Advanced Programming in the UNIX® Environment

ptg10805159

514 Advanced I/O Chapter 14

We use asynchronous I/O when we have other processing to do and we don’t want

to block while performing the I/O operation. However,when we have completed the

processing and find that we still have asynchronous operations outstanding, we can call

theaio_suspendfunction to block until an operation completes.

#include <aio.h>

int aio_suspend(const struct aiocb *constlist[], intnent,

const struct timespec *timeout);

Returns: 0 if OK,−1 on error

One of three things can causeaio_suspendto return. If we areinterrupted by a

signal, it returns−1witherrnoset toEINTR.Ifthe time limit specified by the optional

timeout argument expires without any of the I/O operations completing, then

aio_suspendreturns−1witherrnoset toEAGAIN(we can pass a null pointer for the

timeoutargument if we want to block without a time limit). If any of the I/O operations

complete,aio_suspendreturns 0. If all asynchronous I/O operations arecomplete

when we callaio_suspend,thenaio_suspendwill return without blocking.

Thelistargument is a pointer to an array of AIO control blocks and thenent

argument indicates the number of entries in the array.Null pointers in the array are

skipped; the other entries must point to AIO control blocks that have been used to

initiate asynchronous I/O operations.

When we have pending asynchronous I/O operations that we no longer want to

complete, we can attempt to cancel them with theaio_cancelfunction.

#include <aio.h>

int aio_cancel(intfd,struct aiocb *aiocb);

Returns: (see following)

Thefdargument specifies the file descriptor with the outstanding asynchronous I/O

operations. If theaiocb argument isNULL,then the system attempts to cancel all

outstanding asynchronous I/O operations on the file. Otherwise, the system attempts

to cancel the single asynchronous I/O operation described by the AIO control block.

We say that the system ‘‘attempts’’ to cancel the operations, because there is no

guarantee that the system will be able to cancel any operations that are in progress.

Theaio_cancelfunction can return one of four values:

AIO_ALLDONE All of the operations completed beforethe attempt to cancel

them.

AIO_CANCELED All of the requested operations have been canceled.

AIO_NOTCANCELED At least one of the requested operations could not be

canceled.

−1The call toaio_cancelfailed. The error code will be stored

inerrno.