table of contents
UIO(9) | Kernel Developer's Manual | UIO(9) |
NAME¶
uio
,
uiomove
,
uiomove_nofault
—
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);
int
uiomove_nofault
(void
*buf, int
howmuch, struct
uio *uiop);
DESCRIPTION¶
The functionsuiomove
() and
uiomove_nofault
() are used to transfer data
between buffers and I/O vectors that might possibly 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
() 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
orUIO_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.
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.
RETURN VALUES¶
On successuiomove
() and
uiomove_nofault
() will return 0; on error
they will return an appropriate error code.
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 theuio
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(struct cdev *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); }
ERRORS¶
uiomove
() and
uiomove_nofault
() will fail and return the
following error code if:
- [
EFAULT
] - The invoked copyin(9) or
copyout(9) returned
EFAULT
uiomove_nofault
() will fail and
return the following error code if:
- [
EFAULT
] - A page fault occurs.
SEE ALSO¶
read(2), readv(2), write(2), writev(2), copyin(9), copyout(9), sleep(9)HISTORY¶
Theuio
mechanism appeared in some early
version of UNIX.
AUTHORS¶
This manual page was written by Jörg Wunsch.January 19, 2012 | Debian |