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\" $FreeBSD: /repoman/r/ncvs/src/lib/libc/sys/aio_read.2,v 1.23 2005/12/13 13:43:35 davidxu Exp $
\"
Dd May 17, 2010
Dt AIO_READ 3
Os
Sh NAME
Nm aio_read
Nd asynchronous read from a file (REALTIME)
Sh LIBRARY
Lb librt
Sh SYNOPSIS
In aio.h
Ft int
Fn aio_read "struct aiocb *aiocbp"
Sh DESCRIPTION
The
Fn aio_read
system call allows the calling process to read
Fa aiocbp->aio_nbytes
from the descriptor
Fa aiocbp->aio_fildes
beginning at the offset
Fa aiocbp->aio_offset
into the buffer pointed to by
Fa aiocbp->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.
Pp
If
Dv _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
Fa aiocbp->aio_reqprio .
Pp
The
Fa aiocbp->aio_lio_opcode
argument
is ignored by the
Fn aio_read
system call.
Pp
The
Fa aiocbp
pointer may be subsequently used as an argument to
Fn aio_return
and
Fn aio_error
in order to determine return or error status for the enqueued operation
while it is in progress.
Pp
If the request could not be enqueued (generally due to invalid arguments),
then the call returns without having enqueued the request.
Pp
If the request is successfully enqueued, the value of
Fa aiocbp->aio_offset
can be modified during the request as context, so this value must
not be referenced after the request is enqueued.
Sh RESTRICTIONS
The Asynchronous I/O Control Block structure pointed to by
Fa aiocbp
and the buffer that the
Fa aiocbp->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.
Pp
The asynchronous I/O control buffer
Fa aiocbp
should be zeroed before the
Fn aio_read
call to avoid passing bogus context information to the kernel.
Pp
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.
Pp
If the file offset in
Fa aiocbp->aio_offset
is past the offset maximum for
Fa aiocbp->aio_fildes ,
no I/O will occur.
Sh RETURN VALUES
Rv -std aio_read
Sh ERRORS
The
Fn aio_read
system call will fail if:
Bl -tag -width Er
It Bq Er EAGAIN
The request was not queued because of system resource limitations.
El
Pp
The following conditions may be synchronously detected when the
Fn aio_read
system call is made, or asynchronously, at any time thereafter.
If they
are detected at call time,
Fn aio_read
returns \-1 and sets
Va errno
appropriately; otherwise the
Fn aio_return
system call must be called, and will return \-1, and
Fn aio_error
must be called to determine the actual value that would have been
returned in
Va errno .
Pp
Bl -tag -width Er
It Bq Er EBADF
The
Fa aiocbp->aio_fildes
argument
is invalid.
It Bq Er EINVAL
The offset
Fa aiocbp->aio_offset
is not valid, the priority specified by
Fa aiocbp->aio_reqprio
is not a valid priority, or the number of bytes specified by
Fa aiocbp->aio_nbytes
is not valid.
It Bq Er EOVERFLOW
The file is a regular file,
Fa aiocbp->aio_nbytes
is greater than zero, the starting offset in
Fa aiocbp->aio_offset
is before the end of the file, but is at or beyond the
Fa aiocbp->aio_fildes
offset maximum.
El
Pp
If the request is successfully enqueued, but subsequently cancelled
or an error occurs, the value returned by the
Fn aio_return
system call is per the
Xr read 2
system call, and the value returned by the
Fn aio_error
system call is either one of the error returns from the
Xr read 2
system call, or one of:
Bl -tag -width Er
It Bq Er EBADF
The
Fa aiocbp->aio_fildes
argument
is invalid for reading.
It Bq Er ECANCELED
The request was explicitly cancelled via a call to
Fn aio_cancel .
It Bq Er EINVAL
The offset
Fa aiocbp->aio_offset
would be invalid.
El
Sh SEE ALSO
Xr siginfo 2 ,
Xr aio 3
Sh STANDARDS
The
Fn aio_read
system call is expected to conform to the
St -p1003.1-2001
standard.
Sh HISTORY
The
Fn aio_read
system call first appeared in
Nx 5.0 .

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