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I have a custom command that is executed in the pre linking stage. What I want to do is calculate a checksum of all parts that are going to be part of my executable. Then I want to link link this checksum as a global variable into my executable.

So far I can calculate the checksum of all libraries that are going to be linked into my executable. But I also need the checksum of the already existing (because its already been built) object file of the exe. Is there a way to get access to this object file?

add_custom_command(OUTPUT ${CMAKE_CURRENT_BINARY_DIR}/checksum.cpp 
    PRE_LINK
    COMMAND ${CMAKE_COMMAND} -P "${CMAKE_CURRENT_BINARY_DIR}/checksum_generator.cmake" ${LIB_DEPENDENCIES} <object_file_of_executable_target>
    DEPENDS ${LIB_DEPENDENCIES}
)

add_library(checksum STATIC ${CMAKE_CURRENT_BINARY_DIR}/checksum.cpp)
target_link_libraries(executable_target PUBLIC checksum)

So my question is: How can I get the object file of the executable target?

Every time I try to access some generator expression of the executable_target and pass it, CMake crashes because of cyclic dependencies (which is true for the output executable, but not for the object files since they already exists in the pre link stage).

sfs
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1 Answers1

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Every time I try to access some generator expression of the executable_target and pass it, CMake crashes because of cyclic dependencies

I am assuming you've build your executable as an OBJECT library first, and then the actual executable after using it like so?

add_library(obj OBJECT ${...})

add_executable(exe $<TARGET_OBJECTS:obj>)

Have you tried the $<TARGET_OBJECTS:obj> generator expression in your custom command? If your checksum.cpp is only linked in the executable target (rather than the object library) then this should resolve your cyclic dependency.

Pesho_T
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  • Thanks, I didn't think about creating a extra object library. Still would prefer if it was not needed, but it works now. – sfs Nov 22 '19 at 06:59
  • Glad to have helped. Unfortunately (in your case) in CMake you are encouraged to work with targets and use them to express relationships, rather than files that get generated. Also, while under the hood there are the compiling and linking stages, I'm not aware of a way to separate them in CMake to allow what you're doing without the intermediate OBJECT library. – Pesho_T Nov 22 '19 at 08:13