如何在 jnr ffi 中使用带有结构的结构

Question

我有以下c代码：

#include <stdio.h>

struct Second {
    int a_number;
};

struct Top {
    struct Second second;
};

void lets_go(struct Top *top) {
    printf("The number is %d\n", top->second.a_number);
}

我想从 Java:

int main(void) {
    struct Top top = {{8}};
    lets_go(&top);
}

我也想使用 jnr-ffi，所以我查看了测试结果：

package structs.playing;

import structs.playing.Program.Test.Top;
import structs.playing.Program.Test.Second;
import jnr.ffi.LibraryLoader;
import jnr.ffi.Runtime;
import jnr.ffi.Struct;

public final class Program {

    public static interface Test {

        void lets_go(Top top);

        public static final class Second extends Struct {               
            public final Signed32 a_number = new Signed32();                
            public Second(final Runtime runtime) {
                super(runtime);
            }
        }

        public static final class Top extends Struct {              
            public Second second;                           
            public Top(final Runtime runtime) {
                super(runtime);
            }
        }
    }

    public static void main(final String[] args) {

        Test test = LibraryLoader.create(Test.class).load("test");
        Runtime runtime = Runtime.getRuntime(test);         
        Top top = new Top(runtime);
        Second second = new Second(runtime);
        top.second = second;
        second.a_number.set(7);         
        test.lets_go(top);
    }    
}

问题是 a_number 的值根本没有设置，所以我在输出中得到了一个垃圾值，例如：

The number is 46645760

那么如何获得与我的 C 代码相同的值？

Answer 1

分配结构时，如行

top.second = second;

在您的 Java 代码中，该结构从 second 复制到 top.second，因此它们成为不同区域的独立实体的记忆。稍后，当您在以下行中将 7 分配给 second 的 a_number 属性时：

second.a_number.set(7);

top.second对应的属性保持不变，因为它们不是同一个对象

为了得到与您的 C 代码相同的结果，请尝试将您的 main 方法更改为：

public static void main(final String[] args) {

    Test test = LibraryLoader.create(Test.class).load("test");
    Runtime runtime = Runtime.getRuntime(test);         
    Top top = new Top(runtime);
    top.second.a_number.set(8);         
    test.lets_go(top);
}

不需要初始化新的 Second 对象，因为在初始化 Top 对象的过程中已经为 top.second 分配了内存。

Answer 2

我已经弄明白了（顺便说一句，我知道成员应该是私有的并包装在属性中，但我想让代码片段尽可能小，这不是生产质量代码）。 .

如果将 Pointer 成员变量放入结构中，则可以在构造子结构时使用它的内存，如下所示...

package structs.playing;

import structs.playing.Program.Test.Top;
import jnr.ffi.LibraryLoader;
import jnr.ffi.Runtime;
import jnr.ffi.Struct;

public final class Program {

    public static interface Test {

        void lets_go(Top top);

        public static final class Second extends Struct {

            public final Signed32 a_number = new Signed32();

            public Second(final Runtime runtime) {
                super(runtime);
            }           
        }

        public static final class Top extends Struct {

            private final Pointer secondPointer = new Pointer();            
            public final Second second;

            public Top(final Runtime runtime) {
                super(runtime);                             
                second = new Second(runtime); 
                second.useMemory(secondPointer.getMemory());
            }           
        }
    }

    public static void main(final String[] args) {

         Test test = LibraryLoader.create(Test.class).load("test");
         Runtime runtime = Runtime.getRuntime(test);         
         Top top = new Top(runtime);
         top.second.a_number.set(8);         
         test.lets_go(top);
    }
}

Answer 3

编辑：我花了更多时间查看代码，我对创建结构的理解略有改变。

我认为您应该声明有关结构的所有内容并将其设为最终结构，因为每次您声明一个新成员时，它都会向它所属的结构注册自己。

struct 中有一些适用于每个用例的辅助函数。重载的 array() 方法允许您注册一个成员数组或结构数组。 inner() 方法允许您注册单个结构。否则，您只需定义新的 Member 对象，它们就会自动注册。

例如：

struct Second {
    int a_number;
};

struct Top {
    struct Second second;
    struct Second seconds[5];
    int another_number;
    int more_numbers[5];
};

表示为：

public final class Second extends Struct {
    public final Signed32 a_number = new Signed32();
    public Second(final Runtime runtime) {
        super(runtime);
    }
}

public final class Top extends Struct {              
    public final Second second = inner(new Second(getRuntime()));  
    public final Second[] seconds = array(new Second[5]);
    public final Signed32 another_number = new Signed32();
    public final Signed32[] more_numbers = array(new Signed32[5]);
    public Top(final Runtime runtime) {
        super(runtime);
    }
}

原文：我相信执行此操作的正确方法是使用接受 (Runtime, Struct) 的重载 Struct 构造函数。 https://github.com/jnr/jnr-ffi/blob/master/src/main/java/jnr/ffi/Struct.java#L129

protected Struct(Runtime runtime, Struct enclosing) {
    this(runtime);
    __info.alignment = enclosing.__info.alignment;
}

此构造函数强制封闭结构共享其内存。所以在你的例子中我认为它看起来像这样：

package structs.playing;

import structs.playing.Program.Test.Top;
import structs.playing.Program.Test.Second;
import jnr.ffi.LibraryLoader;
import jnr.ffi.Runtime;
import jnr.ffi.Struct;

public final class Program {

    public static interface Test {

        void lets_go(Top top);

        public static final class Second extends Struct {               
            public final Signed32 a_number = new Signed32();                
            public Second(final Runtime runtime, final Struct enclosing) {
                super(runtime, enclosing);
            }
        }

        public static final class Top extends Struct {              
            public Second second;                           
            public Top(final Runtime runtime) {
                super(runtime);
            }
        }
    }

    public static void main(final String[] args) {

        Test test = LibraryLoader.create(Test.class).load("test");
        Runtime runtime = Runtime.getRuntime(test);         
        Top top = new Top(runtime);
        Second second = new Second(runtime, top);
        top.second = second;
        second.a_number.set(7);         
        test.lets_go(top);
    }    
}

请注意对 Second 构造函数的更改，并注意我将 Top 对象传递给了 Second 对象，因此它知道 top 正在封闭它。这没有经过测试，只是分享我在尝试理解代码时发现的内容。

我认为您的示例中发生的情况是，第二个对象正在分配其自己的内存，而 Top 对此一无所知。

如果这不起作用，我会建议考虑做这样的事情：

public static final class Top extends Struct {              
    public Second second = new Second(getRuntime(), this);                           
    public Top(final Runtime runtime) {
        super(runtime);
    }
}

如何在 jnr ffi 中使用带有结构的结构

How to use a struct with a struct in jnr ffi

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