# Parsing data efficiently (JSON or maybe something else?)

**URL:** <https://discourse.cmake.org/t/parsing-data-efficiently-json-or-maybe-something-else/14403>\
**Category:** Code\
**Created:** [July 21, 2025, 12:36pm UTC](https://discourse.cmake.org/t/parsing-data-efficiently-json-or-maybe-something-else/14403 "2025-07-21T12:36:05Z")\
**Posts on this page:** 3\
**Page:** 1

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**Author:** ![epistax](https://discourse.cmake.org/letter_avatar_proxy/v4/letter/e/eb9ed0/32.png) [@epistax](https://discourse.cmake.org/u/epistax)\
**Post date:** [July 21, 2025, 12:36pm UTC](https://discourse.cmake.org/t/parsing-data-efficiently-json-or-maybe-something-else/14403/1 "2025-07-21T12:36:05Z")

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We have a hybrid setup where we are introducing CMake piece by piece on top of an existing complete build environment. The first goal for us is introducing a tight TDD cycle. We’re making some headway using JSON to share some common data between the build systems, such as warning flags.

We have a (~2000 item) list of interfaces that are mocked as a part unit testing build. We’re using CMake’s json support to read the JSON list to a CMake list, iterate through it and create the custom commands and targets to generate the mocks. Using CMake’s profiler on one system showed the loop taking about 70% of the overall configuration time with 30% just parsing the JSON list.

The overall time is not a lot so we don’t need to change it, but I wanted to see if we were doing things in a reasonable fashion for CMake. We have our dependencies set up so if there is a change to a mocked interface, it gets re-mocked. If there is a change to the JSON list, configure re-runs.

Thanks!

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**Author:** ![vito.gamberini](https://discourse.cmake.org/user_avatar/discourse.cmake.org/vito.gamberini/32/4376_2.png) [@vito.gamberini](https://discourse.cmake.org/u/vito.gamberini)\
**Post date:** [July 21, 2025, 6:59pm UTC](https://discourse.cmake.org/t/parsing-data-efficiently-json-or-maybe-something-else/14403/2 "2025-07-21T18:59:19Z")

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Parsing JSON inside CMakeLang is about the worst case scenario for any pipeline. JSON is ubiquitous, so it’s a necessary feature, but any opportunity to avoid it should be leveraged.

If it became a problem, the answer is to not use CMake to do so. Write your own script in a “real” programming language to parse the JSON into `add_custom_command()/add_custom_target` calls, record them in a file, and `include()` that file from your CMake project.

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**Author:** ![epistax](https://discourse.cmake.org/letter_avatar_proxy/v4/letter/e/eb9ed0/32.png) [@epistax](https://discourse.cmake.org/u/epistax)\
**Post date:** [July 21, 2025, 7:07pm UTC](https://discourse.cmake.org/t/parsing-data-efficiently-json-or-maybe-something-else/14403/3 "2025-07-21T19:07:49Z")

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Thanks! This makes sense.