Advanced Programming in the UNIX® Environment

ptg10805159

Section 14.5 Asynchronous I/O 515

If an asynchronous I/O operation is successfully canceled, calling theaio_error

function on the corresponding AIO control block will return the errorECANCELED.If

the operation can’t be canceled, then the corresponding AIO control block is unchanged

by the call toaio_cancel.

One additional function is included with the asynchronous I/O interfaces, although

it can be used in either a synchronous or an asynchronous manner.Thelio_listio

function submits a set of I/O requests described by a list of AIO control blocks.

#include <aio.h>

int lio_listio(intmode,struct aiocb *restrict const list[restrict],

intnent,struct sigevent *restrictsigev);

Returns: 0 if OK,−1 on error

Themodeargument determines whether the I/O is truly asynchronous. When it is set to

LIO_WAIT,thelio_listiofunction won’t return until all of the I/O operations

specified by the list arecomplete. In this case, thesigevargument is ignored. When the

modeargument is set toLIO_NOWAIT,then thelio_listiofunction returns as soon as

the I/O requests arequeued. The process is notified asynchronously when all of the

I/O operations complete, as specified by thesigevargument. If we don’t want to be

notified, we can set sigevto NULL.Note that the individual AIO control blocks

themselves may also enable asynchronous notification when an individual operation

completes. The asynchronous notification specified by thesigevargument is in addition

to these, and is sent only when all of the I/O operations complete.

The list argument points to a list of AIO control blocks specifying the I/O

operations to perform. Thenentargument specifies the number of elements in the array.

The list of AIO control blocks can containNULLpointers; these entries areignored.

In each AIO control block, theaio_lio_opcode field specifies whether the

operation is a read (LIO_READ), a write (LIO_WRITE), or a no-op (LIO_NOP), which is

ignored. Aread is treated as if the corresponding AIO control block had been passed to

theaio_readfunction. Similarly,awrite is treated as if the AIO control block had

been passed toaio_write.

Implementations can limit the number of asynchronous I/O operations we are

allowed to have outstanding. The limits areruntime invariants, and aresummarized in

Figure14.19.

Name Description Minimum acceptable value

AIO_LISTIO_MAX maximum number of I/O operations _POSIX_AIO_LISTIO_MAX( 2 )

in a single list I/O call

AIO_MAX maximum number of outstanding _POSIX_AIO_MAX( 1 )

asynchronous I/O operations

AIO_PRIO_DELTA_MAX maximum amount by which a process 0

can decrease its asynchronous

I/O priority level

Figure 14.19POSIX.1 runtime invariant values for asynchronous I/O