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.

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.
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.
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.
lseek():
- Moves the offset within the file—can be positive/negative; allows sparse writes.
close():
- Frees the descriptor. Returns 0 or -1 on error.

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

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