How to get a Java string from a char pointer in C++

Question

I am porting the openvr sample to jogl, after we created the binding with jna.

Almost at the end (before rendering the controllers and the tracking base stations), I got stuck trying to translate a char pointer in C to a String in Java.

C++ code here:

//-----------------------------------------------------------------------------
// Purpose: Helper to get a string from a tracked device property and turn it
//          into a std::string
//-----------------------------------------------------------------------------
std::string GetTrackedDeviceString( vr::IVRSystem *pHmd, vr::TrackedDeviceIndex_t unDevice, vr::TrackedDeviceProperty prop, vr::TrackedPropertyError *peError = NULL )
{
    uint32_t unRequiredBufferLen = pHmd->GetStringTrackedDeviceProperty( unDevice, prop, NULL, 0, peError );
    if( unRequiredBufferLen == 0 )
        return "";

    char *pchBuffer = new char[ unRequiredBufferLen ];
    unRequiredBufferLen = pHmd->GetStringTrackedDeviceProperty( unDevice, prop, pchBuffer, unRequiredBufferLen, peError );
    std::string sResult = pchBuffer;
    delete [] pchBuffer;
    return sResult;
}

GetStringTrackedDeviceProperty here:

/** Returns a string property. If the device index is not valid or the property is not a string type this function will 
* return 0. Otherwise it returns the length of the number of bytes necessary to hold this string including the trailing
* null. Strings will generally fit in buffers of k_unTrackingStringSize characters. */
virtual uint32_t GetStringTrackedDeviceProperty( vr::TrackedDeviceIndex_t unDeviceIndex, ETrackedDeviceProperty prop, VR_OUT_STRING() char *pchValue, uint32_t unBufferSize, ETrackedPropertyError *pError = 0L ) = 0;

Where VR_OUT_STRING() is defined here as:

# define VR_CLANG_ATTR(ATTR)

#define VR_OUT_STRING() VR_CLANG_ATTR( "out_string: ;" )

I have already done something similar where I had to call a function that expect the pointer to an array of TrackedDevicePose_t structures:

private TrackedDevicePose_t.ByReference trackedDevicePosesReference = new TrackedDevicePose_t.ByReference();
public TrackedDevicePose_t[] trackedDevicePose
            = (TrackedDevicePose_t[]) trackedDevicePosesReference.toArray(VR.k_unMaxTrackedDeviceCount);

I created first the reference and then from it the actual array.

But here I can't have a class extending the char array..

private String getTrackedDeviceString(IVRSystem hmd, int device, int prop, IntBuffer propError) {

    int requiredBufferLen = hmd.GetStringTrackedDeviceProperty.apply(device, prop, Pointer.NULL, 0, propError);

    if(requiredBufferLen == 0) {
        return "";
    }


    CharArray.ByReference charArrayReference = new CharArray.ByReference();
    char[] cs = charArrayReference.toArray(requiredBufferLen);

    return null;
}

Where apply (here) is:

public interface GetStringTrackedDeviceProperty_callback extends Callback {

    int apply(int unDeviceIndex, int prop, Pointer pchValue, int unBufferSize, IntBuffer pError);
};

CharArray class, crap attempt here

Any ideas?

Your code is C++, not C. These are different languages, and the differences matter. — John Bollinger
– John Bollinger, Commented Aug 30, 2016 at 13:04
Moreover, I don't see where Java comes into it. If you mean to interface directly with Java then that requires JNI, and I see no JNI anywhere. If you have a looser interaction in mind, such as via an intermediary file or pipe, then those details are important. — John Bollinger
– John Bollinger, Commented Aug 30, 2016 at 13:07
Yep, you are right John, thanks, I just fixed it. Java comes into with the binding, I also added that right now. — elect
– elect, Commented Aug 30, 2016 at 13:11
As an aside -- pHmd->GetStringTrackedDeviceProperty -- If the implementation of this function throws, your GetTrackedDeviceString has a memory leak due to the new char[ unRequiredBufferLen ]; — PaulMcKenzie
– PaulMcKenzie, Commented Aug 30, 2016 at 13:17
JNA automatically handles conversion from NUL-terminated C strings into Java String. It also provides Native.toString() to manually perform conversions. — technomage
– technomage, Commented Aug 31, 2016 at 10:47

jonhopkins · Accepted Answer · 2016-08-30 13:21:41Z

1

I've done some porting of C and C++ code to Java, and while it's probably horribly hacky, the best I've come up with to solve cases where a pointer to an int primitive or a char*/String is needed for a function call, is to create a small wrapper class with a single property, pass that object into the function, change the property as needed, and retrieve the new value after the function call. So something like:

public class StringPointer {
    public String value = "";
}

StringPointer pchBuffer = new StringPointer();
unRequiredBufferLen = pHmd.GetStringTrackedDeviceProperty( unDevice, prop, pchBuffer, unRequiredBufferLen, peError );
String sResult = pchBuffer.value;

and inside GetStringTrackedDeviceProperty()

...
pchValue.value = "some string";
...

In this case, you can use a String, since that's what your code is doing with the char* after the function call, but if it actually really needs to be a char[], you can just create char[] pchBuffer = new char[unRequiredBufferLen]; and pass that into the function. It will be just like you were using a char* in C++, and any changes you make inside the array will be visible after the function ends, and you can even do String sResult = new String(pchBuffer);.

edited Aug 30, 2016 at 13:21

answered Aug 30, 2016 at 13:19

jonhopkins

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6 Comments

elect Over a year ago

Interesting idea, I'll give it a try

elect Over a year ago

So, about the first method, I can't to that because GetStringTrackedDeviceProperty is of type interface GetStringTrackedDeviceProperty_callback that extends Callback and therefore it cannot have any body. However I also tried your second tip, by changing the argument from jna Pointer to char[], it works but I get a weird result, 桬扟獡獥慴楴湯癟癩e, on C++ I get lighthouse...

jonhopkins Over a year ago

The C++ part is probably using single-byte characters. If you break each char down into its individual bytes, does that get you the correct answer?

elect Over a year ago

Switching from char to byte buffer worked, thanks dear!

elect Over a year ago

Ps: let me just add this, using jna Native.toString(buffer) instead of new String(buffer) removes automatically any ending space

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