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 one of the following entries to the :deps key of your deps.edn:

org.suskalo/coffi {:mvn/version "0.1.169"}
io.github.IGJoshua/coffi {:git/tag "v0.1.169" :git/sha "7ec2748"}

If you use this library as a git dependency, you will need to prepare the library.

$ clj -X:deps prep

Coffi also includes support for the linter clj-kondo. If you use clj-kondo and this library's macros are not linting correctly, you may need to install the config bundled with the library. You can do so with the following shell command:

$ clj-kondo --copy-configs --dependencies --lint "$(clojure -Spath)"

And then adding "org.suskalo/coffi" to the :config-paths key in your .clj-kondo/config.edn file.

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

(defcfn strlen
  "Given a string, measures its length in bytes."
  strlen [::mem/c-string] ::mem/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" [::mem/c-string] ::mem/long) "hello")
;; => 5

If you want to use functions from libraries other than libc, then you'll need to load them. Two functions are provided for this, load-system-library, and load-library. load-system-library takes a string which represents the name of a library that should be loaded via system lookup.

(ffi/load-system-library "z")

This will load libz from the appropriate place on the user's load path.

Alternatively, load-library takes a file path to a dynamically loaded library.

(ffi/load-library "lib/libz.so")

This will load libz from the lib subdirectory of the current working directory. As you can see this requires the entire filename, including platform-specific file extensions.

If a library is attempted to be loaded but doesn't exist or otherwise can't be loaded, an exception is thrown. This can be convenient as any namespace with a load-library call at the top level cannot be required without the library being able to be loaded.

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" [] [::mem/struct [[:x ::mem/float] [:y ::mem/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
  [::mem/struct
   [[:x ::mem/float]
    [:y ::mem/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.

[::mem/pointer ::mem/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.

[::mem/array ::mem/int 3]

Callbacks

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

[::ffi/fn [::mem/c-string] ::mem/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" [::mem/c-string] ::mem/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 ::mem/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" ::mem/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" ::mem/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" [::mem/pointer ::mem/long] ::mem/void
  native-fn
  [ints]
  (let [arr-len (count ints)
        int-array (serialize ints [::mem/array ::mem/int arr-len]
    (native-fn (mem/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" [::mem/pointer] ::mem/void
  native-fn
  [i]
  (let [int-ptr (serialize i [::mem/pointer ::mem/int])]
    (native-fn int-ptr)
    (deserialize int-ptr [::mem/pointer ::mem/int])))

Scopes

In order to serialize any non-primitive type (such as the previous [::mem/pointer ::mem/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" [::mem/pointer] ::mem/void
  native-fn
  [i]
  (with-open [scope (mem/stack-scope)]
    (let [int-ptr (mem/serialize i [::mem/pointer ::mem/int] scope)]
      (native-fn int-ptr)
      (mem/deserialize int-ptr [::mem/pointer ::mem/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.

Serialization and Deserialization

Custom serializers and deserializers may also be created. This is done using two sets of three multimethods which can be extended by the user. For any given type, only one set need be implemented.

Two examples of custom types are given here, one is a 3d vector, and the other an example of a tagged union.

Vector3

For the vector type, it will serialize to a pointer to an array of three floats.

The multimethod primitive-type returns the primitive type that a given type serializes to. For this example, it should be a pointer.

(defmethod mem/primitive-type ::vector
  [_type]
  ::mem/pointer)

For any type which doesn't serialize to a primitive, it returns nil, and therefore need not be overriden.

Next is serialize* and deserialize*, multimethods that work with types that serialize to primitives.

(defmethod mem/serialize* ::vector
  [obj _type scope]
  (mem/address-of (mem/serialize obj [::mem/array ::mem/float 3] scope)))

(defmethod mem/deserialize* ::vector
  [addr _type]
  (mem/deserialize (mem/slice-global addr (mem/size-of [::mem/array ::mem/float 3]))
                   [::mem/array ::mem/float 3]))

The slice-global function allows you to take an address without an associated scope and get a memory segment which can be deserialized.

In cases like this where we don't know the scope of the pointer, we could use add-close-action! to ensure it's freed. For example if a free-vector! function that takes a pointer exists, we could use this:

(defcfn returns-vector
  "returns_vector" [] ::mem/pointer
  native-fn
  [scope]
  (let [ret-ptr (native-fn)]
    (add-close-action! scope #(free-vector! ret-ptr))
    (deserialize ret-ptr ::vector)))

This function takes a scope and returns the deserialized vector, and it will free the pointer when the scope closes.

Tagged Union

For the tagged union type, we will represent the value as a vector of a keyword naming the tag and the value. The type itself will need to take arguments, similar to struct. For example, if we were to represent a result type like in Rust, we might have the following values:

[:ok 5]
[:err "Invalid number format"]

To represent this, we can have a tagged-union type. For this instance of the result type, it may look like this:

[::tagged-union [:ok :err] {:ok ::mem/int :err ::mem/c-string}]

The native representation of these objects is a struct of the tag and a union of the value. In order to correctly serialize the data and pass it to native code, we need a representation of the native layout of the data. The c-layout multimethod provides that.

(defmethod mem/c-layout ::tagged-union
  [[_tagged-union tags type-map]]
  (mem/c-layout [::mem/struct
                 [[:tag ::mem/long]
                  [:value [::mem/union (vals type-map)]]]]))

Types with type arguments are represented as vectors of the type name and any additional arguments. The type name is what is dispatched on for the multimethods.

Now that we have a native layout, we need to be able to serialize and deserialize the value into and out of memory segments. This is accomplished with serialize-into and deserialize-from.

(defn item-index
  "Gets the index of the first occurance of `item` in `coll`."
  [coll item]
  (first
   (->> coll
        (map-indexed vector)
        (filter (comp #{item} second))
        (map first))))

(defmethod mem/serialize-into ::tagged-union
  [obj [_tagged-union tags type-map] segment scope]
  (mem/serialize-into
   {:tag (item-index tags (first obj))
    :value (second obj)}
   [::mem/struct
    [[:tag ::mem/long]
     [:value (get type-map (first obj))]]]
   segment
   scope))

This serialization method is rather simple, it just turns the vector value into a map, and serializes it as a struct, choosing the type of the value based on the tag.

(defmethod mem/deserialize-from ::tagged-union
  [segment [_tagged-union tags type-map]]
  (let [tag (mem/deserialize-from segment ::mem/long)]
    [(nth tags tag)
     (mem/deserialize-from
      (mem/slice segment (mem/size-of ::mem/long))
      (get type-map tag))]))

Deserialization is a little more complex. First the tag is retrieved from the beginning of the segment, and then the type of the value is decided based on that before it is deserialized.

Unions

In the last section the custom serialization and deserialization of a tagged union used a union from coffi in order to define the native layout, but not for actual serialization or deserialization. This is intentional. A union in coffi is rather limited. It can be serialized, but not deserialized without external information.

[::mem/union
 #{::mem/float ::mem/double}
 :dispatch #(cond
             (float? %) ::mem/float
             (double? %) ::mem/double)]

This is a minimal union in coffi. If the :dispatch keyword argument is not passed, then the union cannot be serialized, as coffi would not know which type to serialize the values as. In the example with a tagged union, a dispatch function was not provided because the type was only used for the native layout.

In addition to a dispatch function, when serializing a union an extract function may also be provided. In the case of the value in the tagged union from before, it could be represented for serialization purposes like so:

[::mem/union
 #{::mem/int ::mem/c-string}
 :dispatch #(case (first %)
              :ok ::mem/int
              :err ::mem/c-string)
 :extract second]

This union however would not include the tag when serialized.

If a union is deserialized, then all that coffi does is to allocate a new segment of the appropriate size with an implicit scope so that it may later be garbage collected, and copies the data from the source segment into it. It's up to the user to call deserialize-from on that segment with the appropriate type.

Unwrapped Native Handles

Sometimes the overhead brought by the automatic serialization and deserialization from the methods explained so far is too much. In cases like these, unwrapped native handles are desirable.

The functions make-downcall and make-varargs-factory are provided to create these raw handles.

(def raw-strlen (ffi/make-downcall "strlen" [::mem/c-string] ::mem/long))
(raw-strlen (mem/serialize "hello" ::mem/c-string))
;; => 5

In these cases, the argument types are expected to exactly match the types expected by the native function. For primitive types, those are primitives. For addresses, that is MemoryAddress, and for composite types like structs and unions, that is MemorySegment. Both MemoryAddress and MemorySegment come from the jdk.incubator.foreign package.

In addition, when a raw handle returns a composite type represented with a MemorySegment, it requires an additional first argument, a SegmentAllocator, which can be acquired with scope-allocator to get one associated with a specific scope. The returned value will live until that scope is released.

In addition, function types can be specified as being raw, in the following manner:

[::ffi/fn [::mem/int] ::mem/int :raw-fn? true]

Clojure functions serialized to this type will have their arguments and return value exactly match the types specified and will not perform any serialization or deserialization at their boundaries.

Data Model

In addition to the macros and functions provided to build a Clojure API for native libraries, facilities are provided for taking data and loading all the symbols specified by it. This can be useful if a library provides (or an external provider maintains) a data representation of their API, as Clojure data to represent it may be programmatically generated from these sources.

The data to represent an API is a map with the following form:

(def strlen-libspec
  {:strlen {:type :function
            :symbol "strlen"
            :function/args [::mem/c-string]
            :function/ret ::mem/long}})

Each key in this map represents a single symbol to be loaded. The value is a map with at least the keys :type and :symbol. These are the currently recognized types:

• function
• varargs-factory
• const
• static-var

Each one has its own set of additional keys which can be added to the map. Both function and varargs-factory have the three keys :function/args, :function/ret, and :function/raw-fn?. The const type has :const/type and static-var has :static-var/type.

This data can be passed to the function reify-libspec, which will take the data and return a map from the same keys as the input map to whatever value is appropriate for a given symbol type (e.g. a Clojure function for function, a value for const, etc.).

(ffi/reify-libspec strlen-libspec)
;; => {:strlen #function[...]}

This functionality can be extended by specifying new types as implementations of the multimethod reify-symbolspec, although it's recommended that for any library authors who do so, namespaced keywords be used to name types.

Known Issues

The project author is aware of these issues and plans to fix them in a future release:

There are currently no known issues! Hooray!

Future Plans

These features are planned for future releases.

• Support for va_args type
• Functions for wrapping structs in padding following various standards
• Header parsing tool for generating a data model?
• Generic type aliases

License

Copyright © 2021 Joshua Suskalo

Distributed under the Eclipse Public License version 1.0.