Synchronizing AIO Operations (The GNU C Library)

#### 13.11.3 Getting into a Consistent State ¶

When dealing with asynchronous operations it is sometimes necessary to get into a consistent state. This would mean for AIO that one wants to know whether a certain request or a group of requests were processed. This could be done by waiting for the notification sent by the system after the operation terminated, but this sometimes would mean wasting resources (mainly computation time). Instead POSIX.1b defines two functions which will help with most kinds of consistency.

The `aio_fsync` and `aio_fsync64` functions are only available if the symbol `_POSIX_SYNCHRONIZED_IO` is defined in `unistd.h`.

Function: `int` aio_fsync `(int op, struct aiocb *aiocbp)` ¶

  Preliminary: | MT-Safe | AS-Unsafe lock heap | AC-Unsafe lock mem | See POSIX Safety Concepts.

  Calling this function forces all I/O operations queued at the time of the function call operating on the file descriptor `aiocbp->aio_fildes` into the synchronized I/O completion state (see Synchronizing I/O operations). The `aio_fsync` function returns immediately but the notification through the method described in `aiocbp->aio_sigevent` will happen only after all requests for this file descriptor have terminated and the file is synchronized. This also means that requests for this very same file descriptor which are queued after the synchronization request are not affected.

  If op is `O_DSYNC` the synchronization happens as with a call to `fdatasync`. Otherwise op should be `O_SYNC` and the synchronization happens as with `fsync`.

  As long as the synchronization has not happened, a call to `aio_error` with the reference to the object pointed to by aiocbp returns `EINPROGRESS`. Once the synchronization is done `aio_error` return 0 if the synchronization was not successful. Otherwise the value returned is the value to which the `fsync` or `fdatasync` function would have set the `errno` variable. In this case nothing can be assumed about the consistency of the data written to this file descriptor.

  The return value of this function is 0 if the request was successfully enqueued. Otherwise the return value is -1 and `errno` is set to one of the following values:

  `EAGAIN`

    The request could not be enqueued due to temporary lack of resources.

  `EBADF`

    The file descriptor `aiocbp->aio_fildes` is not valid.

  `EINVAL`

    The implementation does not support I/O synchronization or the op parameter is other than `O_DSYNC` and `O_SYNC`.

  `ENOSYS`

    This function is not implemented.

  When the sources are compiled with `_FILE_OFFSET_BITS == 64` this function is in fact `aio_fsync64` since the LFS interface transparently replaces the normal implementation.

Function: `int` aio_fsync64 `(int op, struct aiocb64 *aiocbp)` ¶

  Preliminary: | MT-Safe | AS-Unsafe lock heap | AC-Unsafe lock mem | See POSIX Safety Concepts.

  This function is similar to `aio_fsync` with the only difference that the argument is a reference to a variable of type `struct aiocb64`.

  When the sources are compiled with `_FILE_OFFSET_BITS == 64` this function is available under the name `aio_fsync` and so transparently replaces the interface for small files on 32 bit machines.

Another method of synchronization is to wait until one or more requests of a specific set terminated. This could be achieved by the `aio_*` functions to notify the initiating process about the termination but in some situations this is not the ideal solution. In a program which constantly updates clients somehow connected to the server it is not always the best solution to go round robin since some connections might be slow. On the other hand letting the `aio_*` functions notify the caller might also be not the best solution since whenever the process works on preparing data for a client it makes no sense to be interrupted by a notification since the new client will not be handled before the current client is served. For situations like this `aio_suspend` should be used.

Function: `int` aio_suspend `(const struct aiocb *const list[], int nent, const struct timespec *timeout)` ¶

  Preliminary: | MT-Safe | AS-Unsafe lock | AC-Unsafe lock | See POSIX Safety Concepts.

  When calling this function, the calling thread is suspended until at least one of the requests pointed to by the nent elements of the array list has completed. If any of the requests has already completed at the time `aio_suspend` is called, the function returns immediately. Whether a request has terminated or not is determined by comparing the error status of the request with `EINPROGRESS`. If an element of list is `NULL`, the entry is simply ignored.

  If no request has finished, the calling process is suspended. If timeout is `NULL`, the process is not woken until a request has finished. If timeout is not `NULL`, the process remains suspended at least as long as specified in timeout. In this case, `aio_suspend` returns with an error.

  The return value of the function is 0 if one or more requests from the list have terminated. Otherwise the function returns -1 and `errno` is set to one of the following values:

  `EAGAIN`

    None of the requests from the list completed in the time specified by timeout.

  `EINTR`

    A signal interrupted the `aio_suspend` function. This signal might also be sent by the AIO implementation while signalling the termination of one of the requests.

  `ENOSYS`

    The `aio_suspend` function is not implemented.

  When the sources are compiled with `_FILE_OFFSET_BITS == 64` this function is in fact `aio_suspend64` since the LFS interface transparently replaces the normal implementation.

Function: `int` aio_suspend64 `(const struct aiocb64 *const list[], int nent, const struct timespec *timeout)` ¶

  Preliminary: | MT-Safe | AS-Unsafe lock | AC-Unsafe lock | See POSIX Safety Concepts.

  This function is similar to `aio_suspend` with the only difference that the argument is a reference to a variable of type `struct aiocb64`.

  When the sources are compiled with `_FILE_OFFSET_BITS == 64` this function is available under the name `aio_suspend` and so transparently replaces the interface for small files on 32 bit machines.