Foreign Function Interface

Julia has built-in functions and data types to interface with C and Fortran code. There are also libraries to interface with Python and R. We will have a look at the interfaces for C and Python.

Before we continue, we return to the default environment in our Julia REPL:

(OurVectors) pkg> activate
  Activating project at `~/.julia/environments/v1.10`

C Interface

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To call a C function we can either use the function ccall or the macro @ccall. For example:

JULIA

> ccall(:getenv, Cstring, (Cstring,), "LANG")
Cstring(0x00007ffe2e3eddd9)

> unsafe_string(ans)
"en_US.UTF-8"

ccall takes the function name, followed by the return type, a tuple of argument types, and finally the arguments themselves. We can see, that there are special types defined to interface with C, e.g. Cstring.

The macro @ccall offers an alternate syntax:

JULIA

> @ccall getenv("LANG"::Cstring)::Cstring
CString(0x00007ffe2e3eddd9)

The type annotations are found at the arguments and after the argument list for the return type, maybe making the intent a little clearer.

Other than C standard library functions, we can also call any function found on the path:

JULIA

> ccall((:g_basename, "libglib-2.0"), Cstring, (Cstring,), "/usr/bin/bash")
Cstring(0x00007f7173d0a681)

> unsafe_string(ans)
"bash"

Julia will automatically try to load libglib-2.0 in this case.

With the macro @ccall, the same call would look as follows

JULIA

> @ccall "libglib-2.0".g_basename("/usr/bin/bash"::Cstring)::Cstring
Cstring(0x00007f26ff113e81)

PyCall

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To interface with python we can use the PyCall package. First, we need to install it to the current repository:

(@v1.10) pkg> add PyCall
    Installed PrecompileTools ----- v1.2.1
     Updating `~/.julia/environments/v1.10/Project.toml`
  [438e738f] + PyCall v1.96.4
    Updating `~/.julia/environments/v1.10/Manifest.toml`
  [8f4d0f93] + Conda v1.10.0
  …

pThen we can import any installed python library:

JULIA

> using PyCall

> datetime = pyimport("datetime")
PyObject <module 'datetime' from '/usr/lib/python3.10/datetime.py'>

> datetime.MAXYEAR
9999

Python Environments

We can control the version of Python that is used by Julia with the environment variable PYTHON in the Julia process.

We could, for example, start the Julia process from a shell in which we activated a particular environment of the venv Python package. This would give the Julia process access to that environments Python version and installed packages.

We can also execute Python code, through a special syntax:

JULIA

> py"[x for x in [1, 2, 3] if x % 2 == 1]"
2-element Vector{Int64}:
 1
 3

> py"""
import datetime

print(datetime.datetime.now())
"""
2022-06-03 14:38:05.903444

RCall

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A similar package exists for R with RCall. It even gives access to an R-REPL from within the Julia REPL, similar to package and shell mode.