SystemaufrufSchnittstellen

Systemaufruf, the German term for a system call, is a mechanism that allows user space programs to request services from the operating system kernel. It provides a controlled interface for operations such as input/output, process management, and memory allocation, while maintaining protection boundaries between user mode and kernel mode. In practice, a program invoking a system call transitions from user mode to kernel mode, executes code in the kernel, and then returns a result to the caller.

Typically, software in user space issues a system call via a library wrapper, which places arguments in registers or on the stack and triggers a trap instruction or a special interrupt. The kernel validates arguments, applies access checks, and dispatches the request to the appropriate handler. After completion, the kernel copies results back to user space and switches back to user mode. If an error occurs, a negative error code is returned, often mapped to the errno values in POSIX systems.

Systems expose system calls through a dispatch table or service table with numeric identifiers; Linux, BSD, and other Unix-like systems provide a catalog of calls, while Windows uses a different mechanism based on the Native API and Win32 API surface. Language libraries (such as libc) offer higher-level wrappers to simplify usage and portability.

Common examples include read, write, open, close for file operations; mmap; fork or posix_spawn for process creation; and wait for synchronization. Performance considerations include the cost of a mode switch between user and kernel space and, on some architectures, optimized fast paths for common calls. Security relies on kernel-enforced checks and minimal trusted interfaces. The system call interface is essential for the operating system’s abstraction of hardware resources and interprocess coordination.