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(libc.info)Asynchronous I/O
Perform I/O Operations in Parallel
==================================
The POSIX.1b standard defines a new set of I/O operations which can
significantly reduce the time an application spends waiting at I/O. The
new functions allow a program to initiate one or more I/O operations and
then immediately resume normal work while the I/O operations are
executed in parallel. This functionality is available if the
`unistd.h' file defines the symbol `_POSIX_ASYNCHRONOUS_IO'.
These functions are part of the library with realtime functions named
`librt'. They are not actually part of the `libc' binary. The
implementation of these functions can be done using support in the
kernel (if available) or using an implementation based on threads at
userlevel. In the latter case it might be necessary to link
applications with the thread library `libpthread' in addition to
`librt'.
All AIO operations operate on files which were opened previously.
There might be arbitrarily many operations running for one file. The
asynchronous I/O operations are controlled using a data structure named
`struct aiocb' ("AIO control block"). It is defined in `aio.h' as
follows.
- Data Type: struct aiocb
The POSIX.1b standard mandates that the `struct aiocb' structure
contains at least the members described in the following table.
There might be more elements which are used by the implementation,
but depending upon these elements is not portable and is highly
deprecated.
`int aio_fildes'
This element specifies the file descriptor to be used for the
operation. It must be a legal descriptor, otherwise the
operation will fail.
The device on which the file is opened must allow the seek
operation. I.e., it is not possible to use any of the AIO
operations on devices like terminals where an `lseek' call
would lead to an error.
`off_t aio_offset'
This element specifies the offset in the file at which the
operation (input or output) is performed. Since the
operations are carried out in arbitrary order and more than
one operation for one file descriptor can be started, one
cannot expect a current read/write position of the file
descriptor.
`volatile void *aio_buf'
This is a pointer to the buffer with the data to be written
or the place where the read data is stored.
`size_t aio_nbytes'
This element specifies the length of the buffer pointed to by
`aio_buf'.
`int aio_reqprio'
If the platform has defined `_POSIX_PRIORITIZED_IO' and
`_POSIX_PRIORITY_SCHEDULING', the AIO requests are processed
based on the current scheduling priority. The `aio_reqprio'
element can then be used to lower the priority of the AIO
operation.
`struct sigevent aio_sigevent'
This element specifies how the calling process is notified
once the operation terminates. If the `sigev_notify' element
is `SIGEV_NONE', no notification is sent. If it is
`SIGEV_SIGNAL', the signal determined by `sigev_signo' is
sent. Otherwise, `sigev_notify' must be `SIGEV_THREAD'. In
this case, a thread is created which starts executing the
function pointed to by `sigev_notify_function'.
`int aio_lio_opcode'
This element is only used by the `lio_listio' and
`lio_listio64' functions. Since these functions allow an
arbitrary number of operations to start at once, and each
operation can be input or output (or nothing), the
information must be stored in the control block. The
possible values are:
`LIO_READ'
Start a read operation. Read from the file at position
`aio_offset' and store the next `aio_nbytes' bytes in the
buffer pointed to by `aio_buf'.
`LIO_WRITE'
Start a write operation. Write `aio_nbytes' bytes
starting at `aio_buf' into the file starting at position
`aio_offset'.
`LIO_NOP'
Do nothing for this control block. This value is useful
sometimes when an array of `struct aiocb' values
contains holes, i.e., some of the values must not be
handled although the whole array is presented to the
`lio_listio' function.
When the sources are compiled using `_FILE_OFFSET_BITS == 64' on a
32 bit machine, this type is in fact `struct aiocb64', since the
LFS interface transparently replaces the `struct aiocb' definition.
For use with the AIO functions defined in the LFS, there is a
similar type defined which replaces the types of the appropriate
members with larger types but otherwise is equivalent to `struct
aiocb'. Particularly, all member names are the same.
- Data Type: struct aiocb64
`int aio_fildes'
This element specifies the file descriptor which is used for
the operation. It must be a legal descriptor since otherwise
the operation fails for obvious reasons.
The device on which the file is opened must allow the seek
operation. I.e., it is not possible to use any of the AIO
operations on devices like terminals where an `lseek' call
would lead to an error.
`off64_t aio_offset'
This element specifies at which offset in the file the
operation (input or output) is performed. Since the
operation are carried in arbitrary order and more than one
operation for one file descriptor can be started, one cannot
expect a current read/write position of the file descriptor.
`volatile void *aio_buf'
This is a pointer to the buffer with the data to be written
or the place where the read data is stored.
`size_t aio_nbytes'
This element specifies the length of the buffer pointed to by
`aio_buf'.
`int aio_reqprio'
If for the platform `_POSIX_PRIORITIZED_IO' and
`_POSIX_PRIORITY_SCHEDULING' are defined the AIO requests are
processed based on the current scheduling priority. The
`aio_reqprio' element can then be used to lower the priority
of the AIO operation.
`struct sigevent aio_sigevent'
This element specifies how the calling process is notified
once the operation terminates. If the `sigev_notify',
element is `SIGEV_NONE' no notification is sent. If it is
`SIGEV_SIGNAL', the signal determined by `sigev_signo' is
sent. Otherwise, `sigev_notify' must be `SIGEV_THREAD' in
which case a thread which starts executing the function
pointed to by `sigev_notify_function'.
`int aio_lio_opcode'
This element is only used by the `lio_listio' and
`[lio_listio64' functions. Since these functions allow an
arbitrary number of operations to start at once, and since
each operation can be input or output (or nothing), the
information must be stored in the control block. See the
description of `struct aiocb' for a description of the
possible values.
When the sources are compiled using `_FILE_OFFSET_BITS == 64' on a
32 bit machine, this type is available under the name `struct
aiocb64', since the LFS transparently replaces the old interface.
Asynchronous Reads/Writes- Asynchronous Read and Write Operations.
- Status of AIO Operations
- Getting the Status of AIO Operations.
- Synchronizing AIO Operations
- Getting into a consistent state.
- Cancel AIO Operations
- Cancellation of AIO Operations.
- Configuration of AIO
- How to optimize the AIO implementation.
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