The aio_read function requests an asynchronous "n = read(fd, buf,
count)" with fd, buf, count given by aiocbp->aio_fildes,
aiocbp->aio_buf, aiocbp->aio_nbytes, respectively. The
return status n can be retrieved upon completion using aio_return(3).
The data is read starting at the absolute file offset
aiocbp->aio_offset, regardless of the current file position. After
this request, the value of the current file position is unspecified.
The "asynchronous" means that this call returns as soon as the request
has been enqueued; the read may or may not have completed when the call
returns. One tests for completion using aio_error(3).
If _POSIX_PRIORITIZED_IO is defined, and this file supports it, then the
asynchronous operation is submitted at a priority equal to that of the calling
process minus aiocbp->aio_reqprio.
The field aiocbp->aio_lio_opcode is ignored.
No data is read from a regular file beyond its maximum offset.
On success, 0 is returned. On error the request is not enqueued, -1 is returned,
and errno is set appropriately. If an error is first detected later, it
will be reported via aio_return(3) (returns status -1) and
aio_error(3) (error status whatever one would have gotten in
errno, such as EBADF).
It is a good idea to zero out the control block before use. This control block
must not be changed while the read operation is in progress. The buffer area
being read into must not be accessed during the operation or undefined results
may occur. The memory areas involved must remain valid.