About native method

[ygxinqing]

I want to create a new native method, it returns String:
for example:
public static native String myTest();

I want to get a String, for example, "Hello,world!"
In the nativeemul.c, how write it?
I write "push_word("Hello,world!");", but get a error: Segmentation fault
if I want to get an integer, this code is right. But if I want to get a string, this code is wrong, who can help me? Thanks very much!

[gloomyandy]

What is it you are trying to do? The code you have used will push the address of a C string on to the stack. This means nothing to Java, Java can not use addresses directly. Why do you need to create a native method, what will this method actually do?

[ygxinqing]

What is it you are trying to do? The code you have used will push the address of a C string on to the stack. This means nothing to Java, Java can not use addresses directly. Why do you need to create a native method, what will this method actually do?

Thanks for your reply.
I need to transplant the jvm to the platform of uclinux, so I must implement the file system. In reading text file, I want to get the string by line, so I write a new native method that return the string. Now, I think that it can return byte[], unfortunately, I don't konw how to write the code in nativeemlu.c. Can you help me? Thanks very much.

[skoehler]

What is it you are trying to do? The code you have used will push the address of a C string on to the stack. This means nothing to Java, Java can not use addresses directly. Why do you need to create a native method, what will this method actually do?

Thanks for your reply.
I need to transplant the jvm to the platform of uclinux, so I must implement the file system. In reading text file, I want to get the string by line, so I write a new native method that return the string. Now, I think that it can return byte[], unfortunately, I don't konw how to write the code in nativeemlu.c. Can you help me? Thanks very much.

I'm sorry, but that doesn't sound like you have worked with Java before. To access Files, implement FileInputStream and FileOutputStream. Then, use InputStreamReader/Writer to read/write characters. Use BufferedReader to have a readLine() method. Also, in Java, Strings are UTF-16 and also contain a length while you attempted to return a zero-terminated C-string with 8-Bits per character. Also, the UTF-16 character array and length have to wrapped in an instance of java.lang.String before a Java program can actually work with the String.

[ygxinqing]

What is it you are trying to do? The code you have used will push the address of a C string on to the stack. This means nothing to Java, Java can not use addresses directly. Why do you need to create a native method, what will this method actually do?

Thanks for your reply.
I need to transplant the jvm to the platform of uclinux, so I must implement the file system. In reading text file, I want to get the string by line, so I write a new native method that return the string. Now, I think that it can return byte[], unfortunately, I don't konw how to write the code in nativeemlu.c. Can you help me? Thanks very much.

I'm sorry, but that doesn't sound like you have worked with Java before. To access Files, implement FileInputStream and FileOutputStream. Then, use InputStreamReader/Writer to read/write characters. Use BufferedReader to have a readLine() method. Also, in Java, Strings are UTF-16 and also contain a length while you attempted to return a zero-terminated C-string with 8-Bits per character. Also, the UTF-16 character array and length have to wrapped in an instance of java.lang.String before a Java program can actually work with the String.

Thanks for your reply.
In uclinux platform, lejos doesn't implement the function of file reading and writing. So, I must write my own file system. In order to get the String of file, jvm must return string or byte[] in nativeemlu.c . I find the similar code: "push_word((STACKWORD) ptr2word(&display_array));", but the display_array isn't the normal array, it's a struct.

[skoehler]

I find the similar code: "push_word((STACKWORD) ptr2word(&display_array));", but the display_array isn't the normal array, it's a struct.

Yes, of course it's a struct, since in Java an array also has a length attribute. But actually you don't want to write a function that returns an array.
You want to implement something similar to the various read/write methods that leJOS has, which have an array, an offset, and a length as an argument.

[ygxinqing]

I find the similar code: "push_word((STACKWORD) ptr2word(&display_array));", but the display_array isn't the normal array, it's a struct.

Yes, of course it's a struct, since in Java an array also has a length attribute. But actually you don't want to write a function that returns an array.
You want to implement something similar to the various read/write methods that leJOS has, which have an array, an offset, and a length as an argument.

Thanks. I think I find the solution.

[gloomyandy]

In general it is not a good idea to return new objects from native methods in leJOS, Doing this requires allocation of storage which in turns means you need to make the method restart-able so that it can handle allocation failure and a gc run. I would also suggest that you may need to spend some time understanding how the leJOS VM works and how the various Java data types map onto C versions.

As Sven has suggested you are probably better off allocating the space in Java and then using some sort of read method to obtain the data from the VM...