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The Event Loop — Middle

At middle level, focus on this question:

What does a readiness-based API like epoll actually tell the event loop, precisely?

Prerequisite: junior.md.

Registering interest, then polling for readiness

// Conceptual epoll usage (simplified)
int epfd = epoll_create1(0);
epoll_ctl(epfd, EPOLL_CTL_ADD, socket_fd, &event);  // "tell me when
                                                       // socket_fd is
                                                       // readable"

struct epoll_event ready_events[MAX_EVENTS];
int n = epoll_wait(epfd, ready_events, MAX_EVENTS, timeout_ms);
// n = number of file descriptors that are ACTUALLY ready right now,
// out of potentially THOUSANDS registered
flowchart LR Register["Register interest in\nMANY file descriptors\n(sockets), ONCE"] --> Wait["epoll_wait(): ONE call\nreturns ONLY the ones\nthat are ACTUALLY ready -\nO(ready), not O(total\nregistered)"]

The critical efficiency property: epoll_wait (and kqueue's equivalent) returns information proportional to the number of ready descriptors, not the total number registered — checking whether any of 10,000 registered sockets are ready doesn't require iterating through all 10,000 one by one (which the older select/poll APIs did, scaling poorly); the kernel maintains this efficiently internally and returns just the relevant subset.

🎓 Takeaway: "readiness" means "this socket has data available to read, or buffer space available to write, right now" — the event loop doesn't get the actual data through this API; it gets a signal that it's now safe to perform a non-blocking read/write without stalling, which it then does as a separate, fast operation.

Test yourself

  1. Why does epoll_wait scale better than the older select/poll APIs at high registered-descriptor counts?
  2. What does "ready" actually mean for a socket — does it mean data has already been delivered to the application, or something else?
  3. Why does the event loop still need to perform an actual read/write call after being told a descriptor is "ready," rather than the readiness API delivering the data directly?

Continue to senior.md.