Generalized C Foreign Function Interface (FFI) that unifies JNA and JDK-16 FFI architectures.

Users can dynamically define and load libraries and define callbacks that C can then call.

This namespace is meant to work with the struct namespace where you can define C-structs laid out precisely in memory.

Available datatypes for the binding layer:

  • :int8 :int16 :int32 :int64 :float32 :float64 :pointer :pointer? :size-t.

:pointer, :pointer?, :size-t are all types that change their underlying definition depending on if the system is 32 bit or 64 bit.

Note that unsigned types will work but the interface will have to be defined in terms of their signed analogues.


user> (require '[tech.v3.datatype :as dtype]) nil user> (require '[tech.v3.datatype.ffi :as dtype-ffi]) nil user> (def libmem-def (dtype-ffi/define-library {:qsort {:rettype :void :argtypes [['data :pointer] ['nitems :size-t] ['item-size :size-t] ['comparator :pointer]]}})) #'user/libmem-def user> (def libmem-inst (dtype-ffi/instantiate-library libmem-def nil)) #'user/libmem-inst user> (def libmem-fns @libmem-inst) #'user/libmem-fns user> (def qsort (:qsort libmem-fns)) #'user/qsort user> (def comp-iface-def (dtype-ffi/define-foreign-interface :int32 [:pointer :pointer])) #'user/comp-iface-def user> (require '[tech.v3.datatype.native-buffer :as native-buffer]) nil user> (def comp-iface-inst (dtype-ffi/instantiate-foreign-interface comp-iface-def (fn [^tech.v3.datatype.ffi.Pointer lhs ^tech.v3.datatype.ffi.Pointer rhs] (let [lhs (.getDouble (native-buffer/unsafe) (.address lhs)) rhs (.getDouble (native-buffer/unsafe) (.address rhs))] (Double/compare lhs rhs))))) #'user/comp-iface-inst user> (def comp-iface-ptr (dtype-ffi/foreign-interface-instance->c comp-iface-def comp-iface-inst)) #'user/comp-iface-ptr user> (def dbuf (dtype/make-container :native-heap :float64 (shuffle (range 100)))) 09:00:27.900 [tech.resource.gc ref thread] INFO tech.v3.resource.gc - Reference thread starting #'user/dbuf user> dbuf #native-buffer@0x00007F47084E4210<float64>[100] [80.00, 90.00, 96.00, 29.00, 19.00, 12.00, 88.00, 94.00, 81.00, 17.00, 54.00, 52.00, 64.00, 86.00, 10.00, 76.00, 49.00, 5.000, 32.00, 69.00, ...] user> (qsort dbuf (dtype/ecount dbuf) Double/BYTES comp-iface-ptr) nil user> dbuf #native-buffer@0x00007F47084E4210<float64>[100] [0.000, 1.000, 2.000, 3.000, 4.000, 5.000, 6.000, 7.000, 8.000, 9.000, 10.00, 11.00, 12.00, 13.00, 14.00, 15.00, 16.00, 17.00, 18.00, 19.00, ...]


(c->string data & [encoding])

Convert a zero-terminated c-string to a java string. See documentation for string->c for encodings.




(define-foreign-interface rettype argtypes & [{:as options}])

Define a strongly typed instance that can be used with foreign-interface-instance->c. This instance takes a single constructor argument which is the IFn that it is wrapping. It has a single typesafe ‘invoke’ method taking types defined by the underlying binding and transforming them into types that Clojure expects - for instance pointers are transformed to/from tech.v3.datatype.ffi.Pointer upon entry/exit from the wrapped IFn.

  • rettype - The return type of the function.
  • argtypes - A possibly empty sequence of argument types.


  • :classname - Similar to :classname in ‘define-library’. A class will be generated to compile-path and after this statement (import classname) will be a valid call.

See foreign-interface-instance->c for example.


(define-library fn-defs symbols options)(define-library fn-defs options)(define-library fn-defs)

Define a library returning the class. The class can be instantiated with a string naming the library path or nil for the current process. After instantiation the library instance will have strongly typed methods named the same as the keyword keys in fn-defs efficiently bound to symbols of the same exact name in library.

  • fn-defs - map of fn-name -> {:rettype :argtypes}.
    • argtypes -
    • :void - return type only.
    • :int8 :int16 :int32 :int64
    • :float32 :float64
    • :size-t - int32 or int64 depending on cpu architecture.
    • :pointer :pointer? - Something convertible to a Pointer type. Potentially exception when nil.
    • rettype - any argtype plus :void


  • :classname - If classname (a symbol) is provided a .class file is saved to compile-path after which (import classname) will be a validcall meaning that after AOT no further reflection or class generation is required to access the class explicitly. That being said ‘import’ does not reload classes that are already on the classpath so this option is best used after library has stopped changing.


user> (dtype-ffi/define-library {:memset {:rettype :pointer
                                          :argtypes [:pointer :int32 :size-t]}}
                                {:classname 'tech.libmemset})
{:library tech.libmemset}
user> (import 'tech.libmemset)
user> (def inst (tech.libmemset. nil)) ;;nil for current process
user> (def test-buf (dtype/make-container :native-heap :float32 (range 10)))
user> test-buf
[0.000, 1.000, 2.000, 3.000, 4.000, 5.000, 6.000, 7.000, 8.000, 9.000, ]
user> (.memset ^tech.libmemset inst test-buf 0 40)
#object[tech.v3.datatype.ffi.Pointer 0x33013c57 "{:address 0x00007F4E28CE56D0 }"]
user> test-buf
[0.000, 0.000, 0.000, 0.000, 0.000, 0.000, 0.000, 0.000, 0.000, 0.000, ]



(define-library-functions library-def-symbol find-fn check-error)

Define public callable vars that will call library functions marshaling strings back and forth. These vars will call find-fn with the fn-name in a late-bound way and check-error may be provided and called on a return value assuming :check-error? is set in the library definition.

  • library-def-symbol - The fully namespaced symbol that points to the function definitions.
  • find-fn - A function taking one argument - the fn name, and returning the function.
  • check-error - A function or macro that receives two arguments - the fn definition from above and the actual un-evaluated function call allowing you to insert pre/post checks.


(dt-ffi/define-library-functions avclj.ffi/avcodec-fns find-avcodec-fn check-error)


(enable-string->c-stats! enabled?)(enable-string->c-stats!)

Enable tracking how often string->c is getting called.


(find-library libname)

This method is useful to check if loading a library will work. If successful, returns the library name that did finally succeed. This may be different from libname in the case where java.library.path has been used to actively find the library.


(find-symbol libname symbol-name)(find-symbol symbol-name)

Find a symbol in a library. A library symbol is guaranteed to have a conversion to a pointer.


(foreign-interface-instance->c ffi-def foreign-inst)

Convert an instance of the above foreign interface definition into a Pointer suitable to be called from C. Callers must ensure that foreign-inst is visible to the gc the entire time the foreign system has a reference to the pointer.

  • foreign-inst - an instance of the class defined by ‘define-foreign-interface’.


(instantiate-class cls & args)

Utility function to instantiate a class via its first constructor in its list of declared constructors. Works with classes returned from ‘define-library’ and ‘define-foreign-interface’. Uses reflection.


(instantiate-foreign-interface ffi-def ifn)

Instantiate a foreign interface defintion. This returns an instance object which can then be converted into a c-pointer via foreign-interface-instance->c.


(instantiate-library library-def libpath)

Uses reflection to instantiate a library



Is the JDK foreign function interface available?



Is the JDK native memory model available?



Is JNA’s ffi interface available?


(library-loadable? libname)


(library-singleton library-def-var library-sym-var library-def-opts)(library-singleton library-def-var)

Create a singleton object, ideally assigned to a variable with defonce, that you can reset to auto-reload the bindings i.e. with new function definitions at the repl.

  • library-def-var must be something that deref’s to the latest library definition.
(defonce ^:private lib (dt-ffi/library-singleton #'avcodec-fns))

;;Safe to call on uninitialized library.  If the library is initialized, however,
;;a new library instance is created from the latest avcodec-fns
(dt-ffi/library-singelton-reset! lib)

(defn set-library!
  (dt-ffi/library-singelton-set! lib libpath))
(defn- find-avcodec-fn
  (dt-ffi/library-singelton-find-fn lib fn-kwd))


(make-ptr dtype prim-init-value options)(make-ptr dtype prim-init-value)

Make an object convertible to a pointer that points to single value of type dtype.



Protocol to allow easy definition of a library singleton that manages re-creating the library definition when the library data changes and also recreating the library instance if necessary.



(library-singleton-definition lib-singleton)

Return the library definition.


(library-singleton-find-fn lib-singleton fn-name)

Find a bound function in the library. Returns an implementation of clojure.lang.IFn that takes only the specific arguments.


(library-singleton-find-symbol lib-singleton sym-name)

Find a symbol in the library. Returns a pointer.


(library-singleton-library lib-singleton)

Return the library instance


(library-singleton-library-path lib-singleton)

Get the bound library path


(library-singleton-reset! lib-singleton)

Regenerate library bindings and bind a new instance to allow repl-style iterative development.


(library-singleton-set! lib-singleton libpath)

Set the library path, generate the library and create a new instance.


(library-singleton-set-instance! lib-singleton libinst)

In some cases such as graal native pathways you have to hard-set the definition and instance.





(->pointer item)

Convert an item into a Pointer, throwing an exception upon failure.


(convertible-to-pointer? item)

Query whether an item is convertible to a pointer.


(ptr->struct struct-type ptr)

Given a struct type and a pointer return a struct whose data starts at the address of the pointer.

  (let [codec (dt-ffi/ptr->struct (:datatype-name @av-context/codec-def*) codec-ptr)]


(set-ffi-impl! ffi-kwd)

Set the global ffi implementation. Options are * :jdk - namespace 'tech.v3.datatype.ffi.mmodel/ffi-fns - only available with jdk’s foreign function module enabled. * :jna - namespace ''tech.v3.datatype.ffi.jna/ffi-fns - available if JNA version 5+ is in the classpath.


(string->c str-data & [{:keys [encoding], :as options}])

Convert a java String to a zero-padded c-string. Available encodings are

  • :ascii, :utf-8 (default), :utf-16, :utf-16LE, utf-16-BE and :utf-32

String data will be zero padded.




(struct-member-ptr data-struct member)

Get a pointer to a struct data member.

   (swscale/sws_scale sws-ctx
                      (struct-member-ptr input-frame :data)
                      (struct-member-ptr input-frame :linesize)
                      0 (:height input-frame)
                      (struct-member-ptr encoder-frame :data)
                      (struct-member-ptr encoder-frame :linesize))



Get the utf-16 encoding that matches the current platform so you can encode a utf-16 string without a bom.



Get the utf-16 encoding that matches the current platform so you can encode a utf-16 string without a bom.