Generated code dependency fails

Usage
1

Sample code is available in https://github.com/marcpawl/cmake_generated_source.git

I have two directories px and pxlib. px generates code, and pxlib consumes it.

For px I have:
cmake_minimum_required ( VERSION 3.12)
set(CMAKE_EXPORT_COMPILE_COMMANDS ON )
set(CMAKE_BUILD_WITH_INSTALL_RPATH ON )
add_custom_target(px_all ALL
DEPENDS /project/px/build /project/px/build/generated.cpp )
add_custom_command(
OUTPUT /project/px/build /project/px/build/generated.cpp
COMMAND /project/px/generator
MAIN_DEPENDENCY /project/px/generator
)

And for pxlib I have
cmake_minimum_required (VERSION 3.12)
set(CMAKE_EXPORT_COMPILE_COMMANDS ON)
set(CMAKE_BUILD_WITH_INSTALL_RPATH ON)
add_library( pxlib STATIC /project/px/build/generated.cpp)
set_target_properties(pxlib PROPERTIES CXX_STANDARD 17)
add_dependencies(pxlib px_all)

But I cannot generate the makefiles unless the generated code is present
cmake3 -G “Unix Makefiles” …

CMake Error at pxlib/CMakeLists.txt:4 (add_library):
Cannot find source file:

/project/px/build/generated.cpp

Tried extensions .c .C .c++ .cc .cpp .cxx .cu .m .M .mm .h .hh .h++ .hm
.hpp .hxx .in .txx

CMake Error at pxlib/CMakeLists.txt:4 (add_library):
No SOURCES given to target: pxlib

If I run the generator first to provide the C++ flie then I can build my targets. What am I missing to say that the generator script needs to be executed before compiling the library?

P.S. The absolute paths is because I am in the middle of transforming hand written Makefiles to CMakeLists.txt files, they will be made into relative paths in a further step.

2

Instead of a custom target use an OBJECT library and use those objects. As an alternative, put the custom command in the same directory as the static library.

3

That worked, thanks.

For others who have the problem, or more than likely myself in a few months, I have updated https://github.com/marcpawl/cmake_generated_source as a working demo.

px/CMakeLists.txt got
add_library(px_all OBJECT /project/px/build/generated.cpp )
add_custom_command(
OUTPUT /project/px/build /project/px/build/generated.cpp
COMMAND /project/px/generator
MAIN_DEPENDENCY /project/px/generator
)

and pxlib/CMakeLists.txt got
add_library( pxlib STATIC $<TARGET_OBJECTS:px_all>)

4

If you use a newer CMake version, you can link to the object library like any other library using target_link_libraries(). Also you should avoid the hard-coded paths and use the various variables for source and build directories instead.