Lecture 5: System Calls in Operating System [Theory + Example + Hands-on Terminal]

Summary

In this OS placement series lecture, Lakshay delves into the critical concept of system calls, which serve as the interface between user and kernel space, allowing user applications to interact with the hardware through the kernel. He explains the necessity of system calls for tasks like memory allocation and process creation, using examples like 'mkdir' and 'malloc' to illustrate their function. The lecture also covers the types of system calls, including process control, file management, device management, information maintenance, and communication management, providing examples for each category. Lakshay emphasizes the importance of understanding system calls for technical interviews and offers practical examples to solidify the concept.

Takeaways

• 🌐 The lecture discusses the concept of system calls, which are crucial for user space applications to interact with kernel space and access hardware.
• 🔄 System calls provide a mechanism for switching between user mode and kernel mode, allowing applications to request services that they cannot perform in user mode.
• 💾 Kernel space has direct access to hardware, while user space does not, hence the necessity of system calls for hardware interaction.
• 📁 Examples of system calls include creating a directory, which internally uses commands like 'mkdir' in Linux, demonstrating the use of system calls in file management.
• 🛠️ System calls are implemented in C, a low-level language that allows direct hardware access, and are the only way for user applications to perform tasks that require kernel-level permissions.
• 🔧 There are five types of system calls: process control, file management, device management, information maintenance, and communication management, each serving different functionalities.
• 🛂 Process control system calls are used for creating and managing processes, such as 'createprocess()', 'fork()', 'exitprocess()', and 'exit()'.
• 📂 File management system calls include 'open()', 'read()', 'close()', 'write()', 'chmod()', 'umask()', and 'chown()', which handle file operations.
• 🖥️ Device management system calls like 'read()', 'write()', and 'ioctl()' deal with device operations, while information management system calls such as 'getpid()', 'alarm()', and 'sleep()' manage process and system information.
• 🔌 Communication management system calls facilitate inter-process communication, using mechanisms like pipes and shared memory to enable data exchange between processes.

Q & A

What is the primary purpose of system calls in an operating system?

System calls serve as an interface that allows user space applications to request services from the kernel, which has the necessary permissions to access hardware and perform tasks that user space cannot directly execute.

How does the distinction between user space and kernel space function in an operating system?

User space and kernel space are separate areas within an operating system. User space applications run in user mode and cannot directly access hardware. The kernel, running in kernel mode, has unrestricted access to hardware and performs the actual work on behalf of user space applications through system calls.

What is the role of system calls in switching between user mode and kernel mode?

System calls are the mechanism that enables switching from user mode to kernel mode. They provide the means for user space applications to request kernel services, which involve hardware interaction, and thus require the higher privileges of kernel mode.

Can you explain the process of creating a directory using system calls?

Creating a directory involves a system call like 'mkdir' in Linux. When the command is issued, it triggers a system call that transitions from user mode to kernel mode, where the kernel executes the necessary code to create a new directory node in the file system and then returns control to user mode.

What is the significance of the System Call Interface (SCI) in the context of system calls?

The System Call Interface (SCI) is an entry point into the kernel space. It receives requests from user space applications, maps them to the corresponding kernel implementations, and facilitates the execution of those implementations within the kernel.

How are system calls implemented in the kernel?

System calls are typically implemented in C, a lower-level language that allows direct access to hardware. These implementations handle the actual operations requested by user space applications, such as file operations, process management, and device interactions.

What are the different categories of system calls and can you provide some examples for each?

There are five categories of system calls: process control (e.g., fork(), exit()), file management (e.g., open(), read(), write(), chmod()), device management (e.g., read(), write(), ioctl()), information maintenance (e.g., getpid(), alarm(), sleep(), date()), and communication management (e.g., pipe(), shmget(), mmap()).

What is the difference between system calls used in GUI and CLI environments?

In GUI environments, system calls are often abstracted by user-friendly graphical operations, such as clicking buttons to create a folder. In CLI environments, users interact directly with system calls through command-line interfaces, such as using 'mkdir' to create a directory.

How does the process of executing a program involve system calls?

Executing a program involves system calls for process creation (e.g., fork()), memory allocation, and potentially I/O operations. The process starts in user mode, a system call is made to create a new process in kernel mode, and once the process is set up, control returns to user mode where the process can be monitored or interacted with.

Can you provide an example of a system call sequence in a script execution?

In the example provided, a shell script is created and executed. The sequence involves file management system calls (open(), write()) to create and edit the script, process management system calls (fork(), exec()) to create and run the script as a process, and information management system calls (getpid()) to retrieve process information.

Why is it important for developers to understand system calls?

Understanding system calls is crucial for developers because it provides insight into how applications interact with the operating system at a low level. This knowledge is essential for writing efficient, secure, and hardware-accessible code, and it is often a key area of focus in technical interviews for software development roles.