5. System Calls in Operating Systems

1. What is a System Call?

Definition:
A system call is a request made by a user program to the OS kernel to perform tasks that require privileged hardware access (e.g., file creation, memory allocation).

Why Needed?

• User applications run in user mode (restricted access).

• Kernel runs in kernel mode (full hardware access).

• System calls bridge this gap via the System Call Interface (SCI).

Analogy:

• User Mode = Customer at a restaurant (orders food).

• Kernel Mode = Chef in the kitchen (executes orders).

• System Call = Waiter (delivers order to kitchen).

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2. How System Calls Work

Step-by-Step Flow:

1. User Request: e.g., mkdir Movies (CLI) or "New Folder" (GUI).

2. Trigger SCI: Command reaches the System Call Interface.

3. Mode Switch: Software interrupt switches CPU to kernel mode.

4. Kernel Execution: Kernel’s C code creates the folder on disk.

5. Return to User Mode: Confirmation sent back to user space.

Example:

$ mkdir Movies  # User command  
↓  
SCI maps to `sys_mkdir()` in kernel  
↓  
Kernel creates folder → Returns success message

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3. Types of System Calls

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4. Key Examples Explained

(A) Process Control: fork()

• What? Creates a child process (clone of parent).

• Code:

    #include <unistd.h>
    int main() {
        fork();  // Creates child process
        printf("Hello OS!\n");
        return 0;
    }
  

• Output:

    Hello OS!  // Parent  
    Hello OS!  // Child
  

(B) File Management: open() & write()

• What? Creates/writes to a file.

• Terminal Demo:

    $ echo "Hello" > file.txt  # Uses `open()` and `write()` internally.
  

(C) Communication: pipe()

• What? Creates a channel for inter-process communication.

• Use Case:

  • Process A writes to pipe → Process B reads from it.

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5. Practical Demo: From Terminal

Creating a Script (Hello.sh)

1. Write Script:

    #!/bin/bash
    echo "Hello CodeHelp"
    sleep 10  # Uses `sleep()` system call
   

2. Run Script:

    $ chmod +x Hello.sh  # `chmod()` system call
    $ ./Hello.sh         # `fork()` + `exec()`
   

3. Monitor Process:

    $ ps -a              # `getpid()` system call
    $ kill -9 [PID]      # `kill()` system call
   

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6. Why System Calls Matter

1. Security: Prevents apps from directly accessing hardware.

2. Abstraction: Simplifies complex operations (e.g., mkdir vs. manual disk allocation).

3. Standardization: Uniform interface across applications.

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7. Interview Questions

1. How does mkdir work internally?

   • Triggers sys_mkdir() in kernel via SCI.

2. Difference between fork() and exec()?

   • fork() clones a process; exec() replaces it with a new program.

3. What happens during a mode switch?

   • CPU saves user mode state → Switches to kernel mode via interrupt.

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8. Homework

1. Explore man pages:

    $ man 2 kill    # System call documentation
    $ man 2 open
   

2. Experiment:

   • Use strace to trace system calls:

       $ strace ./Hello.sh
     

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Key Takeaways

1. System calls = Gateway between user apps and kernel.

2. 5 Types: Process, File, Device, Info, Communication.

3. Real-World Use: Every CLI/GUI action (e.g., file creation) relies on system calls.