How to define a variable during build with cmake? - cmake

Question

I would like to define a CMake variable BUILD_TIME_VAR in CMakeLists.txt:
computed with a python script during build phase
I can then access its content with ${VAR}
In other words, the equivalent during build phase of:
execute_process(COMMAND bash -c "python $SCRIPT $FILE" OUTPUT_VARIABLE GEN_TIME_VAR)
The variable is then used to generate a file which is a dependency to make binaries.
The goal is to make the code more easy to read since otherwise, the computation occur several times.
Rather than calling n times, the python script, to compute BUILD_TIME_VAR, I would like to use the script once, to factor the code in this way:
if(expression_1)
add_custom_command(OUTPUT foo
COMMAND cmd_1(${BUILD_TIME_VAR}))
...
elseif(expression_2)
# elseif section.
add_custom_command(OUTPUT foo
COMMAND cmd_2(${BUILD_TIME_VAR}))
...
else(expression_n)
# else section.
add_custom_command(OUTPUT foo
COMMAND cmd_n(${BUILD_TIME_VAR}))
...
endif(expression)
add_custom_target(${BINARY} ALL
DEPENDS foo)
Thanks you for your help.

Answer 1

If I understand your question correctly, you're effectively trying to create a "build-time variable." That is something that the build tool (actually all the build tools supported by CMake) would have to support. I know of no such functionality in build tools (make, ninja, VS, ...), and hence of no support for such a thing in CMake either.
You could emulate this by writing the results to a file and reading that file in all subsequent build steps using it.

Answer 2

I believe you've misinterpreted the role of CMake. In short, CMake generates build files for the back-end system of your choice (Make, ninja, etc.). This is concisely reviewed here, where the configure/generate step is briefly documented along with how it precedes the actual build step.
It's not clear what you're trying to achieve exactly, but maybe defining custom commands and using their outputs to link them to custom targets for further dependency chaining (see DEPENDS here) might be helpful, e.g.
cmake_minimum_required(VERSION 3.11)
project(foobar)
set(FOO_FILES "foo.txt")
add_custom_command(OUTPUT ${FOO_FILES}
COMMAND ${CMAKE_COMMAND} -E touch ${FOO_FILES}
COMMAND echo hello >> ${FOO_FILES}
WORKING_DIRECTORY "${CMAKE_CURRENT_BINARY_DIR}"
COMMENT "Creating ${FOO_FILES}"
VERBATIM)
add_custom_target(foo DEPENDS ${FOO_FILES})
set(BAR_FILES "bar_dummy.txt") # this is only use to link the custom command to the corresponding custom target
add_custom_command(OUTPUT ${BAR_FILES}
COMMAND ${CMAKE_COMMAND} -E touch ${BAR_FILES}
COMMAND cat ${FOO_FILES}
WORKING_DIRECTORY "${CMAKE_CURRENT_BINARY_DIR}"
COMMENT "Displaying ${FOO_FILES}"
VERBATIM)
add_custom_target(bar DEPENDS ${BAR_FILES})
add_dependencies(bar foo)
However, you might need to consider the probability of going against the tool, and thus, it might be clearer to have a required step prior to the configuration of your project.