in conjunction with the target exporting/importing feature discussed above.
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| For example, a project might write
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| add_library(mylib STATIC mylib.c mylib.h)
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| install(FILES mylib.h DESTINATION include/myproj)
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| install(TARGETS mylib DESTINATION lib/myproj EXPORT mylib-targets)
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| install(EXPORT mylib-targets DESTINATION lib/myproj)
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| install(FILES mylib-config.cmake DESTINATION lib/myproj)
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| where <code>mylib-config.cmake</code> contains something like
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| get_filename_component(SELF_DIR "${CMAKE_CURRENT_LIST_FILE}" PATH)
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| include(${SELF_DIR}/mylib-targets.cmake)
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| get_filename_component(myproj_INCLUDE_DIRS "${SELF_DIR}/../../include/myproj" ABSOLUTE)
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| After the project is built and installed, an outside project may use it by writing
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| find_package(myproj REQUIRED)
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| include_directories(${myproj_INCLUDE_DIRS})
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| add_executable(myexe myexe.c)
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| target_link_libraries(myexe mylib)
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| ==Package Version Files==
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| The <code>find_package</code> command offers a version request argument. One might write
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| find_package(Foo 1.2)
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| find_package(Bar 4.2 EXACT)
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| in order to get a version of package <code>Foo</code> that is compatible with version 1.2 and exactly package <code>Bar</code> version 4.2.
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| CMake does not attempt to define any convention for the compatibility or exactness of version numbers for a package.
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| It also does not try to map the version number to a directory or file name.
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| Instead packages must provide "version" files next to their package configuration files.
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| This allows maximum flexibility for project authors and package maintainers.
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| A package version file is placed next to the package configuration file.
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| Its name matches that of the configuration file but has either "<code>-version</code>" or "<code>Version</code>" appended to the name before the "<code>.cmake</code>" extension.
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| For example, the files
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| <prefix>/lib/foo-1.3/foo-config.cmake
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| <prefix>/lib/foo-1.3/foo-config-version.cmake
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| and
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| <prefix>/lib/bar-4.2/BarConfig.cmake
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| <prefix>/lib/bar-4.2/BarConfigVersion.cmake
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| are each pairs of package configuration files and corresponding version files.
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| When the <code>find_package</code> command finds a candidate package configuration file it looks next to it for a version file.
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| The version file is loaded to test whether the package version is an acceptable match for the version requested.
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| If the version file claims compatibility the configuration file is accepted. Otherwise it is ignored.
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| When the <code>find_package</code> command loads a version file it first sets the following variables:
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| PACKAGE_FIND_NAME = the <package> name given as the first argument of find_package
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| PACKAGE_FIND_VERSION = full requested version string
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| PACKAGE_FIND_VERSION_MAJOR = requested major version, if any
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| PACKAGE_FIND_VERSION_MINOR = requested minor version, if any
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| PACKAGE_FIND_VERSION_PATCH = requested patch version, if any
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| The version file must use these variables to check whether it is compatible or an exact match for the requested version.
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| It sets the following variables with results:
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| PACKAGE_VERSION = package version (major[.minor[.patch]])
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| PACKAGE_VERSION_EXACT = true if version is exact match
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| PACKAGE_VERSION_COMPATIBLE = true if version is compatible
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| For example, <code>foo-config-version.cmake</code> might contain
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| set(PACKAGE_VERSION 1.3)
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| if("${PACKAGE_FIND_VERSION_MAJOR}" EQUAL 1)
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| set(PACKAGE_VERSION_COMPATIBLE 1) # compatible with any version 1.x
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| if("${PACKAGE_FIND_VERSION_MINOR}" EQUAL 3)
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| set(PACKAGE_VERSION_EXACT 1) # exact match for version 1.3
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| endif("${PACKAGE_FIND_VERSION_MINOR}" EQUAL 3)
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| endif("${PACKAGE_FIND_VERSION_MAJOR}" EQUAL 1)
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| if it is compatible with all "<code>1.x</code>" versions of Foo and exactly matches version "<code>1.3</code>".
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| Note that the input variable names all start in "<code>PACKAGE_FIND_</code>" and the output variable names all start in "<code>PACKAGE_</code>".
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| The names are fixed and do not vary with the package name.
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| Version files are loaded in a nested scope so they are free to set any variables they wish as part of their computation.
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| The <code>find_package</code> command wipes out the scope when the version file has completed and it has checked the output variables.
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| When the version file claims to be an acceptable match for the requested version the <code>find_package</code> command sets the following variables for use by the project:
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| <package>_VERSION = package version (major[.minor[.patch]])
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| <package>_VERSION_MAJOR = major from major[.minor[.patch]], if any
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| <package>_VERSION_MINOR = minor from major[.minor[.patch]], if any
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| <package>_VERSION_PATCH = patch from major[.minor[.patch]], if any
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| The variables report the version of the package that was actually found.
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| The "<code><package></code>" part of their name matches the argument given to the <code>find_package</code> command.
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| =Preprocessor Definitions=
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| Preprocessor definitions may now be added to builds with much finer granularity than in previous versions of CMake. There is a new property called <code>COMPILE_DEFINITIONS</code> that is defined directories, targets, and source files. For example, the code
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| add_library(mylib src1.c src2.c)
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| add_executable(myexe main1.c)
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| set_property(
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| DIRECTORY
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| PROPERTY COMPILE_DEFINITIONS A AV=1
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| )
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| set_property(
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| TARGET mylib
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| PROPERTY COMPILE_DEFINITIONS B BV=2
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| )
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| set_property(
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| SOURCE src1.c
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| PROPERTY COMPILE_DEFINITIONS C CV=3
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| )
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| will build the source files with these definitions:
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| src1.c: -DA -DAV=1 -DB -DBV=2 -DC -DCV=3
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| src2.c: -DA -DAV=1 -DB -DBV=2
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| main2.c: -DA -DAV=1
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| When the <code>add_definitions</code> command is called with flags like "<code>-DX</code>" the definitions are extracted and added to the current directory's <code>COMPILE_DEFINITIONS</code> property. When a new subdirectory is created with <code>add_subdirectory</code> the current state of the directory-level property is used to initialize the same property in the subdirectory.
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| Note in the above example that the <code>set_property</code> command will actually '''set''' the property and replace any existing value. The command provides the <code>APPEND</code> option to help add more definitions without removing existing ones. For example, the code
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| set_property(
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| SOURCE src1.c
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| APPEND PROPERTY COMPILE_DEFINITIONS D DV=4
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| )
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| will add the definitions "<code>-DD -DDV=4</code>" when building <code>src1.c</code>.
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| Definitions may also be added on a per-configuration basis using the <code>COMPILE_DEFINITIONS_<CONFIG></code> property. For example, the code
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| set_property(
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| TARGET mylib
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| PROPERTY COMPILE_DEFINITIONS_DEBUG MYLIB_DEBUG_MODE
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| )
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| will build sources in mylib with <code>-DMYLIB_DEBUG_MODE</code> only when compiling in a <code>Debug</code> configuration.
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| =Link Line Generation=
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| CMake 2.6 implements a new approach to generating link lines for targets.
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| Consider these libraries:
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| /path/to/libfoo.a
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| /path/to/libfoo.so
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| Previously if someone wrote
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| target_link_libraries(myexe /path/to/libfoo.a)
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| CMake would generate this code to link it:
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| ... -L/path/to -Wl,-Bstatic -lfoo -Wl,-Bdynamic ...
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| This worked most of the time, but some platforms (such as OS X) do not
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| support the <code>-Bstatic</code> or equivalent flag. This made it impossible to
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| link to the static version of a library without creating a symlink in
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| another directory and using that one instead.
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| Now CMake will generate this code:
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| ... /path/to/libfoo.a ...
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| This guarantees that the correct library is chosen. However there are some side-effects.
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| ==Missing Linker Search Directories==
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| Projects used to be able to write this (wrong) code and it would work by accident:
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| add_executable(myexe myexe.c)
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| target_link_libraries(myexe /path/to/libA.so B)
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| where "<code>B</code>" is meant to link "<code>/path/to/libB.so</code>". This code is incorrect
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| because it asks CMake to link to <code>B</code> but does not provide the proper
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| linker search path for it. It used to work by accident because the
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| <code>-L/path/to</code> would get added as part of the implementation of linking to
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| A. The correct code would be
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| link_directories(/path/to)
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| add_executable(myexe myexe.c)
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| target_link_libraries(myexe /path/to/libA.so B)
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| or even better
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| add_executable(myexe myexe.c)
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| target_link_libraries(myexe /path/to/libA.so /path/to/libB.so)
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| In order to support projects that have this bug, we've added a
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| compatibility feature that adds the "<code>-L/path/to</code>" paths for all libraries
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| linked with full paths even though the linker will not need those paths
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| to find the main libraries. The compatibility mode is enabled when a
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| link line contains a non-full-path library (like <code>B</code>) and either
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| <code>CMAKE_BACKWARDS_COMPATIBILITY</code> is set to 2.4 or lower or
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| <code>CMAKE_LINK_OLD_PATHS</code> is set to true.
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| If you are trying to build a project and run into this problem, a quick-fix is to run
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| cmake -DCMAKE_LINK_OLD_PATHS:BOOL=ON .
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| in the top of the build tree.
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| ==Linking to System Libraries==
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| System libraries on UNIX-like systems are typically provided in <code>/usr/lib</code> or <code>/lib</code>. These directories are considered implicit linker search paths because linkers automatically search these locations even without a flag like <code>-L/usr/lib</code>. Consider the code
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| find_library(M_LIB m)
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| target_link_libraries(myexe ${M_LIB})
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| Typically the <code>find_library</code> command would find the math library
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| /usr/lib/libm.so
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| In CMake 2.4 the link line generation would split off the link directory <code>/usr/lib</code> and the library <code>libm.so</code> and produce the
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| ... -lm ...
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| Note that the <code>-L/usr/lib</code> option is left out because it is an implicit linker search path. The linker would see <code>-lm</code> and search for the math library. Typically the linker would find /usr/lib/libm.so too. However some platforms provide multiple versions of libraries correesponding to different architectures. For example, on an IRIX machine one might find the libraries
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| /usr/lib/libm.so (ELF o32)
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| /usr/lib32/libm.so (ELF n32)
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| /usr/lib64/libm.so (ELF 64)
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| On a Solaris machine one might find
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| /usr/lib/libm.so (sparcv8 architecture)
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| /usr/lib/sparcv9/libm.so (sparcv9 architecture)
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| When the linker sees <code>-lm</code> it in fact searches the system path corresponding to the current architecture. Internally it might use <code>-L/usr/lib/sparcv9</code> instead of <code>-L/usr/lib</code>.
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| In CMake 2.6 most libraries are given to the linker using their full path. The code
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| target_link_libraries(myexe /my/special/dir/libm.so)
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| generates the link line
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| ... /my/special/dir/libm.so ...
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| Unfortunately if this were done also for <code>/usr/lib/libm.so</code> the linker might complain if <code>/usr/lib/libm.so</code> does not match the architecture it wants.
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| The problem is created because <code>find_library</code> may not know about all the architecture-specific system search paths used by the linker. In fact when it finds <code>/usr/lib/libm.so</code> it may be finding a library of incorrect architecture. The solution is for the link computation to recognize that when it is given a library in a system search path that it should ask the linker to find the correct version of the library at link time. Consider the original example:
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| find_library(M_LIB m)
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| target_link_libraries(myexe ${M_LIB})
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| If <code>find_library</code> finds the library in a directory that is not an implicit link directory the full path will be linked as usual.
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| If <code>find_library</code> finds <code>/usr/lib/libm.so</code> the link computation will realize that this might be a system library and produces the link line
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| ... -lm ...
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| and the linker will locate the correct version of the library. This solution suffers from the original problem of distinguishing between static and shared versions:
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| /usr/lib/libm.a
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| /usr/lib/libm.so
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| When asked to link <code>/usr/lib/libm.a</code> the link computation will produce
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| ... -Wl,-Bstatic -lm ... -Wl,-Bshared ...
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| On platforms that do not have flags such as <code>-Bstatic</code> the link computation will never convert a full path to a <code>-l</code> option.
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