System calls in os

System calls (syscalls) are the programmatic way for an application to request services from the operating system's kernel. They act as a vital bridge between user-space applications (with limited privileges) and the kernel (with full hardware access). 

How System Calls Work:

1. User Mode: The application runs in a restricted "user mode" where it cannot directly touch hardware or memory of other processes.
2. The Request: When it needs a service (like saving a file), it issues a system call instruction (often called a trap or software interrupt).
3. Mode Switch: The CPU switches from user mode to kernel mode, giving the OS full authority to perform the task safely.
4. Execution & Return: The kernel executes the requested service, then switches the CPU back to user mode and returns the results to the application. 

Types of System Calls with Examples

Category	Purpose	Unix/Linux Example	Windows Example
Process Control	Creating, stopping, or managing programs.	fork(), exit(), wait()	CreateProcess(), ExitProcess()
File Management	Opening, reading, writing, or deleting files.	open(), read(), write(), close()	CreateFile(), ReadFile()
Device Management	Interacting with hardware like printers or disks.	ioctl(), read(), write()	SetConsoleMode(), ReadConsole()
Information Maintenance	Getting/setting system time or attributes.	getpid(), sleep(), alarm()	GetCurrentProcessID(), Sleep()
Communication	Exchanging data between different programs.	pipe(), shmget(), mmap()	CreatePipe(), MapViewOfFile()
Protection	Controlling access permissions.	chmod(), chown()	SetFileSecurity()

Real-World Example: Saving a Document

When you click "Save" in a text editor like Notepad:

1. The editor calls an API function (e.g., WriteFile in Windows).
2. This triggers a system call to the kernel.
3. The kernel verifies your permissions and physically writes the data to your SSD/HDD.
4. Once done, the kernel tells the editor "Success," and you can continue typing.