Added explanation for A3 (dll) code.

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

@@ -0,0 +1,38 @@
+# Explanation
+
+This file contains explanation for dynamic linking library program (Assignment-A3)
+
+---
+
+Please note the files referred in this guide:
+- [A3.java](https://git.kska.io/sppu-te-comp-content/SystemsProgrammingAndOperatingSystem/src/branch/main/Codes/Group%20A/Assignment-A3/A3.java)
+- [A3.c](https://git.kska.io/sppu-te-comp-content/SystemsProgrammingAndOperatingSystem/src/branch/main/Codes/Group%20A/Assignment-A3/A3.c)
+
+---
+
+- 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 `A3.c` file** since the functions defined in that file can be found in the header file.
+- 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
+
+---

Codes/Group A/Assignment-A3/README.md

@@ -4,6 +4,8 @@ These are the steps to run code for Assignment-A3.
 
 ---
 
+### Refer [EXPLANATION](https://git.kska.io/sppu-te-comp-content/SystemsProgrammingAndOperatingSystem/src/branch/main/Codes/Group%20A/Assignment-A3/EXPLANATION.md) to understand how everything works.
+
 > [!IMPORTANT]
 > Tested on Linux and Windows.