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Today, I will talk to you about how to increase the system call API in KVM, which may not be well understood by many people. In order to make you understand better, the editor has summarized the following for you. I hope you can get something according to this article.

Different j2me devices also have different system call API and their implementation.

J2ME KVM tutorial on how to add system call API in KVM

As we all know, KVM does not support Native function calls, so if you want to add some API of system calls, you can only add it to the KVM. At the same time, different j2me devices also have different system call API and their implementation. How can we add a system call API to the original KVM source code when we download it from SUN? This paper explains the method of adding KVM system API step by step with the concrete practice step by step.

In fact, adding a system call API to KVM is easier than adding a system call API to linux. The work is roughly divided into two steps. One step is to add your own new class to classes.zip, and the other is to implement the nativeapi function of the new class in KVM's nativeCore.c.

Let's take org.test.MiniTest, a new class, as an example to implement a TestInt () system call function. The function is simple enough to return an integer of 9999.

1. New org.test.MiniTest class

After downloading the j2me_cldc1.1 version of KVM code from SUN. Under the j2me_cldcapi directory, add the package directory of orgestMiniTest.Java and the java file. Then write the following code:

Packageorg.test; publicclassMiniTest {publicstaticnativeintTestInt ();}

two。 Compile * times

According to the KVM compilation method in the previous article, at the command prompt, skip to the directory j2me_cldcuildwin32, and type the make command to compile for * times. However, this compilation process will prompt an error when compiling the * .o file in the link KVM to find the _ Java_org_test_MiniTest_TestInt symbol.

This is because we only defined the native function TestInt in org.test.MiniTest, but did not implement its corresponding function in any of the c files of KVM.

First of all, the compilation process is to compile all the * .java files in j2me_cldc/api with javac, then package the class files into a classes.zip, and then JCC the tool will generate ROMJavaWin.c and nativeFunctionTableWin.c based on classes.zip by default. Instead, declare the native function in ROMJavaWin.c:

ExternvoidJava_org_test_MiniTest_TestInt (void)

3. Implement the Java_org_test_MiniTest_TestInt function

As you can see from the code in KVM, KVM defaults to putting some native functions in the nativeCore.c file. You can also add some C program files yourself, but this example puts the Java_org_test_MiniTest_TestInt in the nativeCore.c file.

Here is the code:

VoidJava_org_test_MiniTest_TestInt (void) {pushStack (9999);}

It is hard to say why the return value is returned using the macro pushStack. The way JAVA runs is actually a stack, and the bytecode of Java is actually a stack language. The intermediate code generation chapter in the compilation principle can find its prototype and its description. In addition, you can also read the book InsideJavaVirtualMachine.

4. Second compilation

The second compilation is that you can generate a real kvm.exe file. In the same way as the * compilation and the previous compilation of KVM, just type in the make command.

5. Test the API of MiniTest.TestInt

I wrote a Test class to test the API:

Importorg.test.*; classTest {publicstaticvoidmain (String [] args) {System.out.println ("TestResult=" + MiniTest.TestInt ());}}

When compiling with javac, you need to copy the previous j2me_cldc/classes.zip and execute:

Javac-classpathclasses.zipTest.java kvm-classpath.Test after reading the above, do you have any further understanding of how to add system call API in KVM? If you want to know more knowledge or related content, please follow the industry information channel, thank you for your support.

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