Add comments explaining practical

Add comments explaining each practical experiment in the code files.

Look at the C code files in the repository, each likely corresponding to a practical experiment. For each file, add a comment at the top explaining what the practical experiment is about and what the code demonstrates. Check if there are any existing comments to follow as a style guide. Running a simple compile test (e.g., `gcc -o test filename.c`) can verify the code still works after adding comments.

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