JNR-FFI

类型转换

基本类型

将 C 的类型和Java类型自动转换。

C Type	Java Type	Size
char	byte	8 bit integer
short	short	16 bit integer
int	int	32 bit integer
long	long	natural long, 32 bits on 32 bit systems, 64 bits on 64 bit systems
float	float	32 bit floating point
double	double	64 bit floating point
const char *	String or Pointer
void *	Pointer or Buffer
int *	IntByReference	其他float*有类似的类
void **	PointerByReference
C enum	Mostly int

结构体/Union

c

struct timespec {
    time_t tv_sec;
    long int tv_nsec;
};

Java

// Union类似，集成Union类即可
public class Timespec extends Struct {
    // 结构体使用自己的基本类型
    // Struct types can be found as inner classes of the Struct class
    public Struct.SignedLong tv_sec = new Struct.SignedLong();
    public Struct.SignedLong tv_nsec = new Struct.SignedLong();

    // Necessary constructor that takes a Runtime
    public Timespec(jnr.ffi.Runtime runtime) {
        super(runtime);
    }

    // You can add your own methods of choice
    public void setTime(long sec, long nsec) {
        tv_sec.set(sec);
        tv_nsec.set(nsec);
    }
}

枚举

c 枚举定义：

enum fuse_buf_flags {
    FUSE_BUF_IS_FD    = (1 << 1),
    FUSE_BUF_FD_SEEK    = (1 << 2),
    FUSE_BUF_FD_RETRY    = (1 << 3),
};

Java对应定义（可以自动转换）

public enum FuseBufFlags implements EnumMapper.IntegerEnum {
    FUSE_BUF_IS_FD(1 << 1),
    FUSE_BUF_FD_SEEK(1 << 2),
    FUSE_BUF_FD_RETRY(1 << 3);

    private final int value;

    FuseBufFlags(int value) {
        this.value = value;
    }

    @Override
    public int intValue() {
        return value;
    }
}

函数类型

typedef int MyCallback(void *data);

int setCallback(MyCallback *callback);

Java对应调用的写法：

public interface ExampleLibrary {
    public interface MyCallback { // type representing callback
        @Delegate
        int invoke(Pointer data); // function name doesn't matter, it just needs to be the only function and have @Delegate
    }

    int setCallback(MyCallback callback);

}

Function pointer is valid as long as the instance stays strongly-referenced.

ObjectReferenceManager referenceManager = Runtime.getRuntime(lib).newObjectReferenceManager();
MyCallback callback = data -> 0;
Pointer key = referenceManager.add(callback);

lib.setCallback(callback);     // Callback is passed internally as a function pointer 

referenceManager.remove(key);  // Free the callback when no more required

内存管理

Pointers

native memory 地址的引用；

示例

maven依赖

<dependency>
  <groupId>com.github.jnr</groupId>
  <artifactId>jnr-ffi</artifactId>
  <version>2.2.12</version>
</dependency>

示例代码：

import jnr.ffi.*;
import jnr.ffi.types.pid_t;

/**
 * Gets the process ID of the current process, and that of its parent.
 */
public class Getpid {
    public interface LibC  {
        // 跟libc.so的getpid的本地方法匹配
        public @pid_t long getpid();
        public @pid_t long getppid();
    }

    public static void main(String[] args) {
        // load libc.so
        LibC libc = LibraryLoader.create(LibC.class).load("c");

        System.out.println("pid=" + libc.getpid() + " parent pid=" + libc.getppid());
    }
}