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DESCRIPTION

The uio argument is a pointer to a struct uio as defined by <sys/uio.h>:

struct uio {
	struct	iovec *uio_iov;	/* pointer to array of iovecs */
	int	uio_iovcnt;	/* number of iovecs in array */
	off_t	uio_offset;	/* offset into file this uio corresponds to */
	size_t	uio_resid;	/* residual i/o count */
	enum	uio_seg uio_segflg;
	enum	uio_rw uio_rw;
	struct	proc *uio_procp;/* associated process or NULL */
};

A struct uio typically describes data in motion. Several of the fields described below reflect that expectation.

uio_iov

Pointer to array of struct iovecs:

struct iovec {
	void	*iov_base;	/* Base address. */
	size_t	 iov_len;	/* Length. */
};

uio_iovcnt

The number of iovecs in the array.

uio_offset

An offset into the corresponding object.

uio_resid

The amount of data remaining to be transferred.

uio_segflg

A flag indicating whether the space described is in user-space (UIO_USERSPACE) or kernel-space (UIO_SYSSPACE).

uio_rw

A flag indicating whether data should be read into the space (UIO_READ) or written from the space (UIO_WRITE).

uio_procp

A pointer to a process whose data area is described by the structure, or which is having the I/O done on its behalf if the area is in kernel-space. uiomove itself does not use this field if the area is in kernel-space, but other functions that take a struct uio may depend on this information.

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

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

uiomove changes fields of the structure pointed to by uio, such that uio->uio_resid is decremented by the amount of data moved, 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 uiomove to easily be used to fill or drain the region of data.