aio_read(3aio)

aio_read, aio_read64 -- asynchronous read

Synopsis

   
   cc [options] -Kthread file
   

   #include <aio.h>
   

   int aio_read(struct aiocb *aiocbp);
   

   int aio_read64(struct aiocb64 *aiocbp);

Description

aio_read supports an asynchronous read capability that allows the calling process to read aiocbp->aio_nbytes from the file associated with the file descriptor aiocbp->aio_fildes into the buffer pointed to by aiocbp->aio_buf.

aiocbp points to an aiocb structure which contains other input parameters as well as completion status members.

If you do not wish to be notified of completion, you should set aio_sigevent.sigev_notify to SIGEV_NONE in the asynchronous I/O control block. If you require notification, you must set the aio_sigevent.sigev_notify to SIGEV_CALLBACK, aio_sigevent.sigev_func to the address of the function to be called, and aio_sigevent.sigev_value to the argument to be passed to the function. You may also want the aio_sigevent.sigev_value to be the address of the asynchronous I/O control block that was used to initiate the I/O request. [See aiocb(5)].

If the control block pointed to by aiocbp or the buffer pointed to by aiocpb->aio_buf becomes an invalid address prior to asynchronous I/O completion, then the behavior is undefined.

The call to aio_read returns 0 when the read request has been initiated or queued to the file or device. If an error condition is encountered during queuing, the call returns -1 without having initiated or queued the request.

Return values

After successfully completing, calls to aio_read and aio_read64 return 0 and the structure member aio_errno is set to EINPROGRESS. If a call to aio_read or aio_read64 fails, -1 is returned with errno set to indicate the error. After the read operation has successfully completed, the aio_error function is set to 0. The return value of the underlying read can be assessed with the aio_return function.

Errors

There are two types of errors that are associated with an asynchronous I/O request. The first occurs during the validity checking of the I/O request submitted by the aio_read and aio_read64 routines. This error is returned to the caller of aio_read or aio_read64. The other occurs during the processing of the actual read operation. The read operation may fail for any of the reasons that a normal read may fail. If the call to aio_read or aio_read64 successfully queues the I/O operation but the operation is canceled or encounters an error, the aio_error function returns one of the values normally returned by the read system call or one of the errors listed below. The time at which the error occurs is implementation-dependent; for example, an invalid file descriptor may not be determined at the time of the call to aio_read or aio_read64 which could successfully complete. However, aio_error then returns EBADF to indicate the error.

Under the following conditions, aio_read fails and sets errno or the aio_errno structure member to:

EOVERFLOW: The file is a regular file, aiocbp->aio_nbytes is greater than 0 and the starting offset in aiocbp->aio_offset is before the end-of-file and is at or beyond the offset maximum in the open file descriptor associated with aiocbp->aio_fildes.

Under the following conditions, aio_read and aio_read64 fail and set errno or the aio_errno structure member to:

EAGAIN: The requested asynchronous I/O operation was not queued because of system resource limitations.
EFAULT: aiocbp or the aio_buf member points outside the allocated address space.
ECANCELED: The requested I/O was canceled before the I/O completed because of aio_cancel.
EINVAL: On SVR4.2, the request does not have the AIO_RAW flag bit set in aio_flags.
ENOMEM: There were no internal kernal aio control blocks available to service the request (number of kernel aio control blocks is tunable via the NUMAIO kernel parameter; see ``Miscellaneous parameters'' in Monitoring and tuning the system).

References

aio_cancel(3aio), aiocb(5), aio_suspend(3aio), aio_write(3aio), intro(2), read(2)

Notices

When developing applications for both SVR4.2 and future versions, the AIO_RAW flag should be set in aio_flags to insure that performance is maintained.

Considerations for large file support

aio_read64 supports large files, but is otherwise identical to aio_read. For details on programming for large file capable applications, see ``Large File Support'' on intro(2).