45 lines
2.7 KiB
Markdown
45 lines
2.7 KiB
Markdown
# Running other programs
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## Running a command at configure time
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Running a command at configure time is relatively easy. Use [`execute_process`][execute_process] to run a process and access the results. It is generally a good idea to avoid hard coding a program path into your CMake; you can use `${CMAKE_COMMAND}`, `find_package(Git)`, or `find_program` to get access to a command to run. Use `RESULT_VARIABLE` to check the return code and `OUTPUT_VARIABLE` to get the output.
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Here is an example that updates all git submodules:
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```cmake
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find_package(Git QUIET)
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if(GIT_FOUND AND EXISTS "${PROJECT_SOURCE_DIR}/.git")
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execute_process(COMMAND ${GIT_EXECUTABLE} submodule update --init --recursive
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WORKING_DIRECTORY ${CMAKE_CURRENT_SOURCE_DIR}
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RESULT_VARIABLE GIT_SUBMOD_RESULT)
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if(NOT GIT_SUBMOD_RESULT EQUAL "0")
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message(FATAL_ERROR "git submodule update --init --recursive failed with ${GIT_SUBMOD_RESULT}, please checkout submodules")
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endif()
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endif()
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```
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## Running a command at build time
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Build time commands are a bit trickier. The main complication comes from the target system; when do you want your command to run? Does it produce an output that another target needs? With this in mind, here is an example that calls a Python script to generate a header file:
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```cmake
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find_package(PythonInterp REQUIRED)
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add_custom_command(OUTPUT "${CMAKE_CURRENT_BINARY_DIR}/include/Generated.hpp"
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COMMAND "${PYTHON_EXECUTABLE}" "${CMAKE_CURRENT_SOURCE_DIR}/scripts/GenerateHeader.py" --argument
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DEPENDS some_target)
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add_custom_target(generate_header ALL
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DEPENDS "${CMAKE_CURRENT_BINARY_DIR}/include/Generated.hpp")
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install(FILES ${CMAKE_CURRENT_BINARY_DIR}/include/Generated.hpp DESTINATION include)
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```
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Here, the generation happens after `some_target` is complete, and happens when you run make without a target (`ALL`). If you make this a dependency of another target with `add_dependencies`, you could avoid the `ALL` keyword. Or, you could require that a user explicitly builds the `generate_header` target when making.
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## Included common utilities
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A useful tool in writing CMake builds that work cross-platform is `cmake -E <mode>` (seen in CMake files as `${CMAKE_COMMAND} -E`). This mode allows CMake to do a variety of things without calling system tools explicitly, like `copy`, `make_directory`, and `remove`. It is mostly used for the build time commands. Note that the very useful `create_symlink` mode used to be Unix only, but was added for Windows in CMake 3.13. [See the docs](https://cmake.org/cmake/help/latest/manual/cmake.1.html#command-line-tool-mode).
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[execute_process]: https://cmake.org/cmake/help/latest/command/execute_process.html
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