# Append variable to \<BUILD\_DIR\> in ExternalProject\_Add command

**URL:** <https://discourse.cmake.org/t/append-variable-to-build-dir-in-externalproject-add-command/2098>\
**Category:** Code\
**Created:** [October 30, 2020, 11:13pm UTC](https://discourse.cmake.org/t/append-variable-to-build-dir-in-externalproject-add-command/2098 "2020-10-30T23:13:11Z")\
**Posts on this page:** 4\
**Page:** 1

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**Author:** ![tomhh](https://discourse.cmake.org/user_avatar/discourse.cmake.org/tomhh/32/493_2.png) [@tomhh](https://discourse.cmake.org/u/tomhh)\
**Post date:** [October 30, 2020, 11:13pm UTC](https://discourse.cmake.org/t/append-variable-to-build-dir-in-externalproject-add-command/2098/1 "2020-10-30T23:13:11Z")

</div>

I have an ExternalProject\_Add call that looks like this…

```auto
ExternalProject_Add(
    SDL2 
    URL https://www.libsdl.org/release/SDL2-2.0.12.tar.gz
    URL_HASH MD5=783b6f2df8ff02b19bb5ce492b99c8ff
    INSTALL_DIR ${CMAKE_CURRENT_BINARY_DIR}
    BINARY_DIR <BINARY_DIR>/${CMAKE_BUILD_TYPE}
    CMAKE_ARGS -D CMAKE_INSTALL_PREFIX=<INSTALL_DIR>
               -D CMAKE_BUILD_TYPE=${CMAKE_BUILD_TYPE})

```

The problem line is `BINARY_DIR <BINARY_DIR>/${CMAKE_BUILD_TYPE}` - This does not seem to expand to what I’d expect. Essentially all I’d like to do is add either `Debug` or `Release` to the default `<BINARY_DIR>` path (which in this case is `SDL2-prefix/src/SDL2-build`).

Is there a way to do this? I would really like for this to work and to not have to manually replicate the path with `${CMAKE_CURRENT_BINARY_DIR}/SDL2-prefix/src/SDL2-build/${CMAKE_BUILD_TYPE}` hardcoded.

Any pointers on how to achieve this or a better idiomatic way for building both Debug and Release configurations using `ExternalProject_Add` please do let me know! 🙂

Thank you very much for your time,

Tom

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<div class="post-metadata">

**Author:** ![ben.boeckel](https://discourse.cmake.org/letter_avatar_proxy/v4/letter/b/ea5d25/32.png) [@ben.boeckel](https://discourse.cmake.org/u/ben.boeckel)\
**Post date:** [October 31, 2020, 1:51am UTC](https://discourse.cmake.org/t/append-variable-to-build-dir-in-externalproject-add-command/2098/2 "2020-10-31T01:51:03Z")

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I think we’d need something like `<DEFAULT_BINARY_DIR>` for such use cases.

---

<div class="post-metadata">

**Author:** ![tomhh](https://discourse.cmake.org/user_avatar/discourse.cmake.org/tomhh/32/493_2.png) [@tomhh](https://discourse.cmake.org/u/tomhh)\
**Post date:** [October 31, 2020, 9:19am UTC](https://discourse.cmake.org/t/append-variable-to-build-dir-in-externalproject-add-command/2098/3 "2020-10-31T09:19:08Z")

</div>

Thanks for getting back to me Ben,

I see, and I take it that currently doesn’t exist? It seems like it would be really useful in certain contexts and follow a similar pattern to other CMake variables.

I might also just be taking the wrong approach and there’s a better way to build multiple configurations using ExternalProject\_Add.

The reason I was trying to do it this way instead of creating a separate build folder would be the library would need to be downloaded twice which seemed redundant/wasteful and if I don’t make a separate configuration folder, then if you build Debug, then Release and then Debug again, the Debug build will start over again which is sometimes a pain as it’ll have been stomped by Release.

I can just fallback to a custom override like `${CURRENT_CMAKE_BINARY_DIR}/SDL2/Debug`, but reusing \<BINARY\_DIR\> felt cleaner.

Thanks!

Tom

---

<div class="post-metadata">

**Author:** ![tomhh](https://discourse.cmake.org/user_avatar/discourse.cmake.org/tomhh/32/493_2.png) [@tomhh](https://discourse.cmake.org/u/tomhh)\
**Post date:** [October 31, 2020, 10:45pm UTC](https://discourse.cmake.org/t/append-variable-to-build-dir-in-externalproject-add-command/2098/4 "2020-10-31T22:45:52Z")

</div>

I’ve been experimenting with getting a reliable cross-platform `ExternalProject_Add` command to work and have now got something that looks like this…

```auto
ExternalProject_Add(
    SDL2
    URL https://www.libsdl.org/release/SDL2-2.0.12.tar.gz
    URL_HASH MD5=783b6f2df8ff02b19bb5ce492b99c8ff
    CMAKE_GENERATOR Ninja
    BINARY_DIR ${CMAKE_CURRENT_BINARY_DIR}/SDL2/build/${CMAKE_BUILD_TYPE}
    INSTALL_DIR ${CMAKE_CURRENT_BINARY_DIR}
    CMAKE_ARGS -D CMAKE_INSTALL_PREFIX=<INSTALL_DIR>
               -D CMAKE_BUILD_TYPE=${CMAKE_BUILD_TYPE})

```

The reason for setting the `CMAKE_GENERATOR` explicitly to `Ninja` is so I can get consistent behaviour across Windows and macOS (the differences between single and multi-config generators I’ve found complicated things…).

It does feel like there should be an easier way to do this though, ideally all I would like to write is this:

```auto
ExternalProject_Add(
    SDL2
    URL https://www.libsdl.org/release/SDL2-2.0.12.tar.gz
    URL_HASH MD5=783b6f2df8ff02b19bb5ce492b99c8ff
    INSTALL_DIR ${CMAKE_CURRENT_BINARY_DIR}
    CMAKE_ARGS -D CMAKE_INSTALL_PREFIX=<INSTALL_DIR>)

```

And then externally build both Debug and Release and ideally have the build artefacts wind up in separate folders. It feels like this must be a common pattern, I just feel like I must be missing something.

Any suggestions on how to improve this solution would be greatly appreciated,

Thanks!

Tom