System Calls and Kernel Interfaces

A system call is an entry point from a user-space program into the operating-system kernel. The kernel owns protected resources such as process state, virtual memory mappings, filesystems, devices, and network protocol stacks. A program requests work on those resources through a defined interface instead of reading or changing kernel memory directly.

On Linux, the system-call interface is one part of the broader user-space application binary interface, or ABI. The ABI defines machine-level details such as system-call numbers, registers, data layouts, and return conventions. A source-level API sits above it. POSIX specifies many portable C interfaces, while Linux also supplies interfaces that are specific to Linux.

Most application code calls a C library function rather than issuing a raw system call. A wrapper such as open() arranges arguments for the target architecture, enters the kernel, translates a kernel error into -1 and errno, and returns to the caller. A wrapper may also select among kernel calls or add library behavior. The wrapper and the raw system call are therefore related interfaces, not guaranteed synonyms.

The request path

A typical request crosses several layers:

1. Application code calls a language runtime or library API.
2. The library wrapper places a system-call number and arguments where the architecture ABI expects them.
3. A processor instruction transfers control from user mode to a kernel entry path.
4. The kernel validates the number, arguments, pointers, credentials, and object state.
5. The selected subsystem performs the operation or returns an error.
6. The entry path restores user-space execution state.
7. The wrapper presents the result in the calling language's convention.