Question

I m working on a project at the moment that requires me to receive a call in Java from a C library. Basically I call a C function that takes a function pointer, the C function then uses the function pointer as a callback. I m using JNA to pass a Java object (I ll call it the Callback object from now on) as the callback function. The callback object has a single method that receives a C structure (called Frame) that contains a 16 element byte array as well as other variables. I ve wrapped this structure in a Java class in the standard JNA way, like this:

class Frame extends Structure {
    public short port;
    public short flags;
    public Pointer name // this is a pointer to the byte array
    public int rateDivisor;
}

追索机制运作良好! 背心物体从C处收到一小块物体,但当我试图利用<代码>,即:getByteArray(0、16)从点子公司获得星体阵列时,该申请因暴力例外而坠毁。然而,如果我用一阵列取代点子:

class Frame extends Structure {
    public short port;
    public short flags;
    public byte[] name = new byte[16];
    public int rateDivisor;
}

Then the code works fine! There s a good reason why I don t want to use this code however. Every time I call the function the returned Frame is actually the same object (it s just being reused) but the byte array is a new object. This callback function is being called many times a second which causes the garbage collector to goes crazy gobbling up thousands of temporary arrays. This project is performance critical so I really don t want any temporary objects in this part of the application.

My guess is that by using a byte array in the Frame class I m causing JNA to create a new copy of the C byte array. What I want to do is have a pointer to the C byte array therefore removing the need to copy the data, hence the experimentation with JNA Pointer.

My question is why can t I get the byte array from the Pointer? Any help would be much appreciated :)

(说明:使这种情况更加困难的是,我没有接触到《来源法》!)

Answer 1

原因可能与以下原因一样简单:名称不是C级结构对应单位/m>。

让我向大家介绍两个例子:

C:

typedef struct{ char * name; }MyCStruct;

地图:

class MyJStruct extends Structure{ Pointer name; }

但在其他情况下(我认为你会去做):

C:

typedef struct{ char name[16]; }MyCStruct;

地图:

class MyJStruct extends Structure{ byte[] name = new byte[16]; }

在第一种情况下,C结构具有点数,其规模是点数(4或8Bytes,视32/64bits而定),第二种情况下,C结构拥有整块体,即面积为16个。

这解释了两幅瓦 Java地图之间的区别:日本宇宙航行局需要了解如何阅读结构领域,也解释了为什么在打电话<代码>时出现错误。

take the first 4 Bytes following the flags field

treat it as a pointer

go into ram to the pointed adress and read 16 Bytes

自记忆区指出后坠毁的,很可能无法达到方案。

您可在C中查到:如果sizeof (struct Frame) 是24,那么,如果其第12(或16)条,则其位置为32个轨道点(或64个轨道点)

关于这一问题的正式文件非常清楚,但很难找到,因此,你可以:

rel=“noreferer”http://t Wall.github.com/jna/3.3.0/javadoc/overview-summary.html

在“Nested ranges”下发现,我确实鼓励你阅读下文“可变规模结构”一节。

希望这一帮助

目录

Answer 2

将你的反馈意见参数改为点。 • 保持自己的私人点和坐标;Frame地图(有或没有薄弱环节),并且只有在输入点基础上,才建立新框架。

e.g.

Map frames = new HashMap<Pointer,Frame>();

void callback(Pointer p) {
   Frame f = frames.get(p);
   if (f == null) {
       f = new Frame(p);
       frames.put(p, f);
   }
   // do whatever...
}

Answer 3

(满足StackTrace期望的新答案;见对我先前的答复的评论)

确实,我不敢肯定,如果有冰冰和干净的方法,......因此,我指出了一个叫<条码>的冰球游戏。

牢记这些途径:

Frame nativeLibFunc(...);        //1
Frame * nativeLibFunc(...);      //2
void nativeLibFunc(Frame f);     //3
void nativeLibFunc(Frame * fptr);//4

我认为,最糟糕的情况是第三,因为如果没有先前解释的正常绘图,你可以这样做。接下来最糟糕的情况是,由于你赢得了区域援助团的一个位置,但直接结果却在顶端,因此,阅读这一结果,需要你把日本宇宙航行局引向点人,并读到你想要跳出的记忆区之后:

class MyMicroFrame extends Structure {
    public short port;
    public short flags;
    //public byte[] name = new byte[16]; ==> commented out, ignored
    //public int rateDivisor;            ==> commented out, read manually, see below
}
//...
MyMicroFrame mmf = NativeLib.INSTANCE.nativeLibFunc(...);
int rateDivisor = mmf.getPointer().getInt(20);

我认为你会这样做。如果你的结构越大,就把它分成小部分,避免你希望通过在第一部分所指出的记忆中进行人工清醒来重新加入。

www.un.org/Depts/DGACM/index_spanish.htm 在以这种方式开展工作时铭记的事项:

Pointer size 32/64 bits -- 4 or 8 Bytes see com.sun.jna.Native.POINTER_SIZE

Structure alignement which I ve not checked on this answer

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