# ExternalProject Build Byproduct Path

**URL:** <https://discourse.cmake.org/t/externalproject-build-byproduct-path/775>\
**Category:** Code\
**Created:** [March 7, 2020, 7:22pm UTC](https://discourse.cmake.org/t/externalproject-build-byproduct-path/775 "2020-03-07T19:22:15Z")\
**Posts on this page:** 6\
**Page:** 1

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**Author:** ![dMaggot](https://discourse.cmake.org/user_avatar/discourse.cmake.org/dmaggot/32/419_2.png) [@dMaggot](https://discourse.cmake.org/u/dMaggot)\
**Post date:** [March 7, 2020, 7:22pm UTC](https://discourse.cmake.org/t/externalproject-build-byproduct-path/775/1 "2020-03-07T19:22:15Z")

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I am using the ExternalProject for two projects and in both cases I need to list build byproducts. The problem is that in both cases I need to list them relative to the build. Thinking about this, it makes little sense to have build byproducts that are _not_ relative to the build. On the other hand, getting the PREFIX property via ExternaProject\_Get\_Property can only be done after the external project is added, but I want to specify the build byproducts as I am adding the project so there is a chicken & egg problem. Why not make the build byproducts relative to the prefix by default?

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**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:** [March 9, 2020, 3:32pm UTC](https://discourse.cmake.org/t/externalproject-build-byproduct-path/775/2 "2020-03-09T15:32:05Z")

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Not all BYPRODUCTS are guaranteed to be in the install tree. For example, I know that sometimes the build tree of the ExternalProject gets used instead of the install tree at times. Wouldn’t it be better to instead just support `<PREFIX>` and `<BINARY_DIR>` expansions in the arguments (keeping relative paths as they are)?

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**Author:** ![dMaggot](https://discourse.cmake.org/user_avatar/discourse.cmake.org/dmaggot/32/419_2.png) [@dMaggot](https://discourse.cmake.org/u/dMaggot)\
**Post date:** [March 16, 2020, 9:42pm UTC](https://discourse.cmake.org/t/externalproject-build-byproduct-path/775/3 "2020-03-16T21:42:48Z")

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I know they are not guaranteed to be in the install (or the build) tree, I was just saying they should default to that. But your suggestion of expanding variables makes more sense yet it will ultimately not support what I would like to see. Currently I am doing

```
set(FOO_BYPRODUCTS foo bar)
list(TRANSFORM FOO_BYPRODUCTS PREPEND "<foo_path>")
ExternalProjectAdd(foo
...
BUILD_BYPRODUCTS "${FOO_BYPRODUCTS}")

add_executable(bar bar.cpp "${FOO_BYPRODUCTS}")

```

where I had to find `foo_path` manually by trial and error. Your suggestion, if I understand correctly, would still require me to use of `list`, but then I would also want those expanded names to be available at the `add_executable` call. With some sort of default, I could probably do this with

```
set(FOO_BYPRODUCTS foo bar)
ExternalProjectAdd(foo
...
BUILD_BYPRODUCTS "${FOO_BYPRODUCTS}")
ExternalProject_Get_Property(foo BUILD_BYPRODUCTS)
add_executable(bar bar.cpp "${BUILD_BYPRODUCTS}")

```

where the `BUILD_BYPRODUCTS` variable now has all the expanded names. What do you think?

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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:** [March 17, 2020, 1:17pm UTC](https://discourse.cmake.org/t/externalproject-build-byproduct-path/775/4 "2020-03-17T13:17:22Z")

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I think it likely that the `BUILD_BYPRODUCTS <foo_path>/foo <foo_path>/bar` and `ExternalProject_Get_Property()` together would work. The relative paths are kind of ambiguous: what are they relative to?

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**Author:** ![dMaggot](https://discourse.cmake.org/user_avatar/discourse.cmake.org/dmaggot/32/419_2.png) [@dMaggot](https://discourse.cmake.org/u/dMaggot)\
**Post date:** [April 1, 2020, 5:24pm UTC](https://discourse.cmake.org/t/externalproject-build-byproduct-path/775/5 "2020-04-01T17:24:54Z")

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The binary dir of the external project would be a sensible default, I think.

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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:** [April 1, 2020, 5:54pm UTC](https://discourse.cmake.org/t/externalproject-build-byproduct-path/775/6 "2020-04-01T17:54:35Z")

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Possibly. But `<INSTALL_DIR>` may be useful as well. Either way, I think this is ready for a write up as an issue (we have the “safe” default of requiring an absolute path for now if the default for relative is undecidable) since there’s a use case.