如何在jnr ffi中使用结构与结构

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);
}

我想用Java来做这个：

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

我还想使用约尔菲，所以我查看了这些测试，最后得到了以下结果：

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C代码相同的信息呢？

Answer 1

我已经想好了(顺便说一句，我知道成员应该是私有的，并且被包裹在属性中，但是我想使代码片段尽可能小，这不是生产质量代码).

如果将指针成员变量放入结构中，则可以在构造副坐标结构时使用它的内存，如……

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 2

编辑:我花了更多的时间看代码，我对创建结构的理解略有改变。

我相信您应该声明关于结构的所有内容并使其成为最终的，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。

对于每个用例，struct中都有一些帮助函数。重载数组()方法允许您注册一个成员或Structs数组。内部()方法允许您注册单个结构。否则，您只需定义新成员对象，它们将自动注册。

例如：

struct Second {
    int a_number;
};

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

代表如下：

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);
    }
}

我相信正确的方法是使用接受的重载的Struct构造函数(Runtime，Struct)。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，我认为应该是这样的：

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);
    }    
}

请注意对第二个构造函数的更改，并注意到我将Top对象传递给了第二个对象，因此它知道top正在包围它。这是没有测试的，只是分享我在试图理解代码时发现的内容。

我认为在你的例子中发生的事情是，第二个对象是分配它自己的内存，托普对此一无所知。

如果这样做不起作用，我建议您做这样的事情：

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

Answer 3

当指定了结构时，如行中所示

top.second = second;

在您的Java代码中，结构是将从second复制到top.second中，以便它们成为不同内存区域中的独立实体。稍后，当您在下面的行中将7赋值给a_number属性second时：

second.a_number.set(7);

top.second的相应属性保持不变，因为它们不是同一个对象。

为了获得与C代码相同的结果，请尝试将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);
}

没有必要初始化一个新的Second对象，因为作为初始化Top对象的一部分，已经为top.second分配了内存。