System calls and their role in Operating Systems - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is a system call?

A system call is a way for a program to ask the operating system to do something on its behalf, like reading a file or creating a new process.

beginner

Why do programs use system calls instead of directly accessing hardware?

Programs use system calls because the operating system controls hardware access to keep the system safe and stable. Direct access could cause errors or security problems.

intermediate

Name three common types of system calls.

Common system calls include: 1) File management (open, read, write), 2) Process control (create, terminate), 3) Communication (send, receive data).

intermediate

How does a system call work in simple terms?

When a program makes a system call, it switches from user mode to kernel mode so the operating system can safely perform the requested task, then returns control back to the program.

intermediate

What role do system calls play in multitasking operating systems?

System calls help manage multiple programs running at once by controlling process creation, scheduling, and communication between programs.

What is the main purpose of a system call?

A To request services from the operating system

B To directly control hardware devices

C To write code faster

D To bypass security checks

System calls allow programs to request services from the operating system safely.

Which mode does the CPU switch to when a system call is made?

A Safe mode

B User mode

C Kernel mode

D Sleep mode

The CPU switches to kernel mode to allow the operating system to perform privileged tasks.

Which of these is NOT a common system call category?

A Graphic design

B Process control

C File management

D Communication

Graphic design is not a system call category; system calls focus on managing files, processes, and communication.

Why can't user programs access hardware directly?

A Because hardware is broken

B To protect the system from errors and security risks

C Because hardware is too slow

D Because user programs are not smart enough

Direct hardware access is restricted to protect the system's stability and security.

What happens after the operating system completes a system call?

A The program crashes

B The program loses data

C The computer shuts down

D Control returns to the program in user mode

After the system call, control returns to the program running in user mode.

Explain what a system call is and why it is important in operating systems.

Think about how programs ask the OS to do things they cannot do themselves.

Describe the process that happens when a program makes a system call.

Focus on the mode change and how the OS handles the request.

Practice

1. What is the main purpose of a system call in an operating system?

easy

A. To write code that runs only on specific hardware

B. To directly access hardware without OS intervention

C. To compile programs into machine code

D. To allow programs to request services from the operating system

Solution

1. Step 1: Understand what system calls do

   System calls provide a way for programs to ask the operating system to perform tasks on their behalf, such as reading files or managing processes.

2. Step 2: Compare options with this role

   Only To allow programs to request services from the operating system correctly describes this role. Other options describe unrelated actions like direct hardware access or compilation.

3. Final Answer:

   To allow programs to request services from the operating system -> Option D

4. Quick Check:

   System call = request OS service [OK]

Hint: System calls let programs ask OS for help [OK]

Common Mistakes:

• Thinking system calls bypass the OS
• Confusing system calls with compiling
• Assuming system calls are hardware instructions

2. Which of the following is the correct way a program typically uses a system call?

easy

A. By calling a special function provided by the operating system

B. By directly writing machine code instructions

C. By modifying the operating system kernel

D. By sending signals to hardware devices

Solution

1. Step 1: Identify how programs interact with system calls

   Programs use system calls by calling special functions (APIs) provided by the OS, which then perform the requested service.

2. Step 2: Eliminate incorrect options

   Direct machine code writing or kernel modification is not how normal programs use system calls. Sending signals to hardware is also not the typical method.

3. Final Answer:

   By calling a special function provided by the operating system -> Option A

4. Quick Check:

   System call usage = OS function call [OK]

Hint: System calls are accessed via OS functions, not direct code [OK]

Common Mistakes:

• Thinking programs write machine code for system calls
• Believing programs modify the OS kernel directly
• Confusing hardware signals with system calls

3. Consider this pseudocode using a system call to open a file:

fd = open_file("data.txt")
if fd == -1:
    print("Error opening file")
else:
    print("File opened successfully")

What will be printed if the file does not exist?

medium

A. No output

B. File opened successfully

C. Error opening file

D. System crash

Solution

1. Step 1: Understand the system call behavior

   The open_file system call returns a file descriptor if successful, or -1 if it fails (e.g., file not found).

2. Step 2: Follow the conditional logic

   If fd == -1, the program prints "Error opening file". Since the file does not exist, fd will be -1.

3. Final Answer:

   Error opening file -> Option C

4. Quick Check:

   File missing -> fd = -1 -> error message [OK]

Hint: Open returns -1 on failure, triggers error print [OK]

Common Mistakes:

• Assuming file opens successfully even if missing
• Expecting no output on failure
• Thinking system calls cause crashes on errors

4. A program tries to read from a file using this code snippet:

bytes_read = read_file(fd, buffer, 100)
if bytes_read == 0:
    print("End of file reached")
elif bytes_read < 0:
    print("Read error")
else:
    print(f"Read {bytes_read} bytes")

But it always prints "Read error" even when the file exists. What is the likely problem?

medium

A. The print statement syntax is incorrect

B. The file descriptor fd is invalid or not opened properly

C. The file is empty

D. The buffer size is too large

Solution

1. Step 1: Analyze the meaning of bytes_read < 0

   A negative return value from read_file indicates an error, often caused by an invalid file descriptor.

2. Step 2: Check other options for plausibility

   Buffer size being large or file empty would not cause a read error. Print syntax errors cause compile/runtime errors, not read errors.

3. Final Answer:

   The file descriptor fd is invalid or not opened properly -> Option B

4. Quick Check:

   Read error -> invalid fd [OK]

Hint: Negative read means invalid file descriptor [OK]

Common Mistakes:

• Blaming buffer size for read errors
• Assuming empty file causes read error
• Confusing print syntax with read errors

5. You want to write a program that creates a new file, writes data to it, and then closes it using system calls. Which sequence correctly represents these steps?

hard

A. Open file -> Write data -> Close file

B. Write data -> Open file -> Close file

C. Close file -> Open file -> Write data

D. Open file -> Close file -> Write data

Solution

1. Step 1: Understand the logical order of file operations

   You must first open the file to get a handle, then write data to it, and finally close it to save and release resources.

2. Step 2: Match the correct sequence

   Only Open file -> Write data -> Close file follows this logical order. Other options have steps in wrong order, which would cause errors or no effect.

3. Final Answer:

   Open file -> Write data -> Close file -> Option A

4. Quick Check:

   Open before write, close last [OK]

Hint: Always open before writing, close after done [OK]

Common Mistakes:

• Trying to write before opening file
• Closing file before writing
• Skipping the close step