将指针传递到指针，以从 Java 通过 JNA 传递到 C 动态库的浮点数

Question

Catboost提供了一个动态C库，理论上可以从任何编程语言中使用。

我正在尝试使用 JNA 通过 Java 调用它。

我在头文件中定义的CalcModelPrediction函数出现问题，如下所示：

EXPORT bool CalcModelPrediction(
ModelCalcerHandle* calcer,
size_t docCount,
const float** floatFeatures, size_t floatFeaturesSize,
const char*** catFeatures, size_t catFeaturesSize,
double* result, size_t resultSize);

在 Java 中，我定义接口函数如下：

public interface CatboostModel extends Library {
public Pointer ModelCalcerCreate();
public String GetErrorString();
public boolean LoadFullModelFromFile(Pointer calcer, String filename);
public boolean CalcModelPrediction(Pointer calcer, int docCount,
PointerByReference floatFeatures, int floatFeaturesSize,
PointerByReference catFeatures, int catFeaturesSize,
Pointer result, int resultSize);
public int GetFloatFeaturesCount(Pointer calcer);
public int GetCatFeaturesCount(Pointer calcer);
}

然后我这样称呼它：

CatboostModel catboost;
Pointer modelHandle;
catboost = Native.loadLibrary("catboostmodel", CatboostModel.class);
modelHandle = catboost.ModelCalcerCreate();
if (!catboost.LoadFullModelFromFile(modelHandle, "catboost_test.model"))
{
throw new RuntimeException("Cannot load Catboost model.");
}
final PointerByReference ppFloatFeatures = new PointerByReference();
final PointerByReference ppCatFeatures = new PointerByReference();
final Pointer pResult = new Memory(Native.getNativeSize(Double.TYPE));
float[] floatFeatures = {0.5f, 0.8f, 0.3f, 0.3f, 0.1f, 0.5f, 0.4f, 0.8f, 0.3f, 0.3f} ;
String[] catFeatures = {"1", "2", "3", "4"};
int catFeaturesLength = 0;
for (String s : catFeatures)
{
catFeaturesLength += s.length() + 1;
}
try
{
final Pointer pFloatFeatures = new Memory(floatFeatures.length * Native.getNativeSize(Float.TYPE));
for (int dloop=0; dloop<floatFeatures.length; dloop++) {
pFloatFeatures.setFloat(dloop * Native.getNativeSize(Float.TYPE), floatFeatures[dloop]);
}
ppFloatFeatures.setValue(pFloatFeatures);
final Pointer pCatFeatures = new Memory(catFeaturesLength * Native.getNativeSize(Character.TYPE));
long offset = 0;
for (final String s : catFeatures) {
pCatFeatures.setString(offset, s);
pCatFeatures.setMemory(offset + s.length(), 1, (byte)(0));
offset += s.length() + 1;
}
ppCatFeatures.setValue(pCatFeatures);
}
catch (Exception e)
{
throw new RuntimeException("Couldn't initialize parameters for catboost");
}
try
{
if (!catboost.CalcModelPrediction(
modelHandle,
1,
ppFloatFeatures, 10,
ppCatFeatures, 4,
pResult, 1
))
{
throw new RuntimeException("No prediction made: " + catboost.GetErrorString());
}
else
{
double[] result = pResult.getDoubleArray(0, 1);
log.info("Catboost prediction: " + String.valueOf(result[0]));
Assert.assertFalse("ERROR: Result empty", result.length == 0);
}
}
catch (Exception e)
{
throw new RuntimeException("Prediction failed: " + e);
}

我尝试将Pointer、PointerByReference和Pointer[]传递给CalcModelPrediction函数来代替float **floatFeatures和char ***catFeatures但没有任何效果。我总是得到一个分段错误，大概是当CalcModelPrediction函数试图通过调用floatFeatures[0][0]和catFeatures[0][0]来获取floatFeatures和catFeatures的元素时。

所以问题是，将多维数组从Java通过JNA传递到C的正确方法是什么，在那里它可以被视为指向值的指针？

有趣的是，仅接受float **floatFeatures然后简单地调用*floatFeatures的CalcModelPredictionFlat函数在传递PointerByReference时工作得很好。

更新 - 5.5.2018

第 1 部分

在尝试通过稍微修改原始 Catboost .cpp 和 .h 文件并重新编译 libcatboost.so 库来调试段错误后，我发现段错误是由于我将 C 语言中的size_t映射到 Java 中的int。修复此问题后，我在 Java 中的接口函数如下所示：

public interface CatboostModel extends Library {
public boolean LoadFullModelFromFile(Pointer calcer, String filename);
public boolean CalcModelPrediction(Pointer calcer, size_t docCount,
Pointer[] floatFeatures, size_t floatFeaturesSize,
String[] catFeatures, size_t catFeaturesSize,
Pointer result, size_t resultSize);
}

其中size_t类定义如下：

public static class size_t extends IntegerType {                                           
public size_t() { this(0); }                                                           
public size_t(long value) { super(Native.SIZE_T_SIZE, value); }                       
}

第 2 部分进一步研究 Catboost 代码，我注意到**floatFeatures由行访问，如floatFeatures[i]，而***catFeatures则由行和列访问，如catFetures[i][catFeatureIdx]。

在将Java中的floatFeatures更改为Pointer数组后，我的代码开始使用没有分类特征的模型训练，即catFeatures长度为零。

但是，此技巧不适用于通过双下标运算符[i][catFeatureidx]访问catFeatures。所以现在，我修改了原始的 Catboost 代码，以便它接受char **catFeatures- 一个字符串数组。在Java接口函数中，我设置了String[] catFeatures。现在我可以一次对一个元素进行预测，这并不理想。

Answer 1

我已经设法使它与原始的Catboost代码一起工作，并libcatboost.so。

Java 接口函数是这样定义的。请注意，为了模拟浮点值和字符串的 2D 数组(或指向指针的指针(，我使用的是Pointer[]类型。

public interface CatboostModel extends Library {
public boolean LoadFullModelFromFile(Pointer calcer, String filename);
public boolean CalcModelPrediction(Pointer calcer, size_t docCount,
Pointer[] floatFeatures, size_t floatFeaturesSize,
Pointer[] catFeatures, size_t catFeaturesSize,
Pointer result, size_t resultSize);
}

之后，我填充floatFeatures并像这样catFeatures参数(这里有一些虚拟数据(。请注意，对于字符串，我使用的是 JNA 的StringArray。

float[] floatFeatures = {0.4f, 0.8f, 0.3f, 0.3f, 0.1f, 0.5f, 0.4f, 0.8f, 0.3f, 0.3f} ;
String[] catFeatures = {"1", "2", "3", "4"};
final Pointer pFloatFeatures = new Memory(floatFeatures.length * Native.getNativeSize(Float.TYPE));
final Pointer[] ppFloatFeatures = new Pointer[2];
for (int dloop=0; dloop<10; dloop++) {
pFloatFeatures.setFloat(dloop * Native.getNativeSize(Float.TYPE), floatFeatures[dloop]);
}
ppFloatFeatures[0] = pFloatFeatures;
ppFloatFeatures[1] = pFloatFeatures;
final Pointer[] ppCatFeatures = new Pointer[catFeatures.length];
final Pointer pCatFeatures = new StringArray(catFeatures);
ppCatFeatures[0] = pCatFeatures;
ppCatFeatures[1] = pCatFeatures;

最后，我将这些参数传递给 Catboost：

if (!catboost.CalcModelPrediction(
modelHandle,
new size_t(2L),
ppFloatFeatures, new size_t((long)floatFeatures.length),
ppCatFeatures, new size_t((long)catFeatures.length),
pResult, new size_t(2L)
))
{
throw new RuntimeException("No prediction made: " + catboost.GetErrorString());
}

为了获得预测，我们可以执行以下操作：

double[] result = pResult.getDoubleArray(0, 2);