GNU Info
(libc.info)Synchronizing AIO Operations
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 request 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)
Calling this function forces all I/O operations operating queued
at the time of the function call operating on the file descriptor
`aiocbp->aio_fildes' into the synchronized I/O completion state
(Note: Synchronizing I/O). 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 for 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 or not
open for writing.
`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_return64' since the LFS interface
transparently replaces the normal implementation.
- Function: int aio_fsync64 (int OP, struct aiocb64 *AIOCBP)
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_*'
function notify the caller might also be not the best solution since
whenever the process works on preparing data for on 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)
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)
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.
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