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System Interface Guide

Solaris Asynchronous I/O

Notification (SIGIO)

When an asynchronous I/O call returns successfully, the I/O operation has only been queued, waiting to be done. The actual operation also has a return value and a potential error identifier, the values that would have been returned to the caller as the result of a synchronous call. When the I/O is finished, the return value and error value 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 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 which request, if any, is the cause of the SIGIO signal. A process receiving a SIGIO should check all its conditions that could be generating the SIGIO signal.

aioread(3AIO)

aioread(3AIO) is the asynchronous version of read(2). In addition to the normal read arguments, aioread(3AIO) takes the arguments specifying a file position and the address of an aio_result_t structure in which the system stores the result information about the operation. 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.

aiowrite(3AIO)

aiowrite(3AIO) is the asynchronous version of write(2). In addition to the normal write arguments, aiowrite(3AIO) takes arguments specifying a file position and the address of an aio_result_t structure in which the system is to store the resulting information about the operation.

The file position specifies a seek 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.

aiocancel(3AIO)

aiocancel(3AIO) 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.

aiowait(3AIO)

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.

poll(2)

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

Use of poll(2) for very large numbers of files is slow. This problem is resolved by poll(7D).

poll(7D)

/dev/poll provides a highly scalable way of polling a large number of file descriptors. This 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). poll(7D) provides details and examples of the /dev/poll API. When used properly, the /dev/poll API scales much better than poll(2). It is especially suited for applications that satisfy the following criteria:

close(2)

Files are closed by calling close(2). close(2) cancels any outstanding asynchronous I/O request that can be. close(2) waits for an operation that cannot be cancelled (see "aiocancel(3AIO)"). When close(2) returns, there is no asynchronous I/O 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.