Casting a C++ long type to a JNI jlong

Question

I am using JNI to pass data between C++ and Java. I need to pass a 'long' type, and am doing so using something like:

 long myLongVal = 100;
 jlong val = (jlong)myLongVal;
 CallStaticVoidMethod(myClass, "(J)V", (jvalue*)val);

However in Java, when the 'long' parameter is retrieved, it gets retrieved as some very large negative number. What am I doing wrong?

Solution 1

When you pass a jlong (which is 64 bit) as a pointer (which is, most likely, 32-bit) you necessarily lose data. I'm not sure what's the convention, but try either this:

CallStaticVoidMethodA(myClass, "(J)V", (jvalue*)&val); //Note address-of!

or this:

CallStaticVoidMethod(myClass, "(J)V", val); 

It's ...A methods that take a jvalue array, the no-postfix methods take C equivalents to scalar Java types.

The first snippet is somewhat unsafe; a better, if more verbose, alternative would be:

jvalue jv;
jv.j = val;
CallStaticVoidMethodA(myClass, "(J)V", &jv);

On some exotic CPU archtectures, the alignment requirements for jlong variables and jvalue unions might be different. When you declare a union explicitly, the compiler takes care of that.

Also note that C++ long datatype is often 32-bit. jlong is 64 bits, on 32-bit platforms the nonstandard C equivalent is long long or __int64.

Solution 2

CallStaticVoidMethod(myClass, "(J)V", (jvalue*)val);

This is undefined behaviour. You are casting an integer to be a pointer. It is not a pointer. You need, at the very least, to pass the address. This code would on most platforms instantly crash.

Comments

[Drew Hall]

"C++ long is 32-bit too"--true on some architectures, not on others. x64 linux usually uses 64-bit long. long long isn't available everywhere either (it's an extension IIRC).

[Seva Alekseyev]

True, that. I'll add a qualification.

[lost_bits1110]

Thanks for your answer - unfortunately I've tried both suggseted approaches and the parameter still appears garbled when retrieved in Java, I also tried with int and float types in addition to long :( Strangely the following does work, and only for int types (though it doesn't make sense) int someInt = 100; CallStaticVoidMethod(myClass, "(I)V", (jvalue*)(jint)someInt );

[lost_bits1110]

you're absolutely right - I should be taking the address, however this still does not work for me (regardless of float, int or long type, as I tried all of these). Strangely, the following works, and only for int types (though it doesn't make any sense) int someInt = 100; CallStaticVoidMethod(myClass, "(I)V", (jvalue*)(jint)someInt );

[lost_bits1110]

DOH! It turns out I was not using CallStaticVoidMethodA, but CallStaticVoidMethod. Appending the 'A' solves the problem. Thank-you!