Node:Synchronizing AIO Operations,
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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 in unistd.h
the symbol _POSIX_SYNCHRONIZED_IO
is
defined.
int aio_fsync (int op, struct aiocb *aiocbp)
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Function |
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
(see 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 filed. 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.
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int aio_fsync64 (int op, struct aiocb64 *aiocbp)
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Function |
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.
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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.
int aio_suspend (const struct aiocb *const list[], int nent, const struct timespec *timeout)
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Function |
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 already has
completed at the time aio_suspend is called the function returns
immediately. Whether a request has terminated or not is done 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 waked until a request
finished. If timeout is not NULL the process remains
suspended at 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.
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int aio_suspend64 (const struct aiocb64 *const list[], int nent, const struct timespec *timeout)
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Function |
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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