Solaris Asynchronous I/O

This section discusses asynchronous I/O operations in the Solaris operating environment.

Notification (SIGIO)

When an asynchronous I/O call returns successfully, the I/O operation has only been queued and waits to be done. The actual operation has a return value and a potential error identifier. This return value and potential error identifier would have been returned to the caller if the call had been synchronous. When the I/O is finished, both the return and error values are stored at a location given by the user at the time of the request as a pointer to an aio_result_t. The structure of the aio_result_t is defined in <sys/asynch.h>:

typedef struct aio_result_t {
 	ssize_t	aio_return; /* return value of read or write */
 	int 		aio_errno;  /* errno generated by the IO */
 } aio_result_t;

When the aio_result_t has been updated, a SIGIO signal is delivered to the process that made the I/O request.

Note that a process with two or more asynchronous I/O operations pending has no certain way to determine the cause of the SIGIO signal. A process that receives a SIGIO should check all its conditions that could be generating the SIGIO signal.

Using aioread

The aioread(3AIO) routine is the asynchronous version of read(2). In addition to the normal read arguments, aioread(3AIO) takes the arguments that specify a file position and the address of an aio_result_t structure. The resulting information about the operation is stored in the aio_result_t structure. The file position specifies a seek to be performed within the file before the operation. Whether the aioread(3AIO) call succeeds or fails, the file pointer is updated.

Using aiowrite

The aiowrite(3AIO) routine is the asynchronous version of write(2). In addition to the normal write arguments, aiowrite(3AIO) takes arguments that specify a file position and the address of an aio_result_t structure. The resulting information about the operation is stored in the aio_result_t structure.

The file position specifies that a seek operation is to be performed within the file before the operation. If the aiowrite(3AIO) call succeeds, the file pointer is updated to the position that would have resulted in a successful seek and write. The file pointer is also updated when a write fails to allow for subsequent write requests.

Using aiocancel

The aiocancel(3AIO) routine attempts to cancel the asynchronous request whose aio_result_t structure is given as an argument. An aiocancel(3AIO) call succeeds only if the request is still queued. If the operation is in progress, aiocancel(3AIO) fails.

Using aiowait

A call to aiowait(3AIO) blocks the calling process until at least one outstanding asynchronous I/O operation is completed. The timeout parameter points to a maximum interval to wait for I/O completion. A timeout value of zero specifies that no wait is wanted. aiowait(3AIO) returns a pointer to the aio_result_t structure for the completed operation.

Using poll()

To determine the completion of an asynchronous I/O event synchronously rather than depend on a SIGIO interrupt, use poll(2). You can also poll to determine the origin of a SIGIO interrupt.

poll(2) is slow when used on very large numbers of files. This problem is resolved by poll(7D).

Using the poll Driver

Using /dev/poll provides a highly scalable way of polling a large number of file descriptors. This scalability is provided through a new set of APIs and a new driver, /dev/poll. The /dev/poll API is an alternative to, not a replacement of, poll(2). Use poll(7D) to provide details and examples of the /dev/poll API. When used properly, the /dev/poll API scales much better than poll(2). This API is especially suited for applications that satisfy the following criteria:

• Applications that repeatedly poll a large number of file descriptors

• Polled file descriptors that are relatively stable, meaning that the descriptors are not constantly closed and reopened

• The set of file descriptors that actually have polled events pending is small, comparing to the total number of file descriptors that are being polled

Using close

Files are closed by calling close(2). The call to close(2) cancels any outstanding asynchronous I/O request that can be closed. close(2) waits for an operation that cannot be cancelled. For more information, see Using aiocancel. When close(2) returns, no asynchronous I/O is pending for the file descriptor. Only asynchronous I/O requests queued to the specified file descriptor are cancelled when a file is closed. Any I/O pending requests for other file descriptors are not cancelled.