31 lines
1.4 KiB
Markdown
Executable File
31 lines
1.4 KiB
Markdown
Executable File
# 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).
|