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uio.9freebsd

BSD mandoc

NAME

uio uiomove - device driver I/O routines

SYNOPSIS

In sys/types.h In 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 */
         int     uio_resid;              /* remaining bytes to copy */
         enum    uio_seg uio_segflg;     /* address space */
         enum    uio_rw uio_rw;          /* operation */
         struct  thread *uio_td;         /* owner */
 };
 

Ft int Fn uiomove void *buf int howmuch struct uio *uiop

DESCRIPTION

The function Fn 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 Vt struct uio being passed. The transfer request is encoded in this structure. The driver itself should use Fn uiomove to get at the data in this structure.

The fields in the Vt 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 Vt 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 Fn uiomove will return 0, on error it will return an appropriate errno.

ERRORS

Fn uiomove will fail and return the following error code if:

Bq Er EFAULT

The invoked copyin(9) or copyout(9) returned Er 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 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 mechanism appeared in some early version of UNIX

AUTHORS

This manual page was written by An Jörg Wunsch .