Question

I am trying to call a method which is in a C DLL through Java. But I am not getting any output nor any exceptions. Below is my C code.

typedef void (*rfid_data)(
    uint32_t data_id,
    uint32_t data_length,
    char *data  );


RFIDLIB_API uint32_t init(
    char *csvParams                 /*[in]*/,
    uint32_t length                 /*[in]*/,
    rfid_data callbackFunction);

Shown below is my JAva Interface related to the callback.

public interface rfid_data extends Callback{

public void callback(int dataId,int dataLength,Pointer data);

}

Below is my library interface.

public interface rfidlib extends Library{

public int init(Pointer csvParams,int length,rfid_data callbackFunction);

}

Shown below is my main method from which I am calling the C code.

public class MainClass {

public rfidlib rlib;

public static void main(String[] args) {
    MainClass mainClass = new MainClass();

    mainClass.rlib = (rfidlib) Native.loadLibrary("rfidlib", rfidlib.class);
    rfid_data rfidData = new rfid_data() {
        @Override
        public void callback(int dataId, int dataLength, Pointer data) {
            byte[] buffer = data.getByteArray(0,dataLength);
            System.out.println("Data Id is : " + dataId + "Data Length is : " + dataLength + " data is : " + new String(buffer));

        }
    };
    String comPort = "COM4";
    Pointer m = new Memory(comPort.length()+1);
    m.setString(); 
    int result1 = mainClass.rlib.init(m, 100, rfidData);
    System.out.println("result is : "+result1);


}

When I run the code nothing gets printed. Not even the string "result is". What am I doing wrong here? Please advice.

Answer 1

char*隐含Pointer，而不是String。你的回调是接收数据的指针，可能是一个字节数组。除非你的文档另有说明，否则你不应该假设传入的数据是一个NUL终止的C字符串（因为传入的长度，我非常怀疑它是一个可打印的字符串）。如果您确定是C字符串，则使用函数Pointer.getString(0)将其转换为Java字符串。

public interface rfid_data extends Callback{
    public void callback(int dataId, int dataLength, Pointer data);
}

rfid_data cb = new rfid_data() {
    public void callback(int dataId, int dataLength, Pointer data) {
        byte[] buffer = data.getByteArray(0, dataLength);
        // Alternatively, if the incoming data is a NUL-terminated C string
        String s = data.getString(0);
        // ...
    }
};

修改

如果RFIDLIB_API宏包含stdcall（可能），那么您的图书馆也应该实施StdCallLibrary和您的回调StdCallCallback。

可以在github project找到最新的JNA文档。

why am I not getting any output when a method with a callback is called?

1 个答案: