sendfile
A kernel-assisted data-transfer operation that can copy file contents to a socket without routing payload bytes through a user-space application buffer.
Linux Context
sendfile lets the kernel transfer file-backed data toward a socket without requiring an application to copy payload bytes through its own user-space buffer. This can reduce copying and syscall overhead in static-file serving and some proxy paths.
Zero-Copy Boundary
Calling sendfile does not guarantee that no physical copy occurs anywhere in the system. TLS processing, filesystem behavior, DMA, socket buffering, and protocol-stack details determine the actual data path.