Shared memory between C++ and Java processes

c++, java, java-native-interface, shared-memory

Solution

If you have shared memory, for example using `CreateFileMapping` (Windows) or `shmget` (Unix), all you need is a native method on the Java side. Then you can create a `ByteBuffer` that directly accesses the shared memory using `NewDirectByteBuffer` like this:

JNIEXPORT jobject JNICALL Java_getSharedBuffer(JNIEnv* env, jobject caller) {
    void* myBuffer;
    int bufferLength;

Now you have to get a pointer to the shared memory. On Windows you would use something like this:

    bufferLength = 1024; // assuming your buffer is 1024 bytes big
    HANDLE mem = OpenFileMapping(FILE_MAP_READ, // assuming you only want to read
           false, "MyBuffer"); // assuming your file mapping is called "MyBuffer"
    myBuffer = MapViewOfFile(mem, FILE_MAP_READ, 0, 0, 0);
    // don't forget to do UnmapViewOfFile when you're finished

Now you can just create a `ByteBuffer` that is backed by this shared memory:

    // put it into a ByteBuffer so the java code can use it
    return env->NewDirectByteBuffer(myBuffer, bufferLength);
}

Problem

My aim is to pass data from a C++ process to a Java process and then to receive a result back. I have achieved this via a named pipe but I would prefer to share the data rather than passing or copying it, assuming the access would be faster. Initially, I thought of creating a shared segment in C++ that I could write to and read with Java, but I'm not sure if this is possible via JNI, let alone safe. I believe it's possible in Java to allocate the memory using ByteBuffer.allocateDirect and then use GetDirectBufferAddress to access the address in C++, but if I'm correct this is for native calls within JNI and I can't get this address in my C++ process? Lost. Many thanks in advance.

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