omkamra.jnr

A Clojure wrapper around JNR-FFI.

Makes it possible to call functions in native libraries without writing explicit wrappers through JNI, SWIG, etc.

So far only tested on 64-bit Linux.

Usage

(ns com.example
  (:require [omkamra.jnr.library :as library]
            [omkamra.jnr.struct :as struct]
            [omkamra.jnr.union :as union]
            [omkamra.jnr.enum :as enum]
            [omkamra.jnr.util :as util)))

C:

#include <stdlib.h>

void *p = malloc(1024);
free(p);

Clojure:

(library/define $c "c"
  (^Pointer malloc [^size_t size])
  (^void free [^Pointer p]))

(let [p (.malloc $c 1024)]
  (.free $c p))

The library/define form creates a var called $c in the current namespace and binds it to an object which implements the interface consisting of the listed methods. Each method corresponds to a native function exported by the library "c" whose actual name is dependent on the platform (libc.so on Linux, c.dll on Windows, etc.)

Types of function parameters and return values are described via metadata:

C type	Clojure metadata
void	^void
char	^char or ^byte
short	^short
int	^int
long	^long
long long	^long-long
float	^float
double	^double
<type>*	^Pointer
char*	^String
int8_t..int64_t	^int8_t..^int64_t
uint8_t..uint64_t	^uint8_t..^uint64_t

Besides these primitive types you can also use aliases like size_t, off_t, pid_t - for a full list check the JNR-FFI sources.

Note that long is a C long (32/64 bits depending on the platform), not a JVM long (which is always 64 bits).

The JVM does not make a distinction between signed and unsigned types.

Pointers are typeless.

Strings

C:

#include <stdio.h>
#include <string.h>

size_t len = strlen("Hello, world!");
printf("Length of the string is %d\n", len);

Clojure:

(library/define $c "c"
  (^size_t strlen [^String s]))

(let [len (.strlen $c "Hello, world!")]
  (printf "Length of the string is %d\n" len))

The Unicode string on the JVM side is encoded into a temporary buffer and strlen gets a pointer to the encoded bytes. The encoding is UTF-8 by default but can be overridden by supplying an :encoding option:

  (^size_t strlen [^String ^{:encoding "iso-8859-2"} s])

The terminating zero byte on the native side is automatically supplied by the FFI.

Buffers

C:

#include <stdio.h>

char buf[4096];

FILE *f = fopen("/dev/urandom", "r");
fread(buf, 4096, 1, f);
fclose(f);

Clojure:

(library/define $c "c"
  (^Pointer fopen [^String filename ^String opentype])
  (^size_t fread [^java.nio.Buffer data ^size_t size ^size_t count ^Pointer stream])
  (^int fclose [^Pointer stream]))

(let [buf (java.nio.ByteBuffer/allocate 4096)
      f (.fopen $c "/dev/urandom" "r")]
  (.fread $c buf 4096 1 f)
  (.fclose $c f))

If you have a java.nio.Buffer argument but the parameter is tagged as a plain Pointer, you must wrap the buffer in a Pointer:

(jnr.ffi.Pointer/wrap (library/runtime $c) buf)

Structures

C:

#include <time.h>
#include <stdio.h>

struct timespec ts;
clock_gettime(CLOCK_REALTIME, &ts);

printf("%d seconds, %ld nanoseconds\n", ts.tv_sec, ts.tv_nsec);

Clojure:

(struct/define timespec
  ^time_t tv_sec
  ^long tv_nsec)

(library/define $c "c"
  (^int clock_gettime [^int clock ^timespec ts]))

(def CLOCK_REALTIME 0)

(let [ts (timespec. (library/runtime $c))]
  (.clock_gettime $c CLOCK_REALTIME ts)
  (printf "%d seconds, %d nanoseconds\n"
           (.. ts tv_sec get)
           (.. ts tv_nsec get)))

Here we:

1. dynamically create a new jnr.ffi.Struct subclass: (struct/define timespec ...),
2. instantiate it: (timespec. (library/runtime $c))
3. pass it to clock_gettime()

Before the call, the FFI creates a native timespec structure (laid out in memory according to platform conventions) and populates its fields from the JVM-side ts object. The clock_gettime() function gets a pointer to this native structure. After the call returns, all fields in the native structure are copied back to the ts object. If you want to avoid the copying in either direction, add an :in or :out option to the relevant struct parameter:

(^int clock_gettime [^int clock ^timespec ^:out ts])

Unions

C:

typedef union epoll_data
{
  void *ptr;
  int fd;
  uint32_t u32;
  uint64_t u64;
} epoll_data_t;

struct epoll_event
{
  uint32_t events;      /* Epoll events */
  epoll_data_t data;    /* User data variable */
} __attribute__ ((__packed__));

Clojure:

(union/define epoll_data_t
  ^Pointer ptr
  ^int fd
  ^uint32_t u32
  ^uint64_t u64)

(struct/define ^:packed epoll_event
  ^uint32_t events
  ^epoll_data_t data)

Passing primitive types by reference

C:

extern int SDL_VideoInit(const char *driver_name);
extern int SDL_GetDisplayDPI(int displayIndex, float * ddpi, float * hdpi, float * vdpi);
extern void SDL_VideoQuit(void);

Clojure:

(library/define $sdl2 "SDL2"
  (^int SDL_VideoInit [^String driver_name])
  (^int SDL_GetDisplayDPI [^int displayIndex
                           ^Float* ^:out ddpi
                           ^Float* ^:out hdpi
                           ^Float* ^:out vdpi])
  (^void SDL_VideoQuit []))

(let [ddpi (jnr.ffi.byref.FloatByReference.)
      hdpi (jnr.ffi.byref.FloatByReference.)
      vdpi (jnr.ffi.byref.FloatByReference.)]
  (assert (zero? (.SDL_VideoInit $sdl2 nil)))
  (.SDL_GetDisplayDPI $sdl2 0 ddpi hdpi vdpi)
  (.SDL_VideoQuit $sdl2)
  (printf "ddpi: %f hdpi: %f vdpi: %f\n"
          (.getValue ddpi)
          (.getValue hdpi)
          (.getValue vdpi)))

Type tags of the form <X>* are automagically expanded to jnr.ffi.byref.<X>ByReference at compile time. Possible values of <X> include Byte, Short, Int, Long, LongLong, Float, Double and Pointer.

If you want to set a value for an :in argument before the call, pass it to the XByReference constructor, e.g. (jnr.ffi.byref.FloatByReference. 5.0).

Enumerations

C:

typedef enum {
  UV_RUN_DEFAULT = 0,
  UV_RUN_ONCE,
  UV_RUN_NOWAIT
} uv_run_mode;

if (mode == UV_RUN_ONCE) {
  do_something();
}

Clojure:

(enum/define uv_run_mode
  [UV_RUN_DEFAULT 0]
  UV_RUN_ONCE
  UV_RUN_NOWAIT)

(if (= mode uv_run_mode/UV_RUN_ONCE)
  (do-something))

The enum/define form dynamically creates a java.lang.Enum subclass with the given name.

Enum values are assigned in the same way as in C.

Errno

C:

#include <fcntl.h>
#include <errno.h>
#include <string.h>
#include <stdio.h>

if (!open("myfile", O_RDWR)) {
  printf("open failed: %s\n", strerror(errno));
}

Clojure:

(def O_RDONLY 0)
(def O_WRONLY 1)
(def O_RDWR 2)

(library/define $c "c"
  (^int open [^String filename ^int flags])
  (^String strerror [^int errnum]))

(when-not (zero? (.open $c "myfile" O_RDWR))
  (println (.strerror $c (library/errno $c))))

License

Copyright © 2021 Balázs Ruzsa

This program and the accompanying materials are made available under the terms of the Eclipse Public License 2.0 which is available at http://www.eclipse.org/legal/epl-2.0.

This Source Code may also be made available under the following Secondary Licenses when the conditions for such availability set forth in the Eclipse Public License, v. 2.0 are satisfied: GNU General Public License as published by the Free Software Foundation, either version 2 of the License, or (at your option) any later version, with the GNU Classpath Exception which is available at https://www.gnu.org/software/classpath/license.html.