# CPack External generator runs CPACK\_EXTERNAL\_PACKAGE\_SCRIPT before creating JSON metadata
**URL:** https://discourse.cmake.org/t/cpack-external-generator-runs-cpack-external-package-script-before-creating-json-metadata/1773
**Category:** Usage
**Created:** [September 2, 2020, 3:04pm UTC](https://discourse.cmake.org/t/cpack-external-generator-runs-cpack-external-package-script-before-creating-json-metadata/1773 "2020-09-02T15:04:39Z")
**Posts on this page:** 6
**Page:** 1
### Author:  [@cheesy\_falafel](https://discourse.cmake.org/u/cheesy_falafel)
#### Post date: [September 2, 2020, 3:04pm UTC](https://discourse.cmake.org/t/cpack-external-generator-runs-cpack-external-package-script-before-creating-json-metadata/1773/1 "2020-09-02T15:04:40Z")
Hi.
I’m having some trouble understanding how the CPack external generator is supposed to work. I’m running CPack via the CMake “package” target. In CMake I’m setting `CPACK_EXTERNAL_PACKAGE_SCRIPT` to point to a CMake script that invokes an external tool using `execute_process`. I want the external process to read the metadata from the CPack generated JSON file and use it to package my library. However, when I run a clean build, the external script runs before the JSON metadata is created causing my packaging step to fail. Is there a way to instruct CPack to generate the JSON metadata before my external script is executed?
Thanks.
---
### Author:  [@ben.boeckel](https://discourse.cmake.org/u/ben.boeckel)
#### Post date: [November 15, 2020, 2:45pm UTC](https://discourse.cmake.org/t/cpack-external-generator-runs-cpack-external-package-script-before-creating-json-metadata/1773/2 "2020-11-15T14:45:34Z")
It looks like the `package.json` is written out before the script is executed. Could you get a trace of why the `package.json` is disappearing perhaps?
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### Author:  [@t1-nva](https://discourse.cmake.org/u/t1-nva)
#### Post date: [July 4, 2024, 9:58am UTC](https://discourse.cmake.org/t/cpack-external-generator-runs-cpack-external-package-script-before-creating-json-metadata/1773/3 "2024-07-04T09:58:15Z")
A long time has passed since the OP, but I’m seeing the same problem, when running the `CPACK_EXTERNAL_PACKAGE_SCRIPT` the JSON metadata file is present, but with partial contents. Running the same script later/manually works without problems i.e. after cpack has finished. I also tried just copying the JSON metadata file to another location at that point and it has partial content at that time.
Could there be some race condition/file flush issue making the script run before the JSON metadata file is fully completed and/or flushed to disk?
---
### Author:  [@Zitrax](https://discourse.cmake.org/u/Zitrax)
#### Post date: [July 18, 2024, 8:56am UTC](https://discourse.cmake.org/t/cpack-external-generator-runs-cpack-external-package-script-before-creating-json-metadata/1773/4 "2024-07-18T08:56:11Z")
We see the same issue here. @ben.boeckel , it’s not that the file does not exist, it does but as @t1-nva mentions it is not complete, like it has not been fully flushed.
In our case it just ends like so:
```auto
⋮
"tests" :
{
"archiveFile" : "",
"dependencies" : [],
"description" : "",
"displayName" : "Unit Tests",
"group" : "Development",
"installationTypes" : [],
"isDisabledByDefault" : true,
"isDownloaded" : false,
"isHidden" : true,
"isRequired" : false,
"name" : "tests"
}
},
"errorOnAbsoluteInstallDes
```
so it ends right in the middle of a word and is thus not even valid json.
---
### Author:  [@ben.boeckel](https://discourse.cmake.org/u/ben.boeckel)
#### Post date: [July 18, 2024, 11:53am UTC](https://discourse.cmake.org/t/cpack-external-generator-runs-cpack-external-package-script-before-creating-json-metadata/1773/5 "2024-07-18T11:53:24Z")
Ah, that makes the problem a lot clearer, thanks for the extra details. Can you test this patch: [https://gitlab.kitware.com/cmake/cmake/-/merge\_requests/9669](https://gitlab.kitware.com/cmake/cmake/-/merge_requests/9669)
---
### Author:  [@t1-nva](https://discourse.cmake.org/u/t1-nva)
#### Post date: [August 30, 2024, 8:21pm UTC](https://discourse.cmake.org/t/cpack-external-generator-runs-cpack-external-package-script-before-creating-json-metadata/1773/6 "2024-08-30T20:21:54Z")
In CMake 3.30.2 this issue has been fixed and `CMAKE_EXTERNAL_PACKAGE_SCRIPT` is working as expected!
Thanks a lot!