# Detect a variable changed between configure runs

**URL:** <https://discourse.cmake.org/t/detect-a-variable-changed-between-configure-runs/15264>\
**Category:** Code\
**Created:** [October 25, 2025, 11:13pm UTC](https://discourse.cmake.org/t/detect-a-variable-changed-between-configure-runs/15264 "2025-10-25T23:13:07Z")\
**Posts on this page:** 3\
**Page:** 1

<div class="post-metadata">

**Author:** ![risto](https://discourse.cmake.org/user_avatar/discourse.cmake.org/risto/32/5860_2.png) [@risto](https://discourse.cmake.org/u/risto)\
**Post date:** [October 25, 2025, 11:13pm UTC](https://discourse.cmake.org/t/detect-a-variable-changed-between-configure-runs/15264/1 "2025-10-25T23:13:07Z")

</div>

Hello,

I am trying to detect if a variable in the `CMakeLists.txt` was changed compared to the previous configure run.

At first I tried using the cache variables, but it didn’t work, then I tried writing it to a file like this:

```cmake
cmake_minimum_required(VERSION 3.25)
project(test_cache NONE)

set(NEW "Hello")

set(cache_file "${CMAKE_CURRENT_BINARY_DIR}/cache.txt")
if(NOT EXISTS "${cache_file}")
    file(WRITE "${cache_file}" "${NEW}")
else()
    file(READ "${cache_file}" OLD)
    message("OLD: ${OLD}")
    message("NEW: ${NEW}")
    if("${OLD}" STREQUAL "${NEW}")
        message("They are equal, nothing to do")
    else()
        message("They are NOT equal, overwritting")
        file(WRITE "${cache_file}" "${NEW}")
    endif()
endif()

```

The idea is that I can edit manually the `NEW` variable in the `CMakeLists.txt`, and in case the variable changed since the last configure I would execute a process at configure time.  
The goal is to not execute this process unless necessary, as it slows down the configure phase.

There might be other ways of doing this, but I am really confused why this doesn’t work?

When I run this code first time, nothing gets printed as expected.  
On the second run I get the following

```auto
OLD: Hello
NEW: Hello
They are equal, nothing to do

```

Now, I modify variable `set(NEW “World”)`

If I rerun the configure again I get:

```auto
OLD: WORLD
NEW: WORLD
They are equal, nothing to do

```

How is this even possible? I could be completely misunderstanding something.  
I would appreciate some help to clarify this.

Best regards,  
Risto.

---

<div class="post-metadata">

**Author:** ![risto](https://discourse.cmake.org/user_avatar/discourse.cmake.org/risto/32/5860_2.png) [@risto](https://discourse.cmake.org/u/risto)\
**Post date:** [October 25, 2025, 11:48pm UTC](https://discourse.cmake.org/t/detect-a-variable-changed-between-configure-runs/15264/2 "2025-10-25T23:48:35Z")

</div>

The weirdness goes further with this one.

I noticed on my qemu virtual machine, I get the behavior I would expect.  
I started playing with the original script, and if I generate a random string for NEW like this:

```cmake
# set(NEW "Hello")
string(RANDOM LENGTH 5 NEW)

```

This works perfectly.

Also if I have a sleep between the READ and WRITE, it also fixes the issue.

```cmake
    file(READ "${cache_file}" OLD)

    execute_process(COMMAND sleep 1) # This helps
    message("OLD: ${OLD}")
    message("NEW: ${NEW}")
    if("${OLD}" STREQUAL "${NEW}")
        message("They are equal, nothing to do")
    else()
        message("They are NOT equal, overwritting")
        file(WRITE "${cache_file}" "${NEW}")
    endif()
endif()

```

So it seems like the write comes before the read, how should I prevent that.  
I was getting the same behaviour with the CACHE variables.

---

<div class="post-metadata">

**Author:** ![risto](https://discourse.cmake.org/user_avatar/discourse.cmake.org/risto/32/5860_2.png) [@risto](https://discourse.cmake.org/u/risto)\
**Post date:** [October 26, 2025, 7:42am UTC](https://discourse.cmake.org/t/detect-a-variable-changed-between-configure-runs/15264/3 "2025-10-26T07:42:35Z")

</div>

I found the issue, it’s a bit embarrassing.  
There is no issue with CMake here or any kind of file buffering.

The problem was a LSP server running in my text editor.  
I had `cmake-language-server` running in the background, and it seems like it triggered reconfigure on every file save.

I might as well switch to `neocmake`at this point.