# CMake Cross Compiling

Cross compiling is supported by CMake starting with version 2.6.0.

Cross compiling means that the software is built for a different system than the one which does the build. This means

• CMake cannot autodetect the target system
• Usually the executables don't run on the build host
• The build process has to use a different set of include files and libraries for building, i.e. not the native ones

## Setting up the system and toolchain

When cross compiling, CMake cannot guess the target system, so you have to preset some CMake variables, e.g. using a toolchain file. The following variables have to be preset:

CMAKE_SYSTEM_NAME
this one is mandatory, it is the name of the target system, i.e. the same as CMAKE_SYSTEM_NAME would have if CMake would run on the target system. Typical examples are "Linux" and "Windows". This variable is used for constructing the file names of the platform files like Linux.cmake or Windows-gcc.cmake. If your target is an embedded system without OS set CMAKE_SYSTEM_NAME to "Generic". If CMAKE_SYSTEM_NAME is preset, the CMake variable CMAKE_CROSSCOMPILING is automatically set to TRUE, so this can be used for testing in the CMake files.
CMAKE_SYSTEM_VERSION
optional, version of your target system, not used very much.
CMAKE_SYSTEM_PROCESSOR
optional, processor (or hardware) of the target system. This variable is not used very much except for one purpose, it is used to load a CMAKE_SYSTEM_NAME-compiler-CMAKE_SYSTEM_PROCESSOR.cmake file, which can be used to modify settings like compiler flags etc. for the target. You probably only have to set this one if you are using a cross compiler where every target hardware needs special build settings.

Since CMake cannot guess the target system, it also cannot guess which compiler it should use, so you have to preset this too:

CMAKE_C_COMPILER
the C compiler executable, may be the full path or just the filename. If it is specified with full path, then this path will be prefered when searching the C++ compiler and the other tools (binutils, linker, etc.). If this compiler is a gcc-cross compiler with a prefixed name (e.g. "arm-elf-gcc") CMake will detect this and automatically find the corresponding C++ compiler (i.e. "arm-elf-c++"). The compiler can also be preset via the CC environment variables.
CMAKE_CXX_COMPILER
the C++ compiler executable, may be the full path or just the filename. It is handled the same way as CMAKE_C_COMPILER. If the toolchain is a GNU toolchain, you only need to set one of both.

Once the system and the compiler are determined by CMake, it loads the corresponding files in the following order:

• Platform/${CMAKE_SYSTEM_NAME}.cmake (optional) • Platform/${CMAKE_SYSTEM_NAME}-<compiler>.cmake (optional)
• Platform/${CMAKE_SYSTEM_NAME}-<compiler>-${CMAKE_SYSTEM_PROCESSOR}.cmake (optional)

<compiler> is either the basename of the compiler executable, e.g. "gcc" (this is also used if gcc has a different name) or "cl", or by a compiler id, which is detected by compiling a test source file.

For testing the host system, there is a corresponding set of variables, which is set automatically by CMake:

• CMAKE_HOST_SYSTEM_NAME
• CMAKE_HOST_SYSTEM_VERSION
• CMAKE_HOST_SYSTEM_PROCESSOR
• CMAKE_HOST_SYSTEM

Without cross compiling the variables for the host system and the target system are identical. In most cases you will want to test for the target system, then the same way as without cross compiling use the CMAKE_SYSTEM_xxx variables, this will work both for cross compiling and for native building.

With these variables correctly set, CMake will now use the cross compiling toolchain for building and in the CMakeLists.txt you can still use the CMAKE_SYSTEM_XXX variables for testing for which system you are building. This is already enough to use CMake for cross compiling simple (buildsystem-wise) projects.

## Searching and finding external software

Most non-trivial projects will depend on external libraries or tools. CMake offers the FIND_PROGRAM(), FIND_LIBRARY(), FIND_FILE(), FIND_PATH() and FIND_PACKAGE() commands for this purpose. They search the file system in common places for files and return the results. FIND_PACKAGE() is a bit different in that it actually doesn't search itself, but "only" executes FindXXX.cmake modules, which usually call the FIND_PROGRAM(), FIND_LIBRARY(), FIND_FILE() and FIND_PATH() commands then.

When cross compiling e.g. for a target with an ARM processor getting /usr/lib/libjpeg.so as the result of a FIND_PACKAGE(JPEG) wouldn't be much of a help, since this would be the JPEG library for the host system, e.g. an x86 Linux box. So you need to tell CMake to search in other locations. This can be done by setting the following variables:

CMAKE_FIND_ROOT_PATH
~/src/build$cmake -DCMAKE_TOOLCHAIN_FILE=~/Toolchain-eldk-ppc74xx.cmake .. ...  You don't have to write a toolchain file for every piece of software you want to build, the toolchain files are per target platform, i.e. if you are building several software packages all for the same target platform, you have to write only one toolchain file and you can use this for all packages. If your compiler is not able to build a simple program by default without special flags or files (e.g. linker scripts or memory layout files), the toolchain file as shown above doesn't work. Then you have to force the compiler: INCLUDE(CMakeForceCompiler) # this one is important SET(CMAKE_SYSTEM_NAME eCos) # specify the cross compiler CMAKE_FORCE_C_COMPILER(arm-elf-gcc GNU) CMAKE_FORCE_CXX_COMPILER(arm-elf-g++ GNU) # where is the target environment SET(CMAKE_FIND_ROOT_PATH /home/alex/src/ecos/install ) # search for programs in the build host directories SET(CMAKE_FIND_ROOT_PATH_MODE_PROGRAM NEVER) # for libraries and headers in the target directories SET(CMAKE_FIND_ROOT_PATH_MODE_LIBRARY ONLY) SET(CMAKE_FIND_ROOT_PATH_MODE_INCLUDE ONLY)  This is done using the CMAKE_FORCE_XXX_COMPILER() macros. The second argument is the compiler id, which is used by CMake to recognize the compiler. A toolchain for crosscompiling for Win32 using mingw32 might look like this: # the name of the target operating system SET(CMAKE_SYSTEM_NAME Windows) # which compilers to use for C and C++ SET(CMAKE_C_COMPILER i486-mingw32-gcc) SET(CMAKE_CXX_COMPILER i486-mingw32-g++) SET(CMAKE_RC_COMPILER i486-mingw32-windres) # here is the target environment located SET(CMAKE_FIND_ROOT_PATH /usr/i486-mingw32) # adjust the default behaviour of the FIND_XXX() commands: # search headers and libraries in the target environment, search # programs in the host environment set(CMAKE_FIND_ROOT_PATH_MODE_PROGRAM NEVER) set(CMAKE_FIND_ROOT_PATH_MODE_LIBRARY ONLY) set(CMAKE_FIND_ROOT_PATH_MODE_INCLUDE ONLY)  ## System introspection Many non-trivial software projects have a set of system introspection tests for finding out properties of the (target) system. In CMake there are macros provided for this purpose like e.g. CHECK_INCLUDE_FILES() or CHECK_C_SOURCE_RUNS(). Most of these tests will internally use either the TRY_COMPILE() or the TRY_RUN() CMake commands. The TRY_COMPILE() commands work as expected also when cross compiling, they will try to compile the piece of code with the cross compiling toolchain, which will give the expected result. All tests using TRY_RUN() internally cannot work, since the created executables cannot run on the build host system. At first TRY_RUN() tries to compile the software, which will work the same way when cross compiling. If this succeeded, it will check the variable CMAKE_CROSSCOMPILING whether the resulting executable is runnable or not. If not, it will create two cache variables, which then have to be set by the user or via the CMake cache. Let's say the command looks like this: TRY_RUN(SHARED_LIBRARY_PATH_TYPE SHARED_LIBRARY_PATH_INFO_COMPILED${PROJECT_BINARY_DIR}/CMakeTmp
${PROJECT_SOURCE_DIR}/CMake/SharedLibraryPathInfo.cxx OUTPUT_VARIABLE OUTPUT ARGS "LDPATH")  The variable SHARED_LIBRARY_PATH_INFO_COMPILED will be set to the result of the build (i.e. TRUE or FALSE). CMake will create a cache variable SHARED_LIBRARY_PATH_TYPE and preset it to PLEASE_FILL_OUT-FAILED_TO_RUN. This one has to be set to the exit code of the executable if it would have been executed on the target. It will also create a cache variable SHARED_LIBRARY_PATH_TYPE__TRYRUN_OUTPUT and preset it to PLEASE_FILL_OUT-NOTFOUND. This one has to be set to the output the executable prints to stdout and stderr if it is executed on the target. This variable is only created if the TRY_RUN() command was used with the RUN_OUTPUT_VARIABLE or the OUTPUT_VARIABLE argument. You have to fill in appropriate values for these variables. To help you with this CMake tries its best to give you useful information. To do so CMake creates a file${CMAKE_BINARY_DIR}/TryRunResults.cmake. There you will find all variables which CMake could not determine, from which CMake file they were called, the source file, the arguments for the executable and the path to the executable. CMake will also copy the executables in the build directory, they have the names cmTryCompileExec-<name of the variable>, e.g. cmTryCompileExec-SHARED_LIBRARY_PATH_TYPE. You can then try to run this executable manually on the actual target platform and check the results.

Once you have these results, they have to get in the CMake cache. You can either use ccmake/CMakeSetup/"make edit_cache" and edit the variables directly in the cache. Then you won't be able to reuse your changes in another build directory or if you remove CMakeCache.txt. The second option is to use the TryRunResults.cmake file. Copy it to a safe location (i.e. where it is not deleted if you delete the build dir) and give it a useful name, e.g. MyProjectTryRunResults-eldk-ppc.cmake. Then edit it so that the SET() commands set the required values. You can the use this file to preload the CMake cache by using the -C option of cmake:

src/build/ $cmake -C ~/MyProjectTryRunResults-eldk-ppc.cmake .  You don't have to use the other CMake options again, they are now already in the cache. This way you can use MyProjectTryRunResults-eldk-ppc.cmake in multiple build trees and it could also be distributed with your project so it gets easier for other users who want to compile it. This script may be helpful to automatically populate the TRY_RUN results with those placed in a CMakeCache.txt that were created on the target. ## Using executables in the build created during the build In some cases during a build executables are created which are then used in ADD_CUSTOM_COMMAND() or ADD_CUSTOM_TARGET() during the same build process. When cross compiling this won't work without modifications because the executables cannot run on the build host. Starting with CMake 2.6 it is possible to "import" executable targets into a CMake project. When cross compiling this has to be used to import executables built in a native build into the cross-build. This can be done like this: # when crosscompiling import the executable targets from a file IF(CMAKE_CROSSCOMPILING) SET(IMPORT_EXECUTABLES "IMPORTFILE-NOTFOUND" CACHE FILEPATH "Point it to the export file from a native build") INCLUDE(${IMPORT_EXECUTABLES})
ENDIF(CMAKE_CROSSCOMPILING)

...

# only build the generator if not crosscompiling
IF(NOT CMAKE_CROSSCOMPILING)
TARGET_LINK_LIBRARIES(mygenerator ${SOME_LIBS}) ENDIF(NOT CMAKE_CROSSCOMPILING) # then use the target name as COMMAND, CMake >= 2.6 knows how to handle this ADD_CUSTOM_COMMAND(OUTPUT${CMAKE_CURRENT_BINARY_DIR}/generated.c
COMMAND mygenerator foo.dat -o ${CMAKE_CURRENT_BINARY_DIR}/generated.c DEPENDS foo.dat ) ... # export the generator target to a file, so it can be imported (see above) by another build # the IF() is not necessary, but makes the intention clearer IF(NOT CMAKE_CROSSCOMPILING) EXPORT(TARGETS mygenerator FILE${CMAKE_BINARY_DIR}/ImportExecutables.cmake )
ENDIF(NOT CMAKE_CROSSCOMPILING)



So during the native build the target "mygenerator" will be built and used in ADD_CUSTOM_COMMAND(). As command only the target name is used. CMake >= 2.6.0 recognizes this and creates the dependencies and will use the path to the created executable when executing the command. At the end the EXPORT() function (since CMake 2.6.0) is called, which "exports" the listed targets to the file ${CMAKE_BINARY_DIR}/ImportExecutables.cmake, which will look like this: ADD_EXECUTABLE(mygenerator IMPORT) SET_TARGET_PROPERTIES(mygenerator PROPERTIES LOCATION /home/alex/build-native/bin/mygenerator )  This file is then included when cross compiling, it either has to be specified using -D or via the cmake GUI. Then later on the command for actually building mygenerator is excluded. In ADD_CUSTOM_COMMAND() mygenerator will be recognized as an imported target and it will be used when executing the command. If the executable mygenerator also has to be built when cross compiling, then some more logic needs to be added, e.g. like this: # when crosscompiling import the executable targets from a file IF(CMAKE_CROSSCOMPILING) SET(IMPORT_EXECUTABLES "IMPORTFILE-NOTFOUND" CACHE FILEPATH "Point it to the export file from a native build") INCLUDE(${IMPORT_EXECUTABLES})
ENDIF(CMAKE_CROSSCOMPILING)

...

# always build the executable
TARGET_LINK_LIBRARIES(mygenerator ${SOME_LIBS}) # but use different names for the command IF(CMAKE_CROSSCOMPILING) SET(mygenerator_EXE native-mygenerator) ELSE(CMAKE_CROSSCOMPILING) SET(mygenerator_EXE mygenerator) ENDIF(CMAKE_CROSSCOMPILING) # then use the target name as COMMAND, CMake >= 2.6 knows how to handle this ADD_CUSTOM_COMMAND(OUTPUT${CMAKE_CURRENT_BINARY_DIR}/generated.c
COMMAND ${mygenerator_EXE} foo.dat -o${CMAKE_CURRENT_BINARY_DIR}/generated.c
DEPENDS foo.dat )

...
# export the generator target to a file, so it can be imported (see above) by another build
# the IF() is not necessary, but makes the intention clearer
# use the NAMESPACE option of EXPORT() to get a different target name for mygenerator when exporting
IF(NOT CMAKE_CROSSCOMPILING)
EXPORT(TARGETS mygenerator FILE \${CMAKE_BINARY_DIR}/ImportExecutables.cmake NAMESPACE native- )
ENDIF(NOT CMAKE_CROSSCOMPILING)



## Cross compilation for Windows CE

Building for Windows CE requires Visual Studio 2005 or 2008 (No Express Edition!) with at least one installed SDK. If you don't have a specific installation file for your target device it is possible to use the Windows CE 5.0 Standard SDK from http://www.microsoft.com/downloads/details.aspx?familyid=fa1a3d66-3f61-4ddc-9510-ae450e2318c3.

CMake supports Windows CE out of the box since version 2.8.10 when used with the NMake Makefiles generator. To use it you first need the corresponding environment variables set, for which the CMake command env_vs8_wince has been added in the following version. Using 2.8.10 is possible too, if the environment is set manually. To get there start a command prompt and type the following commands:

"%VS80COMNTOOLS%vsvars32.bat"
cmake -E env_vs8_wince "STANDARDSDK_500 (ARMV4I)" > env.bat
env.bat


Then the Makefiles can be generated and built with the following commands:

cmake -G "NMake Makefiles" -DCMAKE_SYSTEM_NAME=WindowsCE -DCMAKE_SYSTEM_PROCESSOR=ARMV4I \path\to\source
cmake --build .


Starting with CMake 2.8.11 it is also possible to create Visual Studio solution for Windows CE targets. Depending on the installed SDKs CMake will accept additional generators. The following command will create Visual Studio 2005 files for the WinCE standard SDK:

cmake -G "Visual Studio 8 2005 STANDARDSDK_500 (ARMV4I)" \path\to\source


To use VS2008 instead of VS2005 simple replace "VS80COMNTOOLS" with "VS90COMNTOOLS", "vs8" with "vs9" and "8 2005" with "9 2008".

## Information how to set up various cross compiling toolchains

• [1], detailed instructions for using CMake for the iPhone (external, thirdparty website).
• eldk, embedded Linux cross compiling toolchain from Denx
• mingw - gcc for cross compiling from Linux to Windows
• SDCC - the small devices C compiler
• eCos - the embedded Configurable operating system
• ADSP - the Analog Devices toolchain for their DSPs
• IBM BlueGene/L
• Cray XT3 / Catamount
• Crosstool NG - may be used to easily build various cross compiler toolchain. The produced toolchain seems to work well with CMake cross-compiling.
• MXE - Builds compiler and libraries for cross compiling from Linux to Windows. Comes with CMake toolchain file!

## FAQ/Potential Problems

• On mixed 32/64 bit Linux installations cross compilation cannot be used to build for 32/64 bit only.
• FindXXX.cmake modules, which rely on executing a binary tool like pkg-config may have problems, since the pkg-config of the target platform cannot be executed on the host. Tools like pkg-config should be used only optional in FindXXX.cmake files.
• What about Scratchbox ? CMake should work without problems in scratchbox, then it will just work in native mode.
• Can it build software for PS3 ? If you build software for PS3, you build software for two architectures, for the PowerPC and for the cells. This is done using two different toolchains. Currently CMake doesn't support using multiple toolchains in one buildtree or building for multiple target architectures in one build tree. So building for PS3 doesn't work out-of-the-box. It should work using ADD_CUSTOM_COMMAND() or by using two buildtrees.