40 lines
2.9 KiB
Markdown
40 lines
2.9 KiB
Markdown
# Explanation
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This file contains explanation for dynamic linking library program (Assignment-A3)
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---
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Please note the files referred in this guide:
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- [A3.java](https://git.kska.io/sppu-te-comp-content/SystemsProgrammingAndOperatingSystem/src/branch/main/Codes/Group%20A/Assignment-A3/A3.java)
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- [A3.c](https://git.kska.io/sppu-te-comp-content/SystemsProgrammingAndOperatingSystem/src/branch/main/Codes/Group%20A/Assignment-A3/A3.c)
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- [A3.h](https://git.kska.io/sppu-te-comp-content/SystemsProgrammingAndOperatingSystem/src/branch/main/Codes/Group%20A/Assignment-A3/A3.h)
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---
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- We're using Java for writing the main program.
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- To demonstrate Dynamic Linking Library (DLL) in Java, we'll be declaring functions in Java and implementing them in C (can be C/C++).
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- First, we create a Java program `A3.java`,
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- This is the main Java file containing function definition and the main function.
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- 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.
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- 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).
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- Now, we will generate the header file. For this, run `javac -h . A3.java`.
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- There will be a new file called `A3.h` in your current working directory,
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- This is the header file.
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- It contains signatures for native functions we created in the Java file.
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- 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](https://git.kska.io/sppu-te-comp-content/SystemsProgrammingAndOperatingSystem/src/branch/main/Codes/Group%20A/Assignment-A3/A3.h) file in this folder for reference. Note that it is automatically generated.
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- Create a new `A3.c` file which is the C program file containing function definitions (for add, sub, mul, div)
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- Define all the functions (add, sub, mul, div)
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- 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`,
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- `gcc` -> GNU compiler for C program
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- `-shared` -> tells the compiler to create a shared file (.so) instead of a regular executable file
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- `-o libA3.so` -> tells the compiler to save the output to `libA3.so` file
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- `fPIC` -> stands for Position-Independent Code. Needed for creating shared libraries.
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- `-I"$JAVA_HOME/include"` and `-I"$JAVA_HOME/include/linux"` -> `-I` flag used for specifiying directories to include. Values in double quotes are directories
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- `A3.c` -> name of the C program file to compile
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- Lastly, run the program using `java -Djava.library.path=. A3`
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- `java` -> Loads Java Virtual Machine (JVM)
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- `-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.
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- `A3` -> name of the Java class containing the main method
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---
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