# POSIX Polling — epoll Backend

```c
#define FIO_POLL_ENGINE_EPOLL
#define FIO_POLL
#include "fio-stl.h"
```

Implementation of the portable polling API on top of Linux `epoll`. Normally selected automatically on Linux; define `FIO_POLL_ENGINE_EPOLL` to force it.

The public API is documented in [`102 poll api.md`](https://facil.io/0.8.x/posix-portable-polling-api/). This file describes backend-specific behavior.

---

## Backend Details

#### `struct fio_poll_s`

```c
struct fio_poll_s {
  fio_poll_settings_s settings;
  struct pollfd fds[2];
  int fd[2];
};
```

Two internal `epoll` file descriptors are used: one for `POLLOUT` readiness and one for `POLLIN`/error events. A `pollfd` pair wakes the internal `poll` used to wait on both epoll FDs at once.

#### `fio_poll_engine`

Returns `"epoll"`.

#### Fork Safety

A state callback re-creates the epoll FDs in the child process after `fork`.

---

## Notes

- Events are monitored in one-shot mode (`EPOLLONESHOT`). After an event fires, the descriptor must be re-armed with `fio_poll_monitor`.
- Error and hang-up events are dispatched through the read-side epoll FD and delivered to `on_close`.
- `EPOLLRDHUP` and `EPOLLHUP` are always included in the monitored events.

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