Lecture Notes: System Calls

Introduction

• System Call: A programmatic way to shift from user mode to kernel mode to access operating system functionality.
• Used when an application or program needs to access kernel-level operations.

User Mode vs Kernel Mode

• User Mode: Where applications run; limited access to system resources.
• Kernel Mode: Full access to hardware and resources; accessed via system calls.

Example Use Cases

• Printing on a monitor or accessing a printer requires kernel mode.
• Accessing or modifying files on a hard disk also requires kernel mode.

System Calls in Different Operating Systems

• Linux: Direct use of system calls like read, write, open, close, fork, etc.
• Windows: Around 700 system calls.
• Many systems use APIs or libraries that internally use system calls (e.g., printf).

Categories of System Calls

1. File Related

• Operations like creating, opening, reading, writing files.
• Programs become processes which require kernel privileges to access files.

2. Device Related

• Access hardware like hard disks, printers, monitors through system calls.
• Privileges for device access are managed by the operating system.

3. Information Related

• Access process or device information, like metadata (size, permissions).
• Examples include getpid (process ID), getppid (parent process ID).

4. Process Control

• Managing process execution, loading into memory.
• Fork System Call: Used in multiprocessing environments to create child processes.
• Wait & Signal: Used for process synchronization, semaphore management.

5. Communication

• Inter-process Communication (IPC): Processes communicate via methods like Pipe() or shared memory (Shmget()).

6. Protection and Security

• Manage file permissions and security levels (e.g., chmod, umask).

Conclusion

• Understanding system calls is crucial for understanding how programs interact with the kernel.
• Importance in ensuring program access to necessary resources and hardware.

Thank you for listening!