How to build cmake ExternalProject while configurating main one?

Question

It can be a pain to refrence ExternalProjects when their install targets are messed up. So one may want to build and install ExternalProjects once before generating main project files for given project. Is it possible with CMake and how to do it?

Answer 1

You may use cmake call within execute_process for configure and build CMake project, which contains ExternalProject:

other_project/CMakeLists.txt:

project(other_project)
include(ExternalProject)

ExternalProject_Add(<project_name> <options...>)

CMakeLists.txt:

# Configure external project
execute_process(
    COMMAND ${CMAKE_COMMAND} ${CMAKE_SOURCE_DIR}/other_project
    WORKING_DIRECTORY ${CMAKE_BINARY_DIR}/other_project
)

# Build external project
execute_process(
    COMMAND ${CMAKE_COMMAND} --build ${CMAKE_BINARY_DIR}/other_project
)

Such a way other_project will be configured and built in directory ${CMAKE_BINARY_DIR}/other_project. If you do not disable installation in ExternalProject_Add call, then it will performed when building other_project.

Normally, you want some options to ExternalProject, like SOURCE_DIR, BINARY_DIR, INSTALL_DIR, to be deduced from variables in the main project. You have two ways for achive that:

1. Create CMakeLists.txt for other_project with configure_file, called from main project (before execute_process command).

2. Pass variables from main project as -D parameters to ${CMAKE_COMMAND}.

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Having separated execute_process calls for sequential COMMANDS is important. Otherwise, if use single execute_process with several COMMANDS, these commands will be just "piped" (executed concurrently but with output of the first command being treated as input for the second).