# How to define a Julia function from C?

**URL:** <https://discourse.julialang.org/t/how-to-define-a-julia-function-from-c/2566>\
**Category:** General Usage\
**Tags:** embedding\
**Created:** [March 9, 2017, 4:14pm UTC](https://discourse.julialang.org/t/how-to-define-a-julia-function-from-c/2566 "2017-03-09T16:14:36Z")\
**Posts on this page:** 9\
**Page:** 1

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**Author:** ![izaid](https://avatars.discourse-cdn.com/v4/letter/i/ee7513/32.png) [@izaid](https://discourse.julialang.org/u/izaid)\
**Post date:** [March 9, 2017, 4:14pm UTC](https://discourse.julialang.org/t/how-to-define-a-julia-function-from-c/2566/1 "2017-03-09T16:14:36Z")

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Hi all,

I’m a bit new to the Julia community, coming more from the Python / NumPy space. I’ve been using Julia internally for a while, and have spun part of my work out into a open-source package I hope to announce soon.

I’m doing some cross-language stuff (notably Julia and JavaScript), so a fair amount of this work is in C. I’d like to be able to define a new Julia function in C itself. Is this possible? I haven’t found good examples of it online.

Cheers,

Irwin

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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:** [March 9, 2017, 4:37pm UTC](https://discourse.julialang.org/t/how-to-define-a-julia-function-from-c/2566/2 "2017-03-09T16:37:39Z")

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See the [embedding example](https://github.com/JuliaLang/julia/blob/9e6b5fd48e978d70518194733e4fc38f293b6655/examples/embedding.c#L76-L82) or search for `jl_eval_string` on StackOverflow.

If you mean define a C function that accepts/returns Julia values (`jl_value_t*`), then a large fraction of the code in `src/` can serve as an example… Just start grepping for `ccall(:jl_` in `base/`. (e.g.: `jl_parse_string`, `jl_new_array`, etc.)

You may also be interested in [node-julia](https://github.com/waTeim/node-julia). I think it’s unmaintained at the moment, but forking and updating it should be a lot less work than building everything from scratch.

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**Author:** ![izaid](https://avatars.discourse-cdn.com/v4/letter/i/ee7513/32.png) [@izaid](https://discourse.julialang.org/u/izaid)\
**Post date:** [March 9, 2017, 5:36pm UTC](https://discourse.julialang.org/t/how-to-define-a-julia-function-from-c/2566/3 "2017-03-09T17:36:44Z")

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Thanks! A search for `ccall(:jl` showed me exactly what I wanted, which was kind of obvious in hindsight – just a function with some jl\_value\_t args.

While ccall from Julia is great, is it possible to get the method defined as a global through the C API itself? In my case, I already have a C init function that needs to be called, so I may as well get my global defs set there. Basically, given an appropriate C function, is it possible to combine jl\_set\_global along with something like jl\_method\_def / jl\_generic\_function\_def / … (I have no idea what those do) to create a Julia function? Basically I’m after ccall in the C API.

As for node-julia, I’m very aware. It appears great, but also appears unmaintained as you mentioned – it didn’t work for me and I tried to fix it, but couldn’t. I just built new JS bindings from scratch, which may or may not be useful – I’ll announce when I feel they are ready.

Irwin

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**Author:** ![StefanKarpinski](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/stefankarpinski/32/24_2.png) [@StefanKarpinski](https://discourse.julialang.org/u/StefanKarpinski)\
**Post date:** [March 9, 2017, 6:08pm UTC](https://discourse.julialang.org/t/how-to-define-a-julia-function-from-c/2566/4 "2017-03-09T18:08:38Z")

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> [@izaid](#):
>
> Basically I’m after ccall in the C API.

I’m confused – isn’t the C API for ccall just calling a function in C?

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**Author:** ![yuyichao](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/yuyichao/32/20_2.png) [@yuyichao](https://discourse.julialang.org/u/yuyichao)\
**Post date:** [March 9, 2017, 6:16pm UTC](https://discourse.julialang.org/t/how-to-define-a-julia-function-from-c/2566/5 "2017-03-09T18:16:42Z")

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I think he wants to define the wrapper in C.

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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:** [March 9, 2017, 6:17pm UTC](https://discourse.julialang.org/t/how-to-define-a-julia-function-from-c/2566/6 "2017-03-09T18:17:29Z")

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> [@izaid](#):
>
> Basically I’m after ccall in the C API.

In principle you could do something like PyMethodDef, but it seems like a lot of unnecessary pain when you are carrying around a giant JIT anyway. If you really need that, the place to start is probably reading the section for `method_sym` in `interpreter.c`.

If you just need a C-callable pointer to a Julia function, then use `jl_function_ptr` (see the definition of `cfunction` in base.jl).

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<div class="post-metadata">

**Author:** ![izaid](https://avatars.discourse-cdn.com/v4/letter/i/ee7513/32.png) [@izaid](https://discourse.julialang.org/u/izaid)\
**Post date:** [March 9, 2017, 6:52pm UTC](https://discourse.julialang.org/t/how-to-define-a-julia-function-from-c/2566/7 "2017-03-09T18:52:56Z")

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yuyichao and ihnorton got it exactly – maybe it’s because I’m used to Python but, yeah, I was looking for the equivalent of PyMethodDef. interpreter.c is a good place to look, thanks for that – but I’ll see if I can do something with ccall.

Thanks!

Irwin

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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:** [March 9, 2017, 7:34pm UTC](https://discourse.julialang.org/t/how-to-define-a-julia-function-from-c/2566/8 "2017-03-09T19:34:36Z")

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[edit: as barche pointed out, there are a handful of C-defined `jl_f_` functions in builtins.c, for bootstrapping]

PyMethodDef exists in Python because it’s the only viable option, and similarly in other interpreted languages. (well, extension interop could be done with ctypes/libffi, but the marshaling overhead would probably be huge).

What I’m suggesting is basically:

```julia
jl_init(...)
...
jl_value_t* myfunc = jl_eval_string("myfunc(args) = ccall((:myfunc, :mylib), Any, (...), args)")

```

You can store that handle, call it with `jl_call[0/1...]` etc.

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<div class="post-metadata">

**Author:** ![barche](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/barche/32/79_2.png) [@barche](https://discourse.julialang.org/u/barche)\
**Post date:** [March 9, 2017, 7:49pm UTC](https://discourse.julialang.org/t/how-to-define-a-julia-function-from-c/2566/9 "2017-03-09T19:49:53Z")

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It seems the C API for defining functions is not as complete as the one for defining types using `jl_new_datatype`. I once tried applying what’s done in `add_builtin_func`, but the result was a lot less flexible than what can be done by defining a function with different methods in Julia.

CxxWrap.jl allows defining functions from C++, but it cheats by feeding the data (function pointer, return and argument types) back to Julia and then using metaprogramming to create a new Julia function that makes the ccall. In case this fits your workflow, see here for a simple example:  
C++:  
[https://github.com/JuliaInterop/CxxWrap.jl/blob/master/deps/src/examples/hello.cpp](https://github.com/JuliaInterop/CxxWrap.jl/blob/master/deps/src/examples/hello.cpp)

Julia:

[https://github.com/JuliaInterop/CxxWrap.jl/blob/master/test/hello.jl](https://github.com/JuliaInterop/CxxWrap.jl/blob/master/test/hello.jl)