järjestelmäkäskyjä

Järjestelmäkäskyt, known in English as system calls, are the fundamental interface between a computer program and the operating system. They are special functions that a program can invoke to request services from the operating system kernel. These services can include tasks such as reading or writing files, creating or managing processes, allocating memory, and communicating with hardware devices.

When a program needs to perform an operation that requires privileged access or direct interaction with the system's resources, it cannot do so directly. Instead, it makes a system call. This triggers a transition from user mode, where programs have limited privileges, to kernel mode, where the operating system has full control. The kernel then executes the requested service on behalf of the program. Once the operation is complete, the kernel returns control to the program, often with the results of the operation.

Common examples of system calls include `open` for opening a file, `read` for reading data from a file or device, `write` for writing data, `fork` for creating a new process, and `exit` for terminating a process. Each operating system has its own set of system calls, although many are standardized across different Unix-like systems and Windows. Understanding system calls is crucial for programmers who need to interact with the underlying operating system to build efficient and robust applications. They represent the controlled pathway by which user-level applications can leverage the power and security of the operating system kernel.