Copying Data from Java to C++ Objects Array Using JNI

Question

I have Java code that requires heavy calculations which I would like to forward into C++ using JNI.
My main concern is having all the data serialized in memory, and next to forward the computation to GPU.
Since the data is received in Java, but the main calculations are done using C++, I thought of arranging all the data continuously in a raw array (ByteBuffer or raw bytes from Unsafe), in the same structure as the C++ object.
For example, suppose I have a point with x and y. In C++ the object has a size of 24 bytes. 8 bytes for (I guess) VTable, 8 bytes for x and 8 bytes for y.
So in Java, I would arrange all the data in the same structure and pass the buffer to C++ using JNI, and in C++ cast it to an array of points.

This worked fine, and I am allowing myself to assume that I will always use the same C++ compiler, same JDK, same OS and same HW (at least for testing the feasibility of the solution).

My question is if these assumptions are correct, or there is a better way to pass serialized data between Java and C++ (I must use JNI and not some kind of IPC)?

JNI/JNA is the only way to invoke a function written in C++. With Unsafe you can allocate a native offheap memory and pass it into the JNI function so in C you simply cast it to void* — Some Name
– Some Name, Commented Feb 18, 2019 at 22:32
The main question here is if i can rely on the C++ structure (offset, alignment of the fields, etc.), so I can fill the buffer accordingly, and in the JNI call, just cast it to Point *? — Guy Yafe
– Guy Yafe, Commented Feb 19, 2019 at 6:09

Some Name · Accepted Answer · 2019-02-21 09:29:25Z

1

if I can rely on the C++ structure (offset, alignment of the fields, etc.)

No, unless you know what your compiler on your specific platform will do in that case. It yields undefined behavior.

Aliasing the content of ByteBuffer (aka char *) with Point * to later access its members is not possible in idiomatic C. Take a look at the C Standard N1570 6.5 (p7):

An object shall have its stored value accessed only by an lvalue expression that has one of the following types:88)

— a type compatible with the effective type of the object,

Assuming you know that void * returned by GetDirectBufferAddress is itself returned by a call to malloc(size) or friends (it actually uses malloc) where size_t size = sizeof(struct Point) than you can cast it to Point * initialize its member from native code and later use it. That would be a conforming way (one of).

edited Feb 21, 2019 at 9:29

answered Feb 21, 2019 at 7:27

Some Name

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2 Comments

Guy Yafe Over a year ago

What about the opposite way: Allocating an array of Point * in C++ and sending it to Java, and then filling this array, when assuming I know the correct offsets?

Some Name Over a year ago

@GuyYafe Sure, the Standard allows you to copy the object representation (aka char[sizeof(Point)]) and do with them what you need (assuming you know what this object representation means). But if you modify this object representation and will send it back to native code you probably will get UB (I need to dig in the Standard for normative reference). Anyway volatile modifier (in C, not in Java) is your friend in that case.

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