system-programming/04_AdvancedFileIO/README.md

Chapter 4 – Advanced File I/O

Scatter/Gather I/O

• Scatter/gather I/O allows reading or writing data to/from multiple buffers in a single system call.
• Uses readv() and writev():
  • readv(fd, iov, iovcnt): reads into multiple buffers (iov array).
  • writev(fd, iov, iovcnt): writes from multiple buffers.
• Improves efficiency by reducing system calls.

Event Poll (select(), poll(), epoll)

• Traditional multiplexing: select(), poll() (limited scalability).
• Linux-specific: epoll for efficient event notification.
  • epoll_create(): create epoll instance.
  • epoll_ctl(): control interest list (add, modify, remove fds).
  • epoll_wait(): wait for events.
• Edge-triggered vs level-triggered modes.

Memory-Mapped Files (mmap())

• Maps files or devices into memory for direct access.
• System calls: mmap(), munmap().
• Benefits: faster I/O, shared memory.
• Use msync() to synchronize changes.
• Resizing mappings, changing protections.
• Advisory calls:
  • posix_fadvise(): give advice about expected I/O patterns.
  • readahead(): prefetch file data.
• Pros and cons of memory mapping.

Asynchronous I/O and I/O Scheduling

• Distinction between synchronous and asynchronous I/O.
• Linux AIO APIs for asynchronous operations.
• I/O schedulers:
  • Manage disk request ordering.
  • Examples: CFQ, noop, deadline.
• Optimizing I/O performance by choosing schedulers.
• Concepts of disk addressing and request life cycle.

Miscellaneous

• ioctl(): device-specific operations.
• Zero-copy transfers: sendfile(), splice().

---

This chapter covers advanced Linux I/O techniques aimed at improving performance and flexibility in system programming.