4.I/O & System Calls

How user-space programs enter the kernel to request privileged services, how this differs from an ordinary function call, and why libc wrappers matter.

Blocking, non-blocking, readiness-based multiplexing, signal-driven I/O, asynchronous completion, and Reactor-style server architectures.

How Linux readiness APIs differ in descriptor representation, scanning cost, registration model, and level- vs. edge-triggered behavior.

File descriptors, page cache, buffered and direct I/O, blocking, readiness, and durable writes.

Why zero-copy techniques reduce CPU-mediated data movement, with mmap, sendfile, page cache, DMA, and the limits of the term 'zero copy'.