Manual Pages  — UIO

As a result of any read(2), write(2), readv(2), or writev(2) system call that is being passed to a character-device driver, the appropriate driver d_read or d_write entry will be called with a pointer to a struct uio being passed. The transfer request is encoded in this structure. The driver itself should use uiomove() or uiomove_nofault() to get at the data in this structure.

The fields in the uio structure are:

uio_iov
	The array of I/O vectors to be processed. In the case of scatter/gather I/O, this will be more than one vector.
uio_iovcnt
	The number of I/O vectors present.
uio_offset
	The offset into the device.
uio_resid
	The remaining number of bytes to process, updated after transfer.
uio_segflg
	One of the following flags:

UIO_USERSPACE
	The I/O vector points into a process's address space.
UIO_SYSSPACE	The I/O vector points into the kernel address space.
UIO_NOCOPY	Do not copy, already in object.

uio_rw
	The direction of the desired transfer, either UIO_READ or UIO_WRITE.
uio_td
	The pointer to a struct thread for the associated thread; used if uio_segflg indicates that the transfer is to be made from/to a process's address space.

The function uiomove_nofault() requires that the buffer and I/O vectors be accessible without incurring a page fault. The source and destination addresses must be physically mapped for read and write access, respectively, and neither the source nor destination addresses may be pageable. Thus, the function uiomove_nofault() can be called from contexts where acquiring virtual memory system locks or sleeping are prohibited.

The uiomove_frombuf() function is a convenience wrapper around uiomove() for drivers that serve data which is wholly contained within an existing buffer in memory. It validates the uio_offset and uio_resid values against the size of the existing buffer, handling short transfers when the request partially overlaps the buffer. When uio_offset is greater than or equal to the buffer size, the result is success with no bytes transferred, effectively signaling EOF.