Asynchronous I/O

In most situations, asynchronous I/O is not required because its effects can be achieved with the use of threads, with each thread execution of synchronous I/O. However, in a few situations, threads cannot achieve what asynchronous I/O can.

The most straightforward example is writing to a tape drive to make the tape drive stream. Streaming prevents the tape drive from stopping while the drive is being written to. The tape moves forward at high speed while supplying a constant stream of data that is written to tape.

To support streaming, the tape driver in the kernel should use threads. The tape driver in the kernel must issue a queued write request when the tape driver responds to an interrupt. The interrupt indicates that the previous tape-write operation has completed.

Threads cannot guarantee that asynchronous writes are ordered because the order in which threads execute is indeterminate. You cannot, for example, specify the order of a write to a tape.

Asynchronous I/O Operations

#include <aio.h>

int aio_read(struct aiocb *aiocbp);

int aio_write(struct aiocb *aiocbp);

int aio_error(const struct aiocb *aiocbp);

ssize_t aio_return(struct aiocb *aiocbp);

int aio_suspend(struct aiocb *list[], int nent,
    const struct timespec *timeout);

int aio_waitn(struct aiocb *list[], uint_t nent, uint_t *nwait,
    const struct timespec *timeout);

int aio_cancel(int fildes, struct aiocb *aiocbp);

aio_read(3RT) and aio_write(3RT) are similar in concept to pread(2) and pwrite(2), except that the parameters of the I/O operation are stored in an asynchronous I/O control block (aiocbp) that is passed to aio_read() or aio_write():

    aiocbp->aio_fildes;    /* file descriptor */
    aiocbp->aio_buf;       /* buffer */
    aiocbp->aio_nbytes;    /* I/O request size */
    aiocbp->aio_offset;    /* file offset */

In addition, if desired, an asynchronous notification type (most commonly a queued signal) can be specified in the 'struct sigevent' member:

    aiocbp->aio_sigevent;  /* notification type */

A call to aio_read() or aio_write() results in the initiation or queueing of an I/O operation. The call returns without blocking.

The aiocbp value may be used as an argument to aio_error(3RT) and aio_return(3RT) in order to determine the error status and return status of the asynchronous operation while it is proceeding.

Waiting for I/O Operation to Complete

You can wait for one or more outstanding asynchronous I/O operations to complete by calling aio_suspend() or aio_waitn(). Use aio_error() and aio_return() on the completed asynchronous I/O control blocks to determine the success or failure of the I/O operation.

The aio_suspend() and aio_waitn() functions take a timeout argument, which indicates how long the caller is willing to wait. A NULL pointer means that the caller is willing to wait indefinitely. A pointer to a structure containing a zero value means that the caller is unwilling to wait at all.

You might start an asynchronous I/O operation, do some work, then call aio_suspend() or aio_waitn() to wait for the request to complete. Or you can rely on the asynchronous notification event specified in aio_sigevent() to occur to notify you when the operation completes.

Finally, a pending asynchronous I/O operation can be cancelled by calling aio_cancel(). This function is called with the address of the I/O control block that was used to initiate the I/O operation.