blocks|key|1422073|text|Java不知道如何处理指针，但它应该能够存储本机函数返回值中的指针，然后将其交给另一个本机函数处理。C指针只不过是核心的数字值。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|1422074|另一个contibutor必须告诉您指向图形内存的指针是否会在两次JNI调用之间被清除，以及是否会有任何变通方法。|1422075|entityMap^0|0|0^^$0|@$1|2|3|4|5|6|7|F|8|@]|9|@]|A|$]]|$1|B|3|C|5|6|7|G|8|@]|9|@]|A|$]]|$1|D|3|-4|5|6|7|H|8|@]|9|@]|A|$]]]|E|$]]

Java wouldn't know what to do with a pointer, but it should be able to store a pointer from a native function's return value then hand it off to another native function for it to deal with. C pointers are nothing more than numeric values at the core.

Another contibutor would have to tell you whether or not the pointed to graphics memory would be cleared between JNI invocations and if there would be any work-arounds.

blocks|key|1207027|text|如果在本机函数内动态分配内存(在堆上)，则不会删除它。换句话说，您可以使用指针、静态变量等在本机函数的不同调用之间保留状态。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|1207028|换一种方式想一想:在从另一个C%2B%2B程序调用的函数调用中，您可以安全地保留什么？同样的事情也适用于这里。当一个函数退出时，该函数调用的堆栈上的任何东西都会被销毁；但是堆上的任何东西都会保留下来，除非您显式地删除它。|1207029|简而言之:只要您不释放返回到调用函数的结果，它将在以后重新进入时仍然有效。做完后一定要把它清理干净。|1207030|entityMap^0|0|0|0^^$0|@$1|2|3|4|5|6|7|H|8|@]|9|@]|A|$]]|$1|B|3|C|5|6|7|I|8|@]|9|@]|A|$]]|$1|D|3|E|5|6|7|J|8|@]|9|@]|A|$]]|$1|F|3|-4|5|6|7|K|8|@]|9|@]|A|$]]]|G|$]]

If you are allocating memory dynamically (on the heap) inside of the native function, it is not deleted. In other words, you are able to retain state between different calls into native functions, using pointers, static vars, etc. 

Think of it a different way: what could you do safely keep in an function call, called from another C++ program? The same things apply here. When a function is exited, anything on the stack for that function call is destroyed; but anything on the heap is retained unless you explicitly delete it. 

Short answer: as long as you don't deallocate the result you're returning to the calling function, it will remain valid for re-entrance later. Just make sure to clean it up when you're done.

blocks|key|1422195|text|我使用了以下方法：|type|unstyled|depth|inlineStyleRanges|entityRanges|data|1422196|在JNI代码中，创建一个包含对所需对象的引用的结构。当您第一次创建此结构时，将其指针作为long返回给java。然后，在java中，你只需调用任何带有这个long作为参数的方法，然后在C中将它转换为指向你的结构体的指针。|offset|length|style|CODE|1422197|该结构将在堆中，因此它不会在不同的JNI调用之间被清除。|1422198|编辑:我不认为你可以使用long+ptr+=+(long)&address;，因为address是一个静态变量。按照Gunslinger47建议的方式使用它，即创建类或结构的新实例(使用new或malloc)并传递其指针。|1422199|entityMap^0|0|18|4|25|4|0|0|N|F|0^^$0|@$1|2|3|4|5|6|7|N|8|@]|9|@]|A|$]]|$1|B|3|C|5|6|7|O|8|@$D|P|E|Q|F|G]|$D|R|E|S|F|G]]|9|@]|A|$]]|$1|H|3|I|5|6|7|T|8|@]|9|@]|A|$]]|$1|J|3|K|5|6|7|U|8|@$D|V|E|W|F|G]]|9|@]|A|$]]|$1|L|3|-4|5|6|7|X|8|@]|9|@]|A|$]]]|M|$]]

I used the following approach:

in your JNI code, create a struct that would hold references to objects you need. When you first create this struct, return its pointer to java as a <code>long</code>. Then, from java you just call any method with this <code>long</code> as a parameter, and in C cast it to a pointer to your struct. 

The structure will be in the heap, so it will not be cleared between different JNI calls.

EDIT: I don't think you can use long ptr = <code>(long)&amp;address;</code> since address is a static variable. Use it the way Gunslinger47 suggested, i.e. create new instance of class or a struct (using new or malloc) and pass its pointer.

blocks|key|1207166|text|我知道这个问题已经得到了正式的回答，但我想添加我的解决方案:将指针放在Java数组中(索引为0)并将其传递给JNI，而不是尝试传递指针。JNI代码可以使用GetIntArrayRegion/SetIntArrayRegion获取和设置数组元素。|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|1207167|在我的代码中，我需要本地层来管理一个文件描述符(一个打开的套接字)。Java类保存一个int[1]数组，并将其传递给本机函数。本机函数可以对其执行任何操作(get/set)，并将结果放回数组中。|1207168|entityMap^0|25|H|2N|H|0|17|6|0^^$0|@$1|2|3|4|5|6|7|J|8|@$9|K|A|L|B|C]|$9|M|A|N|B|C]]|D|@]|E|$]]|$1|F|3|G|5|6|7|O|8|@$9|P|A|Q|B|C]]|D|@]|E|$]]|$1|H|3|-4|5|6|7|R|8|@]|D|@]|E|$]]]|I|$]]

I know this question was already officially answered, but I'd like to add my solution:
Instead of trying to pass a pointer, put the pointer in a Java array (at index 0) and pass that to JNI. JNI code can get and set the array element using <code>GetIntArrayRegion</code>/<code>SetIntArrayRegion</code>.

In my code, I need the native layer to manage a file descriptor (an open socket). The Java class holds a <code>int[1]</code> array and passes it to the native function. The native function can do whatever with it (get/set) and put back the result in the array.

blocks|key|1207212|text|最好完全按照Unsafe.allocateMemory的做法来做。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|1207213|创建您的对象，然后将其键入(uintptr_t)，这是一个32/64位无符号整数。|1207214|return+(uintptr_t)+malloc(50);

void+*+f+=+(uintptr_t)+jlong;|code-block|syntax|javascript|1207215|这是唯一正确的方法。|1207216|下面是Unsafe.allocateMemory所做的健全性检查。|1207217|inline+jlong+addr_to_java(void*+p)+{
++assert(p+==+(void*)(uintptr_t)p,+"must+not+be+odd+high+bits");
++return+(uintptr_t)p;
}

UNSAFE_ENTRY(jlong,+Unsafe_AllocateMemory(JNIEnv+*env,+jobject+unsafe,+jlong+size))
++UnsafeWrapper("Unsafe_AllocateMemory");
++size_t+sz+=+(size_t)size;
++if+(sz+!=+(julong)size+%7C%7C+size+<+0)+{
++++THROW_0(vmSymbols::java_lang_IllegalArgumentException());
++}
++if+(sz+==+0)+{
++++return+0;
++}
++sz+=+round_to(sz,+HeapWordSize);
++void*+x+=+os::malloc(sz,+mtInternal);
++if+(x+==+NULL)+{
++++THROW_0(vmSymbols::java_lang_OutOfMemoryError());
++}
++//Copy::fill_to_words((HeapWord*)x,+sz+/+HeapWordSize);
++return+addr_to_java(x);
UNSAFE_END|1207218|entityMap^0|0|0|0|0|0|0^^$0|@$1|2|3|4|5|6|7|Q|8|@]|9|@]|A|$]]|$1|B|3|C|5|6|7|R|8|@]|9|@]|A|$]]|$1|D|3|E|5|F|7|S|8|@]|9|@]|A|$G|H]]|$1|I|3|J|5|6|7|T|8|@]|9|@]|A|$]]|$1|K|3|L|5|6|7|U|8|@]|9|@]|A|$]]|$1|M|3|N|5|F|7|V|8|@]|9|@]|A|$G|H]]|$1|O|3|-4|5|6|7|W|8|@]|9|@]|A|$]]]|P|$]]

Its best to do this exactly how Unsafe.allocateMemory does.

Create your object then type it to (uintptr_t) which is a 32/64 bit unsigned integer.

<pre><code>return (uintptr_t) malloc(50);

void * f = (uintptr_t) jlong;
</code></pre>

This is the only correct way to do it.

Here is the sanity checking Unsafe.allocateMemory does.

<pre><code>inline jlong addr_to_java(void* p) {
 assert(p == (void*)(uintptr_t)p, "must not be odd high bits");
 return (uintptr_t)p;
}

UNSAFE_ENTRY(jlong, Unsafe_AllocateMemory(JNIEnv *env, jobject unsafe, jlong size))
 UnsafeWrapper("Unsafe_AllocateMemory");
 size_t sz = (size_t)size;
 if (sz != (julong)size || size &lt; 0) {
 THROW_0(vmSymbols::java_lang_IllegalArgumentException());
 }
 if (sz == 0) {
 return 0;
 }
 sz = round_to(sz, HeapWordSize);
 void* x = os::malloc(sz, mtInternal);
 if (x == NULL) {
 THROW_0(vmSymbols::java_lang_OutOfMemoryError());
 }
 //Copy::fill_to_words((HeapWord*)x, sz / HeapWordSize);
 return addr_to_java(x);
UNSAFE_END
</code></pre>

At the moment, i'm trying to create a Java-application which uses CUDA-functionality. The connection between CUDA and Java works fine, but i've got another problem and wanted to ask, if my thoughts about it are correct.

When i call a native function from Java, i pass some data to it, the functions calculates something and returns a result. Is it possible, to let the first function return a reference (pointer) to this result which i can pass to JNI and call another function that does further calculations with the result?

My idea was to reduce the overhead that comes from copying data to and from the GPU by leaving the data in the GPU memory and just passing a reference to it so other functions can use it.

After trying some time, i thought for myself, this shouldn't be possible, because pointers get deleted after the application ends (in this case, when the C-function terminates). Is this correct? Or am i just to bad in C to see the solution? 

Edit: 
Well, to expand the question a little bit (or make it more clearly): Is memory allocated by JNI native functions deallocated when the function ends? Or may i still access it until either the JNI application ends or when i free it manually?

Thanks for your input :)

Passing pointers between C and Java through JNI

Java

目前，我正在尝试创建一个使用CUDA功能的Java应用程序。CUDA和Java之间的连接工作得很好，但我有另一个问题，我想问一下，我的想法是否正确。当我从Java调用一个本机函数时，我将一些数据传递给它，这些函数计算一些东西并返回一个结果。有没有可能让第一个函数返回一个指向这个结果的引用(指针)，我可以将它传递给JNI...

问通过JNI在C和Java之间传递指针
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