使用长从JNI指针传递到Java [英] Passing a pointer from JNI to Java using a long

问题描述

我想传递一个结构，从JNI到Java的指针，以便能够从Java后传回JNI。我已阅读本主题：<一href=\"http://stackoverflow.com/questions/1632367/passing-pointers-between-c-and-java-through-jni\">Passing通过JNI C和Java之间的指针，但我没有成功。

我有一个pretty复杂的结构：结构myStruct_s MYSTRUCT;

从Java，我称之为JNI函数初始化结构和长（指针结构）返回：

  JNIEXPORT jlong​​ JNICALL Java_example_ExampleJNI_getStruct（JNIEnv的* jenv，JCLASS JCLS）{
    结构myStruct_s MYSTRUCT;
    长* LP =（长*）及MYSTRUCT;
    返回LP;
}
 

然后我说法调用Java方法，那么久。在JNI我希望能够使用之前创建的晶格结构。我这样做：

  JNIEEXPORT jint JNICALL Java_example_ExampleJNI_methode1（JNIEnv的* jenv，JCLASS JCLS，jlong​​ jarg）{
    结构myStruct_s * ARG =（结构myStruct_s *）及jarg;
    ...
}
 

那么这是行不通的。我想我长的投入结构是错误的。我应该怎么办呢？
谢谢你。

---

修改：谢谢你的提示，这里的工作职能

  JNIEXPORT jint JNICALL Java_example_ExampleJNI_methode1（JNIEnv的* jenv，JCLASS JCLS，jlong​​ jarg）{
    结构myStruct_s * ARG;
    ARG =（结构myStruct_s *）jarg;
    ...
}JNIEXPORT jlong​​ JNICALL Java_example_ExampleJNI_getStruct（JNIEnv的* jenv，JCLASS JCLS）{
    结构myStruct_s * MYSTRUCT;
    MYSTRUCT =（结构myStruct_s *）malloc的（的sizeof（结构myStruct_s））;
    长LP =（长）MYSTRUCT;
    返回LP;
}
 

解决方案

在您的例子

 结构myStruct_s MYSTRUCT;
 

在堆叠中的局部变量，因此在函数返回之后不可用。 Possubly这只是你的code的切割下来，但如果没有则使用一个malloc（sizeof的（结构myStruct_s）），以让自己的堆分配。

这则引发时，你要释放的分配，注意内存泄漏的问题。

I'm trying to pass a structure as a pointer from JNI to Java to be able to pass it back later from Java to JNI. I have read this thread: Passing pointers between C and Java through JNI, but I did not succeed.

I have a pretty complex structure : struct myStruct_s myStruct;

From Java, I call a JNI function to initialize the structure and to return a long (pointer to the structure):

JNIEXPORT jlong JNICALL Java_example_ExampleJNI_getStruct(JNIEnv *jenv, jclass jcls) {
    struct myStruct_s mystruct;
    long *lp = (long*)&myStruct;
    return lp;
}

Then I call a Java method with that long in argument. In JNI I want to be able to use the strcuture created earlier. I do like this:

JNIEEXPORT jint JNICALL Java_example_ExampleJNI_methode1(JNIEnv *jenv, jclass jcls, jlong jarg) {
    struct myStruct_s *arg = (struct myStruct_s *)&jarg;
    ...
}

Well it doesn't work. I guess my cast of the long into the struct is wrong. How should I do it? Thank you.

---

EDIT : Thanks for the hints, here are the working functions

JNIEXPORT jint JNICALL Java_example_ExampleJNI_methode1(JNIEnv *jenv, jclass jcls, jlong jarg) {
    struct myStruct_s *arg;
    arg = (struct myStruct_s *)jarg;
    ...
} 

JNIEXPORT jlong JNICALL Java_example_ExampleJNI_getStruct(JNIEnv *jenv, jclass jcls) {
    struct myStruct_s *myStruct;
    myStruct = (struct myStruct_s *)malloc(sizeof(struct myStruct_s));
    long lp = (long)myStruct;
    return lp;
}

解决方案

In your example

struct myStruct_s mystruct;

is a local variable on the stack, and therefore not available after the function returns. Possubly that's just a cut-down of your code, but if not then use a malloc(sizeof(struct myStruct_s)) to get yourself a heap allocation.

That then raises the question of when you are going to free that allocation, watch out for memory leaks.

这篇关于使用长从JNI指针传递到Java的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！