31 lines
1.4 KiB
Markdown
31 lines
1.4 KiB
Markdown
# Chapter 2 – File I/O
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## Basic Concepts
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- File I/O is central in Linux: every entity (file, socket, device) is treated as a file.
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- The kernel manages a **file table** per process, with descriptors and metadata like offset and mode.
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## Core Operations
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1. **`open()`**:
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- Opens a file with flags (O_RDONLY, O_WRONLY, O_RDWR, O_CREAT, O_SYNC, O_DIRECT).
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- Returns a file descriptor or -1 on error.
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- Sets owner and permissions for newly created files.
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2. **`read()` / `write()`**:
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- `read(fd, buf, n)` reads up to **n** bytes, returns bytes read or -1.
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- `write(fd, buf, n)` writes, handles partial writes, append mode and non-blocking I/O.
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- Common errors: EAGAIN, EINTR, ECC.
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3. **I/O Synchronization**:
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- `fsync()` and `fdatasync()` flush to disk.
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- `O_SYNC`, `O_DSYNC`, `O_RSYNC` for open-time synchronization.
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- `sync()` syncs all files to disk.
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4. **`lseek()`**:
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- Moves the **offset** within the file—can be positive/negative; allows sparse writes.
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5. **`close()`**:
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- Frees the descriptor. Returns 0 or -1 on error.
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## Positional I/O
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- `pread()` / `pwrite()` allow read/write at specific offsets without altering the shared file offset (useful in multithreaded environments).
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## Multiplexed I/O
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- Functions like `select()` and `poll()` manage multiple file descriptors for asynchronous I/O.
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- More scalable option: `epoll()` (covered in Chapter 4).
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