Define new variable in cmake configure script


Let suppose I have a file called and inside it I have:


message(MY_VAR: ${MY_VAR})

After doing configure_file(${CMAKE_SOURCE_DIR}/ ${CMAKE_CURRENT_BINARY_DIR}/myconf.cmake) I can open configured myconf.cmake and I can see that CMAKE_SOURCE_DIR is set to a source dir (as it was expected) while MY_VAR is empty (unexpected for me).

Thus I think I have some problems when defining new variables in cmake configuring scripts.
Is there a way to overcome this?

cmake 3.19.2

Did you call configure_file() before or after setting MY_VAR? You should set the variable first, then call configure_file().

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If I understand correctly, you want to do:

configure_file(${CMAKE_SOURCE_DIR}/ ${CMAKE_CURRENT_BINARY_DIR}/myconf.cmake @ONLY)



message(MY_VAR: ${MY_VAR})
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Actually I want to set MY_VAR inside configuring script.

I think this is the same that I did. I just tried and I get:

message(MY_VAR: )

MY_VAR is empty

Ah, I get it now. Yes, @fenrir’s suggestion is correct.

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Did you add the @ONLY argument to the configure_file invocation?

Apparently now you did :wink:

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Yeah, the @ONLY argument is important here. Otherwise, ${MY_VAR} gets replaced with the value of ${MY_VAR} from the calling script, which in this case is empty because it hasn’t been set.

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@fenrir, @kyle.edwards thank you!
You were right.

Without @ONLY argument in configure_file() cmake tries to replace MY_VAR with the value of MY_VAR in assumption that it was previously set in my project. But it wasnt in my case and it set to an empty (I didn’t think about that)

So I needed to add @ONLY argument and make difference between ${MY_VAR} and @MY_VAR@ in configuration script

Thank you very much!