coffi

Coffi is a foreign function interface library for Clojure, using the new Project Panama that's a part of the incubator in Java 17. This allows calling native code directly from Clojure without the need for either Java or native code specific to the library, as e.g. the JNI does. Coffi focuses on ease of use, including functions and macros for creating wrappers to allow the resulting native functions to act just like Clojure ones, however this doesn't remove the ability to write systems which minimize the cost of marshaling data and optimize for performance, to make use of the low-level access Panama gives us.

Installation

This library is available on Clojars. Add the following entry to the :deps key of your deps.edn:

org.suskalo/coffi {:mvn/version "0.1.0"}

Usage

There are two major components to coffi and interacting with native code: manipulating off-heap memory, and loading native code for use with Clojure.

In the simplest cases, the native functions you call will work exclusively with built-in types, for example the function strlen from libc.

(require '[coffi.ffi :as ffi :refer [defcfn defalias]])

(defcfn strlen
  "Given a string, measures its length in bytes."
  strlen [::ffi/c-string] ::ffi/long)

(strlen "hello")
;; => 5

The first argument to defcfn is the name of the Clojure var that will hold the native function reference, followed by an optional docstring and attribute map, then the C function identifier, including the name of the native symbol, a vector of argument types, and the return type.

If you wish to use a native function as an anonymous function, it can be done with the cfn function.

((ffi/cfn "strlen" [::ffi/c-string] ::ffi/long) "hello")
;; => 5

Primitive Types

Coffi defines a basic set of primitive types:

• byte
• short
• int
• long
• long-long
• char
• float
• double
• pointer

Each of these types maps to their C counterpart. Values of any of these primitive types except for pointer will be cast with their corresponding Clojure function (with long-long mapping to the long function) when they are passed as arguments to native functions. Additionally, the c-string type is defined, although it is not primitive.

Composite Types

In addition, some composite types are also defined in coffi, including struct and union types (unions will be discussed with serialization and deserialization). For an example c struct and function:

typedef struct point {
    float x;
    float y;
} Point;

Point zero(void) {
    Point res = {};

    res.x = 0.0;
    res.y = 0.0;

    return res;
}

The corresponding coffi definition is like so:

(defcfn zero-point
  "zero" [] [::ffi/struct [[:x ::ffi/float] [:y ::ffi/float]]])

(zero-point)
;; => {:x 0.0,
;;     :y 0.0}

Writing out struct definitions like this every time would get tedious, so the macro defalias is used to define a struct alias.

(defalias ::point
  [::ffi/struct
   [[:x ::ffi/float]
    [:y ::ffi/float]]])

(defcfn zero-point
  "zero" [] ::point)

In cases where a pointer to some data is required to pass as an argument to a native function, but dosn't need to be read back in, the pointer primitive type can take a type argument.

[::ffi/pointer ::ffi/int]

Arrays are also supported via a type argument. Keep in mind that they are the array itself, and not a pointer to the array like you might see in certain cases in C.

[::ffi/array ::ffi/int 3]

Callbacks

In addition to these composite types, there is also support for Clojure functions.

[::ffi/fn [::ffi/c-string] ::ffi/int]

Be aware though that if an exception is thrown out of a callback that is called from C, the JVM will crash. The resulting crash log should include the exception type and message in the registers section, but it's important to be aware of all the same. Ideally you should test your callbacks before actually passing them to native code.

Variadic Functions

Some native functions can take any number of arguments, and in these cases coffi provides vacfn-factory (for "varargs C function factory").

(def printf-factory (ffi/vacfn-factory "printf" [::ffi/c-string] ::ffi/int))

This returns a function of the types of the rest of the arguments which itself returns a native function wrapper.

(def print-int (printf-factory ::ffi/int))

(print-int "Some integer: %d\n" 5)
;; Some integer: 5

At the moment there is no equivalent to defcfn for varargs functions.

Some native functions that are variadic use the type va_list to make it easier for other languages to call them in their FFI. At the time of writing, coffi does not support va-list, however it is a planned feature.

Global Variables

Some libraries include global variables or constants accessible through symbols. To start with, constant values stored in symbols can be fetched with const

(def some-const (ffi/const "some_const" ::ffi/int))

This value is fetched once when you call const and is turned into a Clojure value. If you need to refer to a global variable, then static-variable can be used to create a reference to the native value.

(def some-var (ffi/static-variable "some_var" ::ffi/int))

This variable is an IDeref. Each time you dereference it, the value will be deserialized from the native memory and returned. Additional functions are provided for mutating the variable.

(ffi/freset! some-var 5)
;; => 5
@some-var
;; => 5

Be aware however that there is no synchronization on these types. The value being read is not read atomically, so you may see an inconsistent state if the value is being mutated on another thread.

A parallel function fswap! is also provided, but it does not provide any atomic semantics either.

Complex Wrappers

Some functions require more complex code to map nicely to a Clojure function. The defcfn macro provides facilities to wrap the native function with some Clojure code to make this easier.

(defcfn takes-array
  "takes_array_with_count" [::ffi/pointer ::ffi/long] ::ffi/void
  native-fn
  [ints]
  (let [arr-len (count ints)
        int-array (serialize ints [::ffi/array ::ffi/int arr-len]
    (native-fn (ffi/address-of int-array) arr-len))]))

The symbol native-fn can be any unqualified symbol, and names the native function being wrapped. It must be called in the function body below if you want to call the native code.

This serialize function has a paired deserialize, and allows marshaling Clojure data back and forth to native data structures.

This can be used to implement out variables often seen in native code.

(defcfn out-int
  "out_int" [::ffi/pointer] ::ffi/void
  native-fn
  [i]
  (let [int-ptr (serialize i [::ffi/pointer ::ffi/int])]
    (native-fn int-ptr)
    (deserialize int-ptr [::ffi/pointer ::ffi/int])))

Scopes

In order to serialize any non-primitive type (such as the previous [::ffi/pointer ::ffi/int]), off-heap memory needs to be allocated. When memory is allocated inside the JVM, the memory is associated with a scope. Because none was provided here, the scope is an implicit scope, and the memory will be freed when the serialized object is garbage collected.

In many cases this is not desirable, because the memory is not freed in a deterministic manner, causing garbage collection pauses to become longer, as well as changing allocation performance. Instead of an implicit scope, there are other kinds of scopes as well. A stack-scope is a thread-local scope. Stack scopes are Closeable, which means they should usually be used in a with-open form. When a stack-scope is closed, it immediately frees all the memory associated with it. The previous example, out-int, can be implemented with a stack scope.

(defcfn out-int
  "out_int" [::ffi/pointer] ::ffi/void
  native-fn
  [i]
  (with-open [scope (ffi/stack-scope)]
    (let [int-ptr (ffi/serialize i [::ffi/pointer ::ffi/int] scope)]
      (native-fn int-ptr)
      (ffi/deserialize int-ptr [::ffi/pointer ::ffi/int]))))

This will free the pointer immediately upon leaving the function.

When memory needs to be accessible from multiple threads, there's shared-scope. When using a shared-scope, it should be accessed inside a with-acquired block. When a shared-scope is .closed, it will release all its associated memory when every with-acquired block associated with it is exited.

In addition, two non-Closeable scopes are global-scope, which never frees the resources associated with it, and connected-scope, which is a scope that frees its resources on garbage collection, like an implicit scope.

TODO Serialization and Deserialization

TODO Data Model

License

Copyright © 2021 Joshua Suskalo

Distributed under the Eclipse Public License version 1.0.