通过JNI在C和Java之间的指针 [英] Passing pointers between C and Java through JNI

问题描述

我一直在这个岗位<随后的@tulskiy的建议通过JNI存储在Java中C指针href=\"http://stackoverflow.com/questions/1632367/passing-pointers-between-c-and-java-through-jni/1634310#1634310\">Passing通过JNI C和Java之间的指针

I've been storing c pointers in Java through JNI by following the advice of @tulskiy in this post Passing pointers between C and Java through JNI

关键是要投的指针作为jlong​​。因此，从三我有收益率（jlong​​）PTR;

The trick is to cast the pointer as a jlong. So from c I have return (jlong) ptr;

我返回jlong​​（总是64位），因为我希望我的code到两个64位和32位系统上运行。在64位计算机上的64位指针的存储器的大小为64，它遵循一个32位的计算机上，在存储器中的指针的大小是32位。

I'm returning a jlong (always 64 bits) because I want my code to work on both 64 and 32 bit systems. The size in memory of a 64 bit pointer on a 64 bit computer is 64 and it follows that on a 32-bit computer, the size of the pointer in memory is 32 bits.

问题是32位机器上，我得到一个编译器警告说：来自不同大小的指针转换为整数。该警告消失，如果我有
收益率（jlong​​）（int32_t）PTR; 然而，这code是不好的64位机

The problem is that on the 32 bit machine I get a compiler warning saying "casting to integer from pointer of different size." The warnings goes away if I have return (jlong) (int32_t) ptr; However this code is not good for the 64 bit machine.

我想有我的code编译没有警告，所以，如果有一个合法的警告，我会看到它。任何人有什么想法？

I would like to have my code compile without warnings so that if there is a legitimate warning I will see it. Anyone have any ideas?

谢谢，
本

推荐答案

有在各种C.方便的整数类型你想要的可能是使用intptr_t和uintptr_t形式：

There are various handy integer types in C. The one you want is probably intptr_t or uintptr_t:

return (jlong)(intptr_t) ptr;

的区别？

• 从使用intptr_t 铸造到 jlong​​ 和背部是保证工作提供了 jlong​​ 足够大（这你隐含假设它反正）。


• 从 uinttptr_t 铸造到 jlong​​ 和背部避免了符号扩展，但未定义的行为，如果 uintptr_t形式太大，以适应在 jlong​​



• Casting from intptr_t to jlong and back is guaranteed to work provided jlong is big enough (which you're implicitly assuming it is anyway).
• Casting from uinttptr_t to jlong and back avoids a sign-extension, but is undefined behaviour if the uintptr_t is too big to fit in a jlong (but all "sane" architectures/compilers just use two's complement arithmetic)

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