= Kernel
{disambiguate=operating system}
{wiki}
= Operating-system kernel
{synonym}
The <operating-system kernel> is the core of an <operating system>, controlling access to resources and supplying mechanisms used by <operating-system processes>. <Processor privilege levels> let it enforce boundaries that ordinary programs cannot simply overwrite. The <operating-system kernel> connects <process scheduling>, <virtual memory> and <file system> operations; a <system call> requests a service across that boundary. A <monolithic kernel> and a <microkernel> divide services differently, so the location of every <file system> service is a design choice.
For the toy <file system>, opening `a` resolves its <directory entry> to <inode> 42, establishes an <open file description>, and returns <file descriptor> `3`. Later reads use that <file descriptor> to reach the existing <open file description>, without resolving the spelling `a` again. This explains how <unlinking an open file> preserves access: the open reference survives the <directory entry>. The per-<operating-system process> table interpreting `3` is maintained by the <operating-system kernel>; `3` itself is the small <integer> passed by the program, not <inode> 42. https://pubs.opengroup.org/onlinepubs/9799919799/functions/open.html[The Open Group, POSIX.1-2024, open: file descriptor to open file description to file]{ref}
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