如何像 python ctypes 那样指定 JNR 指针

Posted 2023-02-17

【中文标题】如何像 python ctypes 那样指定 JNR 指针

【英文标题】：how to specify a JNR Pointer like that of python ctypes

【发布时间】：2018-12-04 08:02:22

【问题描述】：

使用 python 的 ctypes，可以指定一个带有类型的指针：

class METADATA(Structure):
    _fields_ = [("classes", c_int),
                ("names", POINTER(c_char_p))]

使用 JNR，它看起来像这样：

public static class Metadata extends Struct

  public Metadata(jnr.ffi.Runtime rt) 
    super(rt);
  

  public final Struct.Unsigned32 classes = new Struct.Unsigned32();
  public final Struct.Pointer names = new Struct.Pointer();

但是，是否可以将names 字段键入为指向字符串的指针？

【参考方案1】：

我不熟悉 python 的 ctypes 但假设 names 的类型是 char* 或 char** 您可以尝试使用以下方法之一。

对于共享库，

#include <stdlib.h>
#include <stdio.h>

struct MyStruct 
    int classes;
    char *names;
    char **names2;
;

struct MyStruct *get_my_struct() 
    struct MyStruct *my_struct = malloc(sizeof(struct MyStruct));
    my_struct->classes = 42;
    my_struct->names = "My Names";
    char **names2 = calloc(2, sizeof(char *));
    names2[0] = "Stack";
    names2[1] = "Overflow";
    my_struct->names2 = names2;
    return my_struct;

结构体可以定义如下

public static class MyStruct extends Struct 
  public MyStruct(Runtime runtime) 
    super(runtime);
  

  public final Struct.Signed32 classes = new Struct.Signed32();
  // For char* the support is built-in
  public final Struct.String names = new Struct.UTF8StringRef();
  // For char** you could wrap a pointer and override getStringMemory
  public final UTF8StringRef[] names2 = UTF8StringRefArray(new Struct.Pointer(), 2);

  protected UTF8StringRef[] UTF8StringRefArray(Pointer pointer, int stringLength) 
    UTF8StringRef[] array = new UTF8StringRef[stringLength];
    for (int i = 0; i < array.length; i++) 
      int index = i;
      array[i] = new UTF8StringRef() 
        @Override
        protected jnr.ffi.Pointer getStringMemory() 
          return pointer.get().getPointer(getRuntime().addressSize() * index);
        
      ;
    
    return array;
  

对于上述情况，以下代码将打印42 My Names are Stack, Overflow。

public interface MyLib 
  MyStruct get_my_struct();


public static void main(String[] args) 
  MyLib mylib = LibraryLoader.create(MyLib.class).load("mylib.so");
  MyStruct myStruct = mylib.get_my_struct();
  System.out.printf("%d %s are %s, %s", myStruct.classes.get(), myStruct.names.get(),
      myStruct.names2[0].get(), myStruct.names2[1].get());