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 begin

ning 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 en

queued 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 sup

ports 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 in

valid arguments), 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 ref

erences 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 struc

ture or the
buffer contents after the request has been enqueued, but be

fore 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 sup

ported.

The following conditions may be synchronously detected when

the
aio_read() system call is made, or asynchronously, at any

time thereafter. If they are detected at call time, aio_read() re

turns -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 priority, 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 in

valid.

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@whis

tle.com>.

BUGS

Invalid information in iocb->_aiocb_private may confuse the

kernel.

BSD November 17, 1998

BSD