Skip to content

README.md

Latest commit

 

History

History
107 lines (83 loc) · 3.11 KB

README.md

File metadata and controls

107 lines (83 loc) · 3.11 KB

Lab 4 Callbacks

The goal of this lab is fix a one line bug in the native code that needs to invoke a Java method.

This example is derived from the article at foojay.io Part 4 https://foojay.io/today/project-panama-for-newbies-part-4

Let's familiarize the steps before we go fix the bug.

To begin let's build and compile the native library and Java code. This is the following steps:

  • clean - Cleanup build artifacts
  • compile C - mylib.c
  • jextract - mylib.h
  • compile Java - PanamaCallback.java
  • run - PanamaCallback.class

Linux:

bash clean.sh
bash compile_mylib.c_linux.sh
bash jextract_linux.sh
bash compile_PanamaCallback.java.sh
bash run_PanamaCallback.java.sh

MacOS

bash clean.sh
bash compile_mylib.c_macos.sh
bash jextract_macos.sh
bash compile_PanamaCallback.java.sh
bash run_PanamaCallback.java.sh

Windows

clean_windows.cmd
compile_mylib.c_windows.cmd
jextract_windows.cmd
compile_PanamaCallback.java_windows.cmd
run_PanamaCallback.java_windows.cmd

Output shows the following:

[Java] Callbacks! Panama style
[C] Inside mylib's C function my_callback_function().
[C]   Now invoking Java's callMePlease() static method.
[JAVA] Inside callMePlease() method - I'm being called from C.
[C] Inside mylib's C function my_callback_function2().
[C]   Now invoking Java's doubleMe(int) static method.

So, the Java code below doubleMe() output isn't begin displayed. The bug is inside the native library in the function callback named my_callback_function2(). This function is in the file mylib.c.

    public static void doubleMe(int value) {
        MemorySegment cString = implicitAllocator()
                .allocateUtf8String("[JAVA] Inside doubleMe() method, %d times 2 = %d.\n".formatted(value, value*2));
        printf(cString);
    }

Expected Output

So something is wrong! It's missing the output statement from inside the doubleMe() Java method. It should be the following:

[Java] Callbacks! Panama style
[C] Inside mylib's C function my_callback_function().
[C]   Now invoking Java's callMePlease() static method.
[JAVA] Inside callMePlease() method - I'm being called from C.
[C] Inside mylib's C function my_callback_function2().
[C]   Now invoking Java's doubleMe(int) static method.
[JAVA] Inside doubleMe() method, 123 times 2 = 246.

Notice the last output statement is missing:

[JAVA] Inside doubleMe() method, 123 times 2 = 246.

Exercise 1 Fix the bug.

The my_callback_function2() in C isn't working, it's not invoking your Java code (static method). Look at the implementation the file mylib.c. The expected output should look like this:

Output shows the following:

[Java] Callbacks! Panama style
[C] Inside mylib's C function my_callback_function().
[C]   Now invoking Java's callMePlease() static method.
[JAVA] Inside callMePlease() method - I'm being called from C.
[C] Inside mylib's C function my_callback_function2().
[C]   Now invoking Java's doubleMe(int) static method.
[JAVA] Inside doubleMe() method, 123 times 2 = 246.

Expected output appended:

[JAVA] Inside doubleMe() method, 123 times 2 = 246.