AIO_READ(2)

NAME

     aio_read -- asynchronous read from a file (REALTIME)

LIBRARY

     Standard C Library (libc, -lc)

SYNOPSIS

     #include <aio.h>

     int
     aio_read(struct aiocb *iocb);

DESCRIPTION

     The aio_read() system call allows the calling process to read
     iocb->aio_nbytes from the descriptor iocb->aio_fildes beginning at the
     offset iocb->aio_offset into the buffer pointed to by iocb->aio_buf.  The
     call returns immediately after the read request has been enqueued to the
     descriptor; the read may or may not have completed at the time the call
     returns.

     If _POSIX_PRIORITIZED_IO is defined, and the descriptor supports it, then
     the enqueued operation is submitted at a priority equal to that of the
     calling process minus iocb->aio_reqprio.

     The iocb->aio_lio_opcode argument is ignored by the aio_read() system
     call.

     The iocb pointer may be subsequently used as an argument to aio_return()
     and aio_error() in order to determine return or error status for the
     enqueued operation while it is in progress.

     If the request could not be enqueued (generally due to invalid argu-
     ments), then the call returns without having enqueued the request.

     If the request is successfully enqueued, the value of iocb->aio_offset
     can be modified during the request as context, so this value must not be
     referenced after the request is enqueued.

RESTRICTIONS

     The Asynchronous I/O Control Block structure pointed to by iocb and the
     buffer that the iocb->aio_buf member of that structure references must
     remain valid until the operation has completed.  For this reason, use of
     auto (stack) variables for these objects is discouraged.

     The asynchronous I/O control buffer iocb should be zeroed before the
     aio_read() call to avoid passing bogus context information to the kernel.

     Modifications of the Asynchronous I/O Control Block structure or the
     buffer contents after the request has been enqueued, but before the
     request has completed, are not allowed.

     If the file offset in iocb->aio_offset is past the offset maximum for
     iocb->aio_fildes, no I/O will occur.

RETURN VALUES

     The aio_read() function returns the value 0 if successful; otherwise the
     value -1 is returned and the global variable errno is set to indicate the
     error.

DIAGNOSTICS

     None.

ERRORS

     The aio_read() system call will fail if:

     [EAGAIN]		The request was not queued because of system resource
			limitations.

     [ENOSYS]		The aio_read() system call is not supported.

     The following conditions may be synchronously detected when the
     aio_read() system call is made, or asynchronously, at any time there-
     after.  If they are detected at call time, aio_read() returns -1 and sets
     errno appropriately; otherwise the aio_return() system call must be
     called, and will return -1, and aio_error() must be called to determine
     the actual value that would have been returned in errno.

     [EBADF]		The iocb->aio_fildes argument is invalid.

     [EINVAL]		The offset iocb->aio_offset is not valid, the priority
			specified by iocb->aio_reqprio is not a valid prior-
			ity, or the number of bytes specified by
			iocb->aio_nbytes is not valid.

     [EOVERFLOW]	The file is a regular file, iocb->aio_nbytes is
			greater than zero, the starting offset in
			iocb->aio_offset is before the end of the file, but is
			at or beyond the iocb->aio_fildes offset maximum.

     If the request is successfully enqueued, but subsequently cancelled or an
     error occurs, the value returned by the aio_return() system call is per
     the read(2) system call, and the value returned by the aio_error() system
     call is either one of the error returns from the read(2) system call, or
     one of:

     [EBADF]		The iocb->aio_fildes argument is invalid for reading.

     [ECANCELED]	The request was explicitly cancelled via a call to
			aio_cancel().

     [EINVAL]		The offset iocb->aio_offset would be invalid.

SEE ALSO

     aio_cancel(2), aio_error(2), aio_return(2), aio_suspend(2),
     aio_waitcomplete(2), aio_write(2), aio(4)

STANDARDS

     The aio_read() system call is expected to conform to the IEEE Std 1003.1
     (``POSIX.1'') standard.

HISTORY

     The aio_read() system call first appeared in FreeBSD 3.0.

AUTHORS

     This manual page was written by Terry Lambert <terry@whistle.com>.

BUGS

     Invalid information in iocb->_aiocb_private may confuse the kernel.