System call

A system call is a controlled request by a program for a service provided by the operating system kernel, such as file access or process creation.

System call interface: The set of calling conventions and identifiers programs use to request kernel services. It defines how a program expresses a request and receives its result.
open (system call): A system call that requests access to a filesystem object and returns a file descriptor. It begins many file-access sequences by asking the kernel to open a path.
Operating system kernel: The privileged core of an operating system that manages hardware and provides foundational services to programs. System calls are requests for services implemented by this core.
Standard library: A collection of reusable routines supplied to programs by a language or runtime environment. Many standard-library operations are ordinary code and do not themselves enter the kernel.
Trap (computing): A synchronous transfer of execution to privileged software caused by an instruction or exceptional condition. A trap commonly transfers control from a system-call instruction into the kernel.
read (system call): A system call that transfers bytes from a file descriptor into a program-provided buffer. It illustrates how programs request input through a kernel-managed interface.
CPU privilege levels: Hardware-enforced execution modes that restrict which instructions and resources software can access. Privilege levels let the kernel protect sensitive operations from ordinary programs.
POSIX: A family of standards defining portable interfaces for Unix-like operating systems. POSIX specifies many interfaces that implementations expose through system calls or library wrappers.
User space and kernel space: Two privilege domains that separate ordinary application execution from operating-system kernel execution. A system call crosses this protection boundary under controlled privilege rules.
write (system call): A system call that transfers bytes from a program-provided buffer to a file descriptor. It shows how output is handed to files, devices, or other kernel-managed destinations.

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