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+# Aysnchronous I/O
+
+_NNG_ provides rich support for {{i:asynchronous I/O}}.
+This allows applications to achieve high levels of concurrency with a
+minimum of fuss, optimized for the platform.
+
+Asynchronous I/O is performed without blocking calling application
+threads, so they may continue to perform other work.
+
+## AIO Handles
+
+Applications create an `nng_aio` object with a function to call when
+the operation is done (along with a pointer to application private data),
+then submit the operation.
+
+These `nng_aio` objects are created using the [`nng_aio_alloc()`][nng_aio_alloc],
+and destroyed using [`nng_aio_free()`][nng_aio_free].
+
+The `nng_aio` object itself is declared like this:
+
+```c
+#include <nng/nng.h>
+
+typedef struct nng_aio nng_aio;
+```
+
+Every asynchronous operation uses its own instance an `nng_aio`, and each
+`nng_aio` can only be used with a single operation at a time.
+
+> [!IMPORTANT]
+> Attempting to submit an operation using an `nng_aio` that is already
+> in use for another operation will crash the application.
+> However, it is possible to submit another operation on the `nng_aio` from
+> the callback associated with the same `nng_aio`.
+
+When the operation is complete, whether successfully
+or otherwise, the callback function is executed.
+The callback will be executed exactly once.
+
+## Cancellation
+
+The asynchronous I/O framework also supports cancellation of
+operations that are already in progress
+(see [`nng_aio_cancel()`][nng_aio_cancel]), as well setting a maximum
+timeout for them to complete within
+(see [`nng_aio_set_timeout()`][nng_aio_set_timeout]).
+
+## Waiting for Completion
+
+It is also possible to initiate an asynchronous operation, and wait for it to
+complete [`nng_aio_wait()`][nng_aio_wait].
+
+> [!IMPORTANT]
+> Applications must never call [`nng_aio_wait()`][nng_aio_wait] or
+> [`nng_aio_stop()`][nng_aio_stop] from a callback registered to
+> an `nng_aio` object. Doing so can lead to a deadlock.
+
+## See Also
+
+[nng_aio_abort][nng_aio_abort],
+[nng_aio_alloc][nng_aio_alloc],
+[nng_aio_cancel][nng_aio_cancel],
+[nng_aio_count][nng_aio_count],
+[nng_aio_free][nng_aio_free],
+[nng_aio_get_input][nng_aio_get_input],
+[nng_aio_get_msg][nng_aio_get_msg],
+[nng_aio_get_output][nng_aio_get_output],
+[nng_aio_result][nng_aio_result],
+[nng_aio_set_input][nng_aio_set_input],
+[nng_aio_set_iov][nng_aio_set_iov],
+[nng_aio_set_msg][nng_aio_set_msg],
+[nng_aio_set_timeout][nng_aio_set_timeout],
+[nng_aio_stop][nng_aio_stop],
+[nng_aio_wait][nng_aio_wait]
+
+{{#include ../refs.md}}