Most programs you use or write rely on File Descriptors (FDs) — even if you never think about them.
If a process reads input, writes output, opens a file, or sends data over a socket, it’s using FDs behind the scenes.
🔍 What Exactly Is an FD?
A File Descriptor is simply a small integer the kernel gives to a process.
It’s not the file itself — it’s a handle the kernel uses to manage any I/O resource.
💡 Why They’re So Important
Unified interface: files, pipes, sockets — all use open(), read(), write(), close().
Process isolation: each process has its own FD table.
State tracking: the kernel keeps the read/write offset and mode for each FD, even if multiple processes open the same file.
⚙️ Under the Hood
Linux manages I/O through three layers:
Per-Process FD Table — what your program sees (0, 1, 2, ...).
Open File Table — stores the state of each open resource.
Inode Table — contains the actual file metadata on disk.
This architecture is a big reason why Unix-like systems are so flexible, consistent, and developer-friendly.
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