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 $NetBSD: uiomove.9,v 1.21 2023/05/22 14:07:24 riastradh Exp $

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.Dd May 9, 2023
.Dt UIOMOVE 9
.Os
.Sh NAME
.Nm uiomove
.Nd move data described by a struct uio
.Sh SYNOPSIS
n sys/systm.h .Ft int
.Fn uiomove "void *buf" "size_t n" "struct uio *uio"
.Ft int
.Fn uiopeek "void *buf" "size_t n" "struct uio *uio"
.Ft void
.Fn uioskip "void *buf" "size_t n" "struct uio *uio"
.Sh DESCRIPTION
The
.Fn uiomove
function copies up to
.Fa n
bytes between the kernel-space address pointed
to by
.Fa buf
and the addresses described by
.Fa uio ,
which may be in user-space or kernel-space.

p
The
.Fa uio
argument is a pointer to a
.Va struct uio
as defined by
n sys/uio.h : d -literal -offset indent struct uio {
 struct iovec *uio_iov;
 int uio_iovcnt;
 off_t uio_offset;
 size_t uio_resid;
 enum uio_rw uio_rw;
 struct vmspace *uio_vmspace;
};
.Ed

p
A
.Va struct uio
typically describes data in motion.
Several of the fields described below reflect that expectation.
l -tag -width "uio_vmspace " t Va uio_iov Pointer to array of
.Tn I/O
vectors to be processed.
The
.Va struct iovec
is defined to be:
d -literal -offset indent struct iovec {
 void *iov_base;
 size_t iov_len;
};
.Ed

p
The members in the
.Va struct iovec
should only be initialized.
These are:
l -tag -width "*iov_base " -offset indent t Va iov_base The address for a range of memory to or from which data is transferred.
t Va iov_len The number of bytes of data to be transferred to
or from the range of memory starting at
.Va iov_base .
.El
t Va uio_iovcnt The number of
.Tn I/O
vectors in the
.Va uio_iov
array.
t Va uio_offset An offset into the corresponding object.
t Va uio_resid The amount of space described by the structure; notionally, the amount
of data remaining to be transferred.
t Va uio_rw A flag indicating whether data should be read into the space
(UIO_READ) or written from the space (UIO_WRITE).
t Va uio_vmspace A pointer to the address space which is being transferred to or from.
.El

p
The value of
.Va uio->uio_rw
controls whether
.Fn uiomove
copies data from
.Fa buf
to
.Fa uio
or vice versa.

p
The lesser of
.Fa n
or
.Va uio->uio_resid
bytes are copied.

p
.Fn uiomove
changes fields of the structure pointed to by
.Fa uio ,
such that
.Va uio->uio_resid
is decremented by the amount of data moved,
.Va uio->uio_offset
is incremented by the same amount, and the array of iovecs is adjusted
to point that much farther into the region described.
This allows multiple calls to
.Fn uiomove
to easily be used to fill or drain the region of data.

p
The
.Fn uiopeek
function copies up to
.Fa n
bytes of data without updating
.Fa uio ;
the
.Fn uioskip
function updates
.Fa uio
without copying any data, and is guaranteed never to sleep or fault
even if the buffers are in userspace and memory access via
.Fn uiomove
or
.Fn uiopeek
would trigger paging.
A successful
.Fn uiomove buf n uio
call is equivalent to a successful
.Fn uiopeek buf n uio
followed by
.Fn uioskip n uio .
.Sh RETURN VALUES
Upon successful completion,
.Fn uiomove
and
.Fn uiopeek
return 0.
If a bad address is encountered,
.Er EFAULT
is returned.
.Sh SEE ALSO
.Xr copy 9 ,
.Xr ufetch 9 ,
.Xr ustore 9