where is env_dir of heroku buildpack compile comes from?

Question

I've read the article buildpack-api, but still get confused.

The bin/compile of buildpack has 3 arguments, BUILD_DIR CACHE_DIR ENV_DIR, which is used in the build process.

But how can a user specify the 3 DIRs? Is it comes from herocu config var like this?

heroku config:set CACHE_DIR=cache

and user should mannually create a cache directory in the root directory of his project?

howto specify the BUILD_DIR when it is not the root of the project?

Answer 1

These 3 variables are controlled by Heroku and passed to the buildpack bin/compile script by Heroku, the user doesn't control or set them.

The buildpack should create the cache directory (if it is missing) in the place pointed by the second argument passed to the bin/compile script.