[SOLVED] Cache result of execute_process in CMake

Issue

As part of the configuration process of my project, I need to query a command line tool with execute_process to retrieve a variable required by the rest of the flow. The input to this query depends on a configuration option.

The particular command invoked by execute_process happens to be fairly expensive, so I would like to avoid rerunning it unless the input has changed.

How do I "cache" the result of execute_process so it's only rerun if a certain CMake option changes (i.e., instead of whenever anything changes that would trigger CMake to run again)?

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Example code:

set(MY_INPUT_VARIABLE "my_default" CACHE STRING "Used by command line tool.")

# This should only be re-run if MY_INPUT_VARIABLE changed since last time it was run
execute_process(COMMAND slow_executable ${MY_INPUT_VARIABLE} OUTPUT_VARIABLE RESULT)

Solution

Copy the value to a INTERNAL cache variable and compare it with the current version of the cache variable.

set(MY_INPUT_VARIABLE "my_default" CACHE STRING "Used by command line tool.")
set(MY_INPUT_VARIABLE_INTERNAL "" CACHE INTERNAL "for internal use only; do not modify")

if (NOT MY_INPUT_VARIABLE STREQUAL MY_INPUT_VARIABLE_INTERNAL)
    execute_process(COMMAND slow_executable ${MY_INPUT_VARIABLE} OUTPUT_VARIABLE RESULT)
    set(MY_INPUT_VARIABLE_INTERNAL ${MY_INPUT_VARIABLE} CACHE INTERNAL "for internal use only; do not modify")
endif()

Note that you could also use a different kind of cache variable, but you'd need to use FORCE to overwrite the value. In addition to this such a cache variable would be displayed by cmake-gui.

Answered By - fabian