HP-UX Reference (11i v1 05/09) - 5 Miscellaneous Topics (vol 9)

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aio(5) aio(5)

Manifest Constants

Certain values as deﬁned by the POSIX standard are declared in

aio.h.

The following values are returned by the

aio_cancel()

function:

AIO_CANCELED All speciﬁed asynchronous I/O operations were successfully canceled.

AIO_NOTCANCELED

At least one speciﬁed asynchronous I/O operations was not successfully canceled.

AIO_ALLDONE All speciﬁed asynchronous I/O operations were completed before the request was

processed.

The following values are valid values of the

flags ﬁeld that controls return from the lio_listio()

function:

LIO_WAIT Wait for all speciﬁed operations to complete.

LIO_NOWAIT Return without waiting for operations to complete.

The following values are operation codes supplied in the

aio_lio_opcode ﬁeld the designate the type of

an operation started with

lio_listio() .

LIO_READ The aiocb speciﬁes an asynchronous read operation.

LIO_WRITE The aiocb speciﬁes an asynchronous write operation.

LIO_NOP The aiocb speciﬁes no operation and is silently ignored.

Enqueuing of Operations

If an error condition is detected that prevents an operation from being started, aio_read() and

aio_write() do not enqueue a request. Instead they immediately return -1 and set errno to indicate

the cause of the failure. Once an operation has been successfully enqueued, calls to

aio_error() and

aio_return() must be used to determine the status of the operation and any error conditions, including

those normally reported by read() and write(). The request remains enqueued and consumes process

and system resources until aio_return() is called.

Error reporting of operations enqueued by lio_listio() may be less immediate than that of operations

enqueued by aio_read() and aio_write() . With the exception of resource shortages, errors for

which aio_read() and aio_write() would immediately return -1 and an errno value do not cause

lio_listio() to stop enqueuing the current or subsequent requests in its list. Instead, partial success

occurs. In this case, the application must use aio_error() to determine which operations in its list

have been enqueued and which resulted in errors.

Asynchronous I/O operations are said to be complete when one of the following is true:

• The I/O transfer is performed successfully.

• An error is detected in one or more parameters of the operation.

• The operation is canceled.

If a valid sigevent is speciﬁed in the aiocb used to start the operation, then that signal is delivered

when the operation completes.

Reading and Writing Asynchronously

Asynchronous read and write operations are started using the aio_read(), aio_write() , and

lio_listio() interfaces. The parameters for each operation are provided in the aiocb used to start

the operation. A list of aiocb pointers can be provided to the lio_listio() function call, in which

case the type (read or write) of the operation is determined from the aio_lio_opcode ﬁeld of the

aiocb. Once started, the I/O operations may proceed concurrently with execution of the process or thread

that initiated the operation.

With the implementation of HP-UX kernel threads, an application may achieve asynchronous I/O behavior

by using synchronous read() and write() functions from independent threads within the process.

However, the application may have to implement many of the status management facilities provided in the

POSIX Asynchronous I/O facility.

Waiting for Completion

The POSIX Asynchronous I/O facility supports both polling and notiﬁcation models. The polling model is

implemented by repeatedly calling the aio_error() function to test the status of an operation. The

HP-UX 11i Version 1: September 2005 − 2 − Hewlett-Packard Company Section 5−−17