Casting a C++ long type to a JNI jlong
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 retrie开发者_运维技巧ved, it gets retrieved as some very large negative number. What am I doing wrong?
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
.
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.
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