the open-file table

When you open a file, the OS does not look it up afresh on every single read. That would be like re-finding a library book on the shelf for every word you read. Instead, when you open a file the OS makes a small in-memory bookmark that remembers everything it needs while the file is in use: where the file's metadata is, where you currently are in it, what you are allowed to do. The open-file table is the collection of these bookmarks.

There are actually two levels of table working together. There is a system-wide open-file table with one entry per distinct open file across the whole machine, holding shared facts like the file's location on disk and a count of how many openers are currently using it (its reference count). And there is a per-process table, one for each running program, holding that program's own handles. A handle in your per-process table points at an entry in the system-wide table, which in turn points at the file's metadata. The crucial per-open detail, the current position you are reading or writing at, lives in the shared open-file structure, which is why two separate opens of the same file each get their own independent position.

Why it matters: this two-level design is what lets many programs (and many parts of one program) share files cleanly and lets the OS reclaim resources at exactly the right moment. The system-wide entry's reference count is the safety mechanism: the OS only truly closes the file and frees its in-memory structures when the LAST holder closes it. A common confusion is thinking your file descriptor stores the position itself, it does not, it indexes into these tables where the real state lives.