# Embedding Jullia, jl\_init(), julia\_init() and JULIA\_HOME

**URL:** <https://discourse.julialang.org/t/embedding-jullia-jl-init-julia-init-and-julia-home/4032>\
**Category:** General Usage\
**Tags:** embedding\
**Created:** [June 1, 2017, 12:45pm UTC](https://discourse.julialang.org/t/embedding-jullia-jl-init-julia-init-and-julia-home/4032 "2017-06-01T12:45:44Z")\
**Posts on this page:** 7\
**Page:** 1

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**Author:** ![oschulz](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/oschulz/32/2998_2.png) [@oschulz](https://discourse.julialang.org/u/oschulz)\
**Post date:** [June 1, 2017, 12:45pm UTC](https://discourse.julialang.org/t/embedding-jullia-jl-init-julia-init-and-julia-home/4032/1 "2017-06-01T12:45:44Z")

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I need a bit of help from the experts. I’m continuing Joosep Pata’s work on [ROOT.jl](https://github.com/JuliaHEP/ROOT.jl), a package that makes it possible to use [CERN ROOT](https://root.cern.ch/) from Julia (via Cxx.jl). Cxx.jl alone is not enough, however, because ROOT-6 brings it’s own LLVM-based C++ backend, and really doesn’t like to see Julia’s LLVM instance. Joosep managed to work around the problem by making ROOT.jl create a special Julia executable that first initializes ROOT and then starts Julia: [https://github.com/JuliaHEP/ROOT.jl/blob/master/deps/ui.cc](https://github.com/JuliaHEP/ROOT.jl/blob/master/deps/ui.cc) (for the history, see [this thread](https://groups.google.com/d/topic/julia-users/N0ErjdtlwZk/discussion).)

I’m trying to update this for Julia v0.6 - and it almost works as it is, but I ran into a problem with `JULIA_HOME`: If I use the default Julia executable, `JULIA_HOME` is set correctly (naturally). If I copy the standard Julia executable to a different path and set `JULIA_HOME` via the environment variable `$JULIA_HOME`, everything is fine too, of course. But if I run the Julia executable built from ROOT.jl’s “ui.cc”, then `JULIA_HOME` is set to  
“[…]/julia/usr/lib” instead of “[…]/julia/usr/bin” and `$JULIA_HOME` from the environment is ignored. As a result of the wrong value of `JULIA_HOME` (lib dir instead of bin dir), package pre-compilation fails when triggered from the ROOT-compatible Julia executable.

I’ve narrowed this down to the fact that ROOT.jl’s “ui.cc” uses

```
jl_init()

```

which calls

```
jl_init_with_image(libjldir, jl_get_default_sysimg_path())

```

(in [“repl.c”](https://github.com/JuliaLang/julia/blob/master/ui/repl.c)) which in turn sets `jl_options.julia_home` to `libjldir`. This is a bit confusing, actually, as the first argument of `jl_init_with_image` is actually named `julia_home_dir` - I don’t quite get why `libjldir` passed as the value. Then, when `jl_resolve_sysimg_location is run`, it won’t touch `jl_options.julia_home` because it’s already set.

The default Julia executable, however, uses

```
julia_init(jl_options.image_file_specified ? JL_IMAGE_CWD : JL_IMAGE_JULIA_HOME)

```

which bypasses `jl_init` and `jl_init_with_image`, so that `jl_resolve_sysimg_location` finds `jl_options.julia_home` unset and then sets it correctly.

So, here’s my two questions for the experts:

- Is the behavior of `jl_init` to ignore `JULIA_HOME` and set `jl_options.julia_home` to the lib instead of the bin directory intentional? Somehow this was different in v0.5.

- Should I use `jl_init` (according to the “Embedding Julia” section in the Julia docs) or `julia_init` (and if the latter, what do I need to link in to get access to `jl_options`)?

Any advice would be very much appreciated.

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**Author:** ![oschulz](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/oschulz/32/2998_2.png) [@oschulz](https://discourse.julialang.org/u/oschulz)\
**Post date:** [June 1, 2017, 2:50pm UTC](https://discourse.julialang.org/t/embedding-jullia-jl-init-julia-init-and-julia-home/4032/2 "2017-06-01T14:50:46Z")

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@yuyichao, you helped Joosep with the initial concept - could you give me a hint?

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**Author:** ![ihnorton](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/ihnorton/32/26_2.png) [@ihnorton](https://discourse.julialang.org/u/ihnorton)\
**Post date:** [June 1, 2017, 3:59pm UTC](https://discourse.julialang.org/t/embedding-jullia-jl-init-julia-init-and-julia-home/4032/3 "2017-06-01T15:59:55Z")

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> [@oschulz](#):
>
> Is the behavior of jl\_init to ignore JULIA\_HOME and set jl\_options.julia\_home to the lib instead of the bin directory intentional? Somehow this was different in v0.5.

In 0.6, `jl_init()` is supposed to resolve the system image location automatically relative to the location of `libjulia`, via `jl_get_default_sysimg_path()` uses, which is set in the build system by `JL_SYSTEM_IMAGE_PATH`.

> [@oschulz](#):
>
> Should I use jl\_init (according to the “Embedding Julia” section in the Julia docs)

If you want to use a different sysimg location, you need to use `jl_init_with_image()` (I believe this is mentioned in the docs).

> [@oschulz](#):
>
> I don’t quite get why libjldir passed as the value.

There’s a long history of confusion about what the meaning of “HOME” is, unfortunately… `libjldir` in this case is the _directory path_ of `libjulia` (look at the resolution code in `jl_init()`.

> [@oschulz](#):
>
> if the latter, what do I need to link in to get access to jl\_options

Hopefully it’s not necessary, but if you decided you need that for some reason, you could use `dlsym` to get the address of the global. Note that it’s not a public API so the layout could change, which should be fine because you are linking julia.h (presumably consistently with the libjulia you are opening…)

Related: [https://github.com/JuliaLang/julia/pull/21299](https://github.com/JuliaLang/julia/pull/21299)

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**Author:** ![oschulz](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/oschulz/32/2998_2.png) [@oschulz](https://discourse.julialang.org/u/oschulz)\
**Post date:** [June 1, 2017, 7:54pm UTC](https://discourse.julialang.org/t/embedding-jullia-jl-init-julia-init-and-julia-home/4032/4 "2017-06-01T19:54:39Z")

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@ihnorton, thanks for the quick anwer!

> In 0.6, jl\_init() is supposed to resolve the system image location automatically relative to the location of libjulia

But shouldn’t it still set `JULIA_HOME` the same way it’s normally set (to the bin directory)?

> There’s a long history of confusion about what the meaning of “HOME” is, unfortunately

Well, Julia package precompilation seems to have a very clear opinion on it, because it uses `JULIA_HOME` to construct the path to a Julia binary it wants to start … so, if JULIA\_HOME is set to a lib directory, Julia won’t work (at least packages precompilation won’t):

```julia
ERROR: could not spawn `[...]/julia/usr/lib/julia -Cnative -J[...]/julia/usr/lib/julia/sys.so --compile=yes --depwarn=yes -O0 --output-ji /user/.julia/lib/v0.6/Compat.ji --output-incremental=yes --startup-file=no --history-file=no --color=no --eval 'while !eof(STDIN)
    eval(Main, deserialize(STDIN))

```

Won’t that cause problems when embedding Julia, in general?

In any case, if I understand you correctly, I _should_ use `julia_init`, not `jl_init`? I’m just asking because the recently updates official embedding example does use `jl_init()`, so I’m a bit confused.

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**Author:** ![ihnorton](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/ihnorton/32/26_2.png) [@ihnorton](https://discourse.julialang.org/u/ihnorton)\
**Post date:** [June 2, 2017, 3:08pm UTC](https://discourse.julialang.org/t/embedding-jullia-jl-init-julia-init-and-julia-home/4032/5 "2017-06-02T15:08:40Z")

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Ok, I think I get it. This is kind of wonky because `JULIA_HOME` environment variable is the path of the julia executable – which may or may not exist in an embedding situation. Even if it can’t be assumed, we should probably try harder for the sake of precompilation.

I re-opened this related issue which has a nice reduced case already (easier than installing ROOT). Please comment there if needed:

[https://github.com/JuliaLang/julia/issues/14577#issuecomment-305814334](https://github.com/JuliaLang/julia/issues/14577#issuecomment-305814334)

(I still think you should use only `jl_init` or `jl_init_with_image` rather than `julia_init`, but we can hopefully do a better job by default for `jl_init`)

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**Author:** ![ExpandingMan](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/expandingman/32/866_2.png) [@ExpandingMan](https://discourse.julialang.org/u/ExpandingMan)\
**Post date:** [June 2, 2017, 3:37pm UTC](https://discourse.julialang.org/t/embedding-jullia-jl-init-julia-init-and-julia-home/4032/6 "2017-06-02T15:37:03Z")

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I just want to say that I’m really happy you are resuming work on ROOT.jl 😄.

I graduated in late 2015 and wound up working as a data scientist, having not discovered Julia until early 2016, and it saddens me deeply that I was not aware of Julia (and it was not in a mature state, as it sort of is now) when I was still doing physics. Sometimes I feel like all the wonderful things I’ve discovered about Julia are going to waste now that I’m _not_ doing physics. During most of the time, and at most of the places I was a student the attitude was typically “anything other than C++ or Fortran is too slow and/or is silly and/or is useless” (an attitude I was an exponent of, as there weren’t really any alternatives at the time), but, somewhat to my horror, by the time I left people were starting to use Python. (Though, at least the people I knew were actually using it _as a scripting language_, like it is supposed to be, unlike in the data science community where it somehow gets used for everything under the sun.) When I imagine reliving all my physics days with Julia I am filled with joy, nostalgia, and enthusiasm for better ways of doing things.

Sorry, I know that was a huge tangent from the subject of this post, I just wanted to share my enthusiasm about ROOT.jl. I would love to be of help some time if I have the time.

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**Author:** ![oschulz](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/oschulz/32/2998_2.png) [@oschulz](https://discourse.julialang.org/u/oschulz)\
**Post date:** [June 2, 2017, 4:11pm UTC](https://discourse.julialang.org/t/embedding-jullia-jl-init-julia-init-and-julia-home/4032/7 "2017-06-02T16:11:24Z")

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Thanks very much for looking into this, @ihnorton!

I have to admit that now that I’m using `julia_init`, I kinda like it for this special case, though - I want the root-enabled julia executable to be as close to the standard julia exe as possible, including honoring `$JULIA_HOME`, if set.