CMake add_custom_command not being run

Question

I'm trying to use add_custom_command to generate a file during the build. The command never seemed to be run, so I made this test file.

cmake_minimum_required( VERSION 2.6 )

add_custom_command(
  OUTPUT hello.txt
  COMMAND touch hello.txt
  DEPENDS hello.txt
)

I tried running:

cmake .  
make

And hello.txt was not generated. What have I done wrong?

Answer 1

The add_custom_target(run ALL ... solution will work for simple cases when you only have one target you're building, but breaks down when you have multiple top level targets, e.g. app and tests.

I ran into this same problem when I was trying to package up some test data files into an object file so my unit tests wouldn't depend on anything external. I solved it using add_custom_command and some additional dependency magic with set_property.

add_custom_command(
  OUTPUT testData.cpp
  COMMAND reswrap 
  ARGS    testData.src > testData.cpp
  DEPENDS testData.src 
)
set_property(SOURCE unit-tests.cpp APPEND PROPERTY OBJECT_DEPENDS testData.cpp)

add_executable(app main.cpp)
add_executable(tests unit-tests.cpp)

So now testData.cpp will generated before unit-tests.cpp is compiled, and any time testData.src changes. If the command you're calling is really slow you get the added bonus that when you build just the app target you won't have to wait around for that command (which only the tests executable needs) to finish.

It's not shown above, but careful application of ${PROJECT_BINARY_DIR}, ${PROJECT_SOURCE_DIR} and include_directories() will keep your source tree clean of generated files.