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_requir         


        

Answer 1

The problem with two existing answers is that they either make the dependency global (add_custom_target(name ALL ...)), or they assign it to a specific, single file (set_property(...)) which gets obnoxious if you have many files that need it as a dependency. Instead what we want is a target that we can make a dependency of another target.

The way to do this is to use add_custom_command to define the rule, and then add_custom_target to define a new target based on that rule. Then you can add that target as a dependency of another target via add_dependencies.

# this defines the build rule for some_file
add_custom_command(
  OUTPUT some_file
  COMMAND ...
)
# create a target that includes some_file, this gives us a name that we can use later
add_custom_target(
  some_target
  DEPENDS some_file
)
# then let's suppose we're creating a library
add_library(some_library some_other_file.c)
# we can add the target as a dependency, and it will affect only this library
add_dependencies(some_library some_target)

The advantages of this approach:

• some_target is not a dependency for ALL, which means you only build it when it's required by a specific target. (Whereas add_custom_target(name ALL ...) would build it unconditionally for all targets.)
• Because some_target is a dependency for the library as a whole, it will get built before all of the files in that library. That means that if there are many files in the library, we don't have to do set_property on every single one of them.
• If we add DEPENDS to add_custom_command then it will only get rebuilt when its inputs change. (Compare this to the approach that uses add_custom_target(name ALL ...) where the command gets run on every build regardless of whether it needs to or not.)

For more information on why things work this way, see this blog post: https://samthursfield.wordpress.com/2015/11/21/cmake-dependencies-between-targets-and-files-and-custom-commands/