Get CMake to declare a target phony

Question

I want to generate some compile time constants. The first answer to another question gets me quite close. From my CMakeLists.txt:

add_library(${PROJECT_NAME} STATIC ${CXX_SRCS} compile_time.hpp)
add_custom_command(OUTPUT compile_time.hpp
    COMMAND ${CMAKE_COMMAND} -P ${CMAKE_CURRENT_SOURCE_DIR}/compile_time.cmake)

This works in the sense that the first time I run make, it generates compile_time.hpp, so that the values of the variables are defined when I run make and not cmake. But compile_time.hpp is not remade when I rerun make or even cmake to redo the makefiles.

How can I make the target compile_time.cpp be marked as phony so that it is always remade? I tried

add_custom_target(compile_time.hpp)

to no effect.

Answer 1

add_custom_target creates a "phony" target: It has no output and is always built. For make some target depended from the "phony" one, use add_dependencies() call:

add_custom_target(compile_time
    COMMAND ${CMAKE_COMMAND} -P ${CMAKE_CURRENT_SOURCE_DIR}/compile_time.cmake
)
# Because we use *target-level* dependency, there is no needs in specifying
# header file for 'add_library()' call.
add_library(${PROJECT_NAME} STATIC ${CXX_SRCS})
add_dependencies(${PROJECT_NAME} compile_time)

Library's dependency from the header compile_time.h will be detected automatically by headers scanning. Because script compile_time.cmake updates this header unconditionally, the library will be rebuilt every time.