SystemsProgrammingAndOperatingSystem/Codes/Group A/Assignment-A3/EXPLANATION.md

Explanation

This file contains explanation for dynamic linking library program (Assignment-A3)

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Please note the files referred in this guide:

• A3.java
• A3.c
• A3.h

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• We're using Java for writing the main program.
• To demonstrate Dynamic Linking Library (DLL) in Java, we'll be declaring functions in Java and implementing them in C (can be C/C++).
• First, we create a Java program A3.java,
  • This is the main Java file containing function definition and the main function.
  • Functions (add, sub, mul, div) in A3 class in this file are native functions, meaning their body is written in C/C++ in a different file.
• After creating this file, you need to compile it. To do so, run javac A3.java (assuming you're already in the directory that contains this file).
• Now, we will generate the header file. For this, run javac -h . A3.java.
• There will be a new file called A3.h in your current working directory,
  • This is the header file.
  • It contains signatures for native functions we created in the Java file.
  • Thus, there's no need to memorized boilerplate in A3.c since the functions defined in that file can be found in the header file. I have included the A3.h file in this folder for reference. Note that it is automatically generated.
• Create a new A3.c file which is the C program file containing function definitions (for add, sub, mul, div)
  • Define all the functions (add, sub, mul, div)
• Then, we have to compile the C program file, i.e. A3.c. For this, run gcc -shared -o libA3.so -fPIC -I"$JAVA_HOME/include" -I"$JAVA_HOME/include/linux" A3.c,
  • gcc -> GNU compiler for C program
  • -shared -> tells the compiler to create a shared file (.so) instead of a regular executable file
  • -o libA3.so -> tells the compiler to save the output to libA3.so file
  • fPIC -> stands for Position-Independent Code. Needed for creating shared libraries.
  • -I"$JAVA_HOME/include" and -I"$JAVA_HOME/include/linux" -> -I flag used for specifiying directories to include. Values in double quotes are directories
  • A3.c -> name of the C program file to compile
• Lastly, run the program using java -Djava.library.path=. A3
  • java -> Loads Java Virtual Machine (JVM)
  • -Djava.library.path=. -> -D is used to set a system property. In this case, we’re setting the java.library.path (for .so or .dll files) property.
  • A3 -> name of the Java class containing the main method

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