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UIO(9) Kernel Developer's Manual UIO(9)

NAME

uio, uiomove — device driver I/O routines

SYNOPSIS

#include <sys/types.h>
#include <sys/uio.h> 
struct uio { 
	struct	iovec *uio_iov;		/* scatter/gather list */ 
	int	uio_iovcnt;		/* length of scatter/gather list */ 
	off_t	uio_offset;		/* offset in target object */ 
	ssize_t	uio_resid;		/* remaining bytes to copy */ 
	enum	uio_seg uio_segflg;	/* address space */ 
	enum	uio_rw uio_rw;		/* operation */ 
	struct	thread *uio_td;		/* owner */ 
};

int
uiomove(void *buf, int howmuch, struct uio *uiop);

DESCRIPTION

The function uiomove() is used to handle transfer of data between buffers and I/O vectors that might possibly also cross the user/kernel space boundary.
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() 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.

RETURN VALUES

On success uiomove() will return 0, on error it will return an appropriate errno.

ERRORS

uiomove() will fail and return the following error code if:
[EFAULT]
The invoked copyin(9) or copyout(9) returned EFAULT

EXAMPLES

The idea is that the driver maintains a private buffer for its data, and processes the request in chunks of maximal the size of this buffer. Note that the buffer handling below is very simplified and will not work (the buffer pointer is not being advanced in case of a partial read), it is just here to demonstrate the uio handling. 
/* MIN() can be found there: */ 
#include <sys/param.h> 
 
#define BUFSIZE 512 
static char buffer[BUFSIZE]; 
 
static int data_available;	/* amount of data that can be read */ 
 
static int 
fooread(dev_t dev, struct uio *uio, int flag) 
{ 
	int rv, amnt; 
 
	rv = 0; 
	while (uio->uio_resid > 0) { 
		if (data_available > 0) { 
			amnt = MIN(uio->uio_resid, data_available); 
			rv = uiomove(buffer, amnt, uio); 
			if (rv != 0) 
				break; 
			data_available -= amnt; 
		} else 
			tsleep(...);	/* wait for a better time */ 
	} 
	if (rv != 0) { 
		/* do error cleanup here */ 
	} 
	return (rv); 
}

SEE ALSO

read(2), readv(2), write(2), writev(2), copyin(9), copyout(9), sleep(9)

HISTORY

The uio mechanism appeared in some early version of UNIX.

AUTHORS

This manual page was written by Jörg Wunsch.
March 21, 2010 Debian