Järjestelmäkutsun

Järjestelmäkutsu (system call) is a fundamental concept in operating systems programming. It represents the programmatic way a user program requests a service from the operating system kernel. These services can range from basic operations like reading from a file or writing to the console, to more complex tasks such as creating a new process or managing memory.

When a user program needs to perform an operation that it does not have direct access to, such as interacting with hardware or accessing protected system resources, it must make a system call. This action transfers control from the user program to the operating system kernel. The kernel then performs the requested service on behalf of the user program and returns control back to the user program, usually with a result or an error code.

System calls are the interface between user space and kernel space. They are essential for maintaining system security and stability, as they ensure that all privileged operations are mediated by the kernel. Without system calls, user programs would have unrestricted access to the system, which could lead to data corruption or security breaches.

The specific set of available system calls varies between different operating systems. However, common categories include process control, file management, device management, and inter-process communication. Programmers typically do not invoke system calls directly but rather use library functions (like those in the C standard library) which, in turn, make the appropriate system calls.