system-programming/02_FileIO/README.md

Chapter 2 – File I/O

Basic Concepts

• File I/O is central in Linux: every entity (file, socket, device) is treated as a file.
• The kernel manages a file table per process, with descriptors and metadata like offset and mode.

Core Operations

1. open():
   • Opens a file with flags (O_RDONLY, O_WRONLY, O_RDWR, O_CREAT, O_SYNC, O_DIRECT).
   • Returns a file descriptor or -1 on error.
   • Sets owner and permissions for newly created files.
2. read() / write():
   • read(fd, buf, n) reads up to n bytes, returns bytes read or -1.
   • write(fd, buf, n) writes, handles partial writes, append mode and non-blocking I/O.
   • Common errors: EAGAIN, EINTR, ECC.
3. I/O Synchronization:
   • fsync() and fdatasync() flush to disk.
   • O_SYNC, O_DSYNC, O_RSYNC for open-time synchronization.
   • sync() syncs all files to disk.
4. lseek():
   • Moves the offset within the file—can be positive/negative; allows sparse writes.
5. close():
   • Frees the descriptor. Returns 0 or -1 on error.

Positional I/O

• pread() / pwrite() allow read/write at specific offsets without altering the shared file offset (useful in multithreaded environments).

Multiplexed I/O

• Functions like select() and poll() manage multiple file descriptors for asynchronous I/O.
• More scalable option: epoll() (covered in Chapter 4).