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.
