Crate jlrs[−][src]
Expand description
jlrs provides access to most of the Julia C API, it can be used to embed Julia in Rust
applications and to use functionality from the Julia C API when writing ccall
able functions.
Currently this crate is only tested on Linux in combination with Julia 1.6 and is not
compatible with earlier versions of Julia.
The documentation assumes you have at least a basic understanding of Julia’s type system.
Features
An incomplete list of features that are currently supported by jlrs:
- Access arbitrary Julia modules and their contents.
- Call Julia functions, including functions that take keyword arguments.
- Include and call your own Julia code.
- Load a custom system image.
- Create values that Julia can use, and convert them back to Rust, from Rust.
- Access the type information and fields of values.
- Create and use n-dimensional arrays.
- Support for mapping Julia structs to Rust structs that can be generated by JlrsReflect.jl.
- Structs that can be mapped to Rust include those with type parameters and bits unions.
- Offload long-running functions to another thread and
.await
the result with the (experimental) async runtime.
Generating the bindings
This crate depends on jl-sys which contains the raw bindings to the Julia C API, these are generated by bindgen. You can find the requirements for using bindgen in their User Guide.
Linux
The recommended way to install Julia is to download the binaries from the official website,
which is distributed in an archive containing a directory called julia-x.y.z
. This directory
contains several other directories, including a bin
directory containing the julia
executable.
In order to ensure the julia.h
header file can be found, either /usr/include/julia/julia.h
must exist, or you have to set the JULIA_DIR
environment variable to /path/to/julia-x.y.z
.
This environment variable can be used to override the default. Similarly, in order to load
libjulia.so
you must add /path/to/julia-x.y.z/lib
to the LD_LIBRARY_PATH
environment
variable.
Windows
If you want to use jlrs on Windows you must use WSL. An installation guide to install WSL on Windows can be found on Microsoft’s website. After installing a Linux distribution, follow the installation instructions for Linux.
Using this crate
The first thing you should do is use
the prelude
-module with an asterisk, this will
bring all the structs and traits you’re likely to need into scope. When embedding Julia, it
must be initialized before you can use it. You can do this by calling Julia::init
which
returns an instance of Julia
. Note that this method can only be called once while the
application is running; if you drop it you won’t be able to create a new instance but have to
restart the application. If you want to use a custom system image, you must call
Julia::init_with_image
instead of Julia::init
. If you’re calling Rust from Julia
everything has already been initialized, you can use CCall
instead.
Calling Julia from Rust
After initialization you have an instance of Julia
, Julia::include
can be used to
include files with custom Julia code. In order to call Julia functions and create new values
that can be used by these functions, Julia::scope
and Julia::scope_with_slots
must be
used. These two methods take a closure with two arguments, a Global
and a mutable
reference to a GcFrame
. Global
is a token that is used to access Julia modules, their
contents and other global values, while GcFrame
is used to root local values. Rooting a
value in a frame prevents it from being freed by the garbage collector until that frame has
been dropped. The frame is created when Julia::scope(_with_slots)
is called and dropped
when that method returns.
Because you can use both a Global
and a mutable reference to a GcFrame
inside the closure,
it’s possible to access the contents of modules and create new values that can be used by
Julia. The methods of Module
let you access the contents of arbitrary modules, several
methods are available to create new values.
The simplest is to call Value::eval_string
, a method that takes two arguments. The first
must implement the Scope
trait, the second is a string which has to contain valid Julia
code. The most important thing to know about the Scope
trait for now is that it’s used
by functions that create new values to ensure the result is rooted. Mutable references to
GcFrame
s implement Scope
, in this case the Value
that is returned is rooted in
that frame, so the result is protected from garbage collection until the frame is dropped when
that scope ends.
In practice, Value::eval_string
is relatively limited. It can be used to evaluate simple
function calls like sqrt(2.0)
, but can’t take any arguments. Its most important use-case is
importing installed packages by evaluating an import
or using
statement. A more
interesting method, Value::new
, can be used with data of any type that implements
IntoJulia
. This trait is implemented by primitive types like i8
and char
. Any type
that implements IntoJulia
also implements Unbox
which is used to extract the contents
of a Julia value.
In addition to evaluating raw commands with Value::eval_string
, it’s possible to call
anything that implements Call
as a Julia function, Value
implements this trait because
any Julia value is potentially callable as a function. Functions can be called with any number
of positional arguments and be provided with keyword arguments. Both Value::eval_string
and
the trait methods of Call
are all unsafe. It’s trivial to write a function like
boom() = unsafe_load(Ptr{Float64}(C_NULL))
, which causes a segfault when it’s called, and
call it with these methods.
As a simple example, let’s convert two numbers to Julia values and add them:
use jlrs::prelude::*; // Initializing Julia is unsafe because it can race with another crate that does // the same. let mut julia = unsafe { Julia::init().unwrap() }; let res = julia.scope(|global, frame| { // Create the two arguments. Note that the first argument, something that // implements Scope, is taken by value and mutable references don't implement // Copy, so it's necessary to mutably reborrow the frame. let i = Value::new(&mut *frame, 2u64)?; let j = Value::new(&mut *frame, 1u32)?; // The `+` function can be found in the base module. let func = Module::base(global).function(&mut *frame, "+")?; // Call the function and unbox the result as a `u64`. The result of the function // call is a nested `Result`; the outer error doesn't contain to any Julia // data, while the inner error contains the exception if one is thrown. Here we // explicitly convert the exception to an error that is compatible with `?`. unsafe { func.call2(&mut *frame, i, j)? .into_jlrs_result()? .unbox::<u64>() } }).unwrap(); assert_eq!(res, 3);
Many more features are available, including creating and accessing n-dimensional Julia arrays
and nesting scopes. To learn how to use them, please see the documentation for the memory
and wrappers
modules.
Calling Rust from Julia
Julia’s ccall
interface can be used to call extern "C"
functions defined in Rust, for most
use-cases you shouldn’t need jlrs. There are two major ways to use ccall
, with a pointer to
the function or a (:function, "library")
pair.
A function can be cast to a void pointer and converted to a Value
:
// This function will be provided to Julia as a pointer, so its name can be mangled. unsafe extern "C" fn call_me(arg: bool) -> isize { if arg { 1 } else { -1 } } let mut julia = unsafe { Julia::init().unwrap() }; julia.scope(|global, frame| unsafe { // Cast the function to a void pointer let call_me_val = Value::new(&mut *frame, call_me as *mut std::ffi::c_void)?; // Value::eval_string can be used to create new functions. let func = Value::eval_string( &mut *frame, "myfunc(callme::Ptr{Cvoid})::Int = ccall(callme, Int, (Bool,), true)" )?.unwrap(); // Call the function and unbox the result. let output = func.call1(&mut *frame, call_me_val)? .into_jlrs_result()? .unbox::<isize>()?; assert_eq!(output, 1); Ok(()) }).unwrap();
You can also use functions defined in dylib
and cdylib
libraries. In order to create such
a library you need to add
[lib]
crate-type = ["dylib"]
or
[lib]
crate-type = ["cdylib"]
respectively to your crate’s Cargo.toml
. Use a dylib
if you want to use the crate in other
Rust crates, but if it’s only intended to be called through ccall
a cdylib
is the better
choice. On Linux, compiling such a crate will be compiled to lib<crate_name>.so
.
The functions you want to use with ccall
must be both extern "C"
functions to ensure the C
ABI is used, and annotated with #[no_mangle]
to prevent name mangling. Julia can find
libraries in directories that are either on the default library search path or included by
setting the LD_LIBRARY_PATH
environment variable on Linux. If the compiled library is not
directly visible to Julia, you can open it with Libdl.dlopen
and acquire function pointers
with Libdl.dlsym
. These pointers can be called the same way as the pointer in the previous
example.
If the library is visible to Julia you can access it with the library name. If call_me
is
defined in a crate called foo
, the following should work if the function is annotated with
#[no_mangle]
:
ccall((:call_me, "libfoo"), Int, (Bool,), false)
One important aspect of calling Rust from other languages in general is that panicking across
an FFI boundary is undefined behaviour. If you’re not sure your code will never panic, wrap it
with std::panic::catch_unwind
.
Most features provided by jlrs including accessing modules, calling functions, and borrowing
array data require a Global
or a frame. You can access these by creating a CCall
first. Another method provided by CCall
is CCall::uv_async_send
, this method can be
used in combination with Base.AsyncCondition
. In particular, it lets you write a ccall
able
function that does its actual work on another thread, return early and wait
on the async
condition, which happens when CCall::uv_async_send
is called when that work is finished.
The advantage of this is that the long-running function will not block the Julia runtime,
There’s an example available on GitHub that shows how to do this.
Async runtime
The experimental async runtime runs Julia in a separate thread and allows multiple tasks to
run in parallel. This works by offloading functions to a new thread in Julia and waiting for
them to complete without blocking the runtime. To use this feature you must enable the async
feature flag:
[dependencies]
jlrs = { version = "0.11", features = ["async"] }
The struct AsyncJulia
is exported by the prelude and lets you initialize the runtime in
two ways, either as a task or as a thread. The first way should be used if you want to
integrate the async runtime into a larger project that uses async_std
. In order for the
runtime to work correctly the JULIA_NUM_THREADS
environment variable must be set to a value
larger than 2.
In order to call Julia with the async runtime you must implement the AsyncTask
trait. The
run
-method of this trait is similar to the closures that are used in the example
above for the sync runtime; it provides you with a Global
and an AsyncGcFrame
which
provides mostly the same functionality as GcFrame
. The AsyncGcFrame
is required to
call CallAsync::call_async
which calls a Julia function on another thread by using
Base.Threads.@spawn
and returns a Future
. While awaiting the result the runtime can handle
another task. If you don’t use CallAsync::call_async
tasks are executed sequentially.
It’s important to keep in mind that allocating memory in Julia uses a lock, so if you execute
multiple functions at the same time that allocate new values frequently the performance will
drop significantly. The garbage collector can only run when all threads have reached a
safepoint, which is the case whenever a function needs to allocate memory. If your function
takes a long time to complete but needs to allocate rarely, you should periodically call
GC.safepoint
in Julia to ensure the garbage collector can run.
You can find basic examples in the examples directory of the repo.
Testing
The restriction that Julia can be initialized once must be taken into account when running
tests that use jlrs
. The recommended approach is to create a thread-local static RefCell
:
use jlrs::prelude::*; use std::cell::RefCell; thread_local! { pub static JULIA: RefCell<Julia> = { let julia = RefCell::new(unsafe { Julia::init().unwrap() }); julia.borrow_mut().scope(|_global, _frame| { /* include everything you need to use */ Ok(()) }).unwrap(); julia }; }
Tests that use this construct can only use one thread for testing, so you must use
cargo test -- --test-threads=1
, otherwise the code above will panic when a test
tries to call Julia::init
a second time from another thread.
If these tests also involve the async runtime, the JULIA_NUM_THREADS
environment
variable must be set to a value larger than 2.
If you want to run jlrs’s tests, both these requirements must be taken into account:
JULIA_NUM_THREADS=3 cargo test -- --test-threads=1
Custom types
In order to map a struct in Rust to one in Julia you can derive ValidLayout
, Unbox
,
and Typecheck
. If the struct in Julia has no type parameters and is a bits type you can
also derive IntoJulia
, which lets you use the type in combination with Value::new
.
You should not implement these structs manually. The JlrsReflect.jl package can generate the correct Rust struct for types that have no tuple or union fields with type parameters. The reason for this restriction is that the layout of tuple and union fields can be very different depending on these parameters in a way that can’t be expressed in Rust.
These custom types can also be used when you call Rust from Julia with ccall
.
Modules
Traits for converting data.
Everything related to errors.
Traits for checking layout compatibility and enforcing layout requirements.
Structs and traits to protect data from being garbage collected.
Reexports structs and traits you’re likely to need.
Wrapper types for Julia data
Macros
Create a new named tuple. You will need a named tuple to call functions with keyword arguments.
Structs
When you call Rust from Julia through ccall
, Julia has already been initialized and trying to
initialize it again would cause a crash. In order to still be able to call Julia from Rust
and to borrow arrays (if you pass them as Array
rather than Ptr{Array}
), you’ll need to
create a frame first. You can use this struct to do so. It must never be used outside
functions called through ccall
, and only once for each ccall
ed function.
A Julia instance. You must create it with Julia::init
or Julia::init_with_image
before you can do anything related to Julia. While this struct exists Julia is active,
dropping it causes the shutdown code to be called but this doesn’t leave Julia in a state from which it can be reinitialized.