xref: /freebsd/lib/libsys/aio_read.2 (revision 1b37af092211f9947d9f089f452232b791b9eafc)

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.Dd March 27, 2025
.Dt AIO_READ 2
.Os
.Sh NAME
.Nm aio_read ,
.Nm aio_read2 ,
.Nm aio_readv
.Nd asynchronous read from a file (REALTIME)
.Sh LIBRARY
.Lb libc
.Sh SYNOPSIS
.In aio.h
.Ft int
.Fn aio_read "struct aiocb *iocb"
.Ft int
.Fn aio_read2 "struct aiocb *iocb" "int flags"
.In sys/uio.h
.Ft int
.Fn aio_readv "struct aiocb *iocb"
.Sh DESCRIPTION
The
.Fn aio_read ,
.Fn aio_read2
and
.Fn aio_readv
system calls allow the calling process to read
from the descriptor
.Fa iocb->aio_fildes .
The syscalls return 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
For the
.Fn aio_read
and
.Fn aio_readv
calls, the read begins at the offset
.Fa iocb->aio_offset .
.Pp
The
.Fn aio_read
call will read
.Fa iocb->aio_nbytes
into the buffer pointed to by
.Fa iocb->aio_buf ,
whereas
.Fn aio_readv
reads the data into the
.Fa iocb->aio_iovcnt
buffers specified by the members of the
.Fa iocb->aio_iov
array.
For
.Fn aio_readv
the
.Fa iovec
structure is defined in
.Xr readv 2 .
.Pp
The
.Fn aio_read2
call takes the
.Fa flags
argument.
If
.Fa flags
is passed as zero, the call behaves identically to
.Fn aio_read .
The following flags can be specified by logical or:
.Bl -tag -width AIO_OP2_VECTORED
.It AIO_OP2_FOFFSET
The read occurs at the file descriptor offset,
which is advanced by the operation as done by the
.Xr read 2
syscall.
The
.Fa iocb->aio_offset
field is ignored.
.It AIO_OP2_VECTORED
Similar to
.Fn aio_readv ,
the read buffers are specified by the
.Fa aiocb->aio_iov
array.
.El
.Pp
The
.Fa iocb
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 iocb->aio_offset
can be modified during the request as context, so this value must
not be referenced after the request is enqueued.
.Pp
The
.Fa iocb->aio_sigevent
structure can be used to request notification of the operation's
completion as described in
.Xr aio 4 .
.Sh RESTRICTIONS
The Asynchronous I/O Control Block structure pointed to by
.Fa iocb
and the buffer that the
.Fa iocb->aio_buf
member of that structure references must remain valid until the
operation has completed.
.Pp
The asynchronous I/O control buffer
.Fa iocb
should be zeroed before the system
calls to avoid passing bogus context information to the kernel.
.Pp
Modifications of the Asynchronous I/O Control Block structure or the
buffer contents are not allowed while the request is queued.
.Pp
If the file offset in
.Fa iocb->aio_offset
is past the offset maximum for
.Fa iocb->aio_fildes ,
no I/O will occur.
.Sh RETURN VALUES
.Rv -std aio_read aio_read2 aio_readv
.Sh DIAGNOSTICS
None.
.Sh ERRORS
The
.Fn aio_read ,
.Fn aio_read2 ,
and
.Fn aio_readv
system calls will fail if:
.Bl -tag -width Er
.It Bq Er EAGAIN
The request was not queued because of system resource limitations.
.It Bq Er EFAULT
Part of
.Fa aio_iov
points outside the process's allocated address space.
.It Bq Er EINVAL
The asynchronous notification method in
.Fa iocb->aio_sigevent.sigev_notify
is invalid or not supported.
.It Bq Er EOPNOTSUPP
Asynchronous read operations on the file descriptor
.Fa iocb->aio_fildes
are unsafe and unsafe asynchronous I/O operations are disabled.
.El
.Pp
The following conditions may be synchronously detected when the
.Fn aio_read
or
.Fn aio_readv
system call is made, or asynchronously, at any time thereafter.
If they
are detected at call time,
The calls return -1 and set
.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 .
.Bl -tag -width Er
.It Bq Er EBADF
The
.Fa iocb->aio_fildes
argument
is invalid.
.It Bq Er EINVAL
The offset
.Fa iocb->aio_offset
is not valid, the priority specified by
.Fa iocb->aio_reqprio
is not a valid priority, or the number of bytes specified by
.Fa iocb->aio_nbytes
is not valid.
.It Bq Er EOVERFLOW
The file is a regular file,
.Fa iocb->aio_nbytes
is greater than zero, the starting offset in
.Fa iocb->aio_offset
is before the end of the file, but is at or beyond the
.Fa iocb->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 iocb->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 iocb->aio_offset
would be invalid.
.El
.Sh SEE ALSO
.Xr aio_cancel 2 ,
.Xr aio_error 2 ,
.Xr aio_return 2 ,
.Xr aio_suspend 2 ,
.Xr aio_waitcomplete 2 ,
.Xr aio_write 2 ,
.Xr sigevent 3 ,
.Xr siginfo 3 ,
.Xr aio 4
.Sh STANDARDS
The
.Fn aio_read
system call is expected to conform to the
.St -p1003.1
standard.
The
.Fn aio_read2
and
.Fn aio_readv
system calls are
.Fx
extensions,
and should not be used in portable code.
.Sh HISTORY
The
.Fn aio_read
system call first appeared in
.Fx 3.0 .
The
.Fn aio_readv
system call first appeared in
.Fx 13.0 .
The
.Fn aio_read2
system call first appeared in
.Fx 14.1 .
.Sh AUTHORS
This
manual page was written by
.An Terry Lambert Aq Mt terry@whistle.com .
.Sh BUGS
Invalid information in
.Fa iocb->_aiocb_private
may confuse the kernel.