Architecture independent level 1 (DDI/DKI)

A uio structure describes an I/O request that can be broken up into different data storage areas (scatter/gather I/O). A request is a list of iovec structures (base-length pairs) indicating where in user space or kernel space the I/O data is to be read or written.

The contents of uio structures passed to the driver through the entry points should not be written by the driver. The uiomove(9F) function takes care of all overhead related to maintaining the state of the uio structure.

uio structures allocated by the driver should be initialized to zero before use, by bzero(9F), kmem_zalloc(9F), or an equivalent.

iovec_t      *uio_iov;      /* pointer to the start of the iovec */
                            /* list for the uio structure */
int          uio_iovcnt;    /* the number of iovecs in the list */
off_t        uio_offset;    /* 32-bit offset into file where data is */
                            /* transferred from or to. See NOTES. */
offset_t     uio_loffset;   /* 64-bit offset into file where data is */
                            /* transferred from or to. See NOTES. */
uio_seg_t    uio_segflg;    /* identifies the type of I/O transfer: */
                            /*    UIO_SYSSPACE:  kernel <-> kernel */
                            /*    UIO_USERSPACE: kernel <-> user */
short        uio_fmode;     /* file mode flags (not driver setable) */
daddr_t      uio_limit;     /* 32-bit ulimit for file (maximum block */
                            /* offset). not driver setable. See NOTES. */
diskaddr_t   uio_llimit;    /* 64-bit ulimit for file (maximum block */
                            /* offset). not driver setable. See NOTES. */
int          uio_resid;     /* residual count */

The uio_iov member is a pointer to the beginning of the iovec(9S) list for the uio. When the uio structure is passed to the driver through an entry point, the driver should not set uio_iov. When the uio structure is created by the driver, uio_iov should be initialized by the driver and not written to afterward.

aread(9E), awrite(9E), read(9E), write(9E), bzero(9F), kmem_zalloc(9F), uiomove(9F), cb_ops(9S), iovec(9S)

Writing Device Drivers

Only one structure, uio_offset or uio_loffset, should be interpreted by the driver. Which field the driver interprets is dependent upon the settings in the cb_ops(9S) structure.

Only one structure, uio_limit or uio_llimit, should be interpreted by the driver. Which field the driver interprets is dependent upon the settings in the cb_ops(9S) structure.

When performing I/O on a seekable device, the driver should not modify either the uio_offset or the uio_loffset field of the uio structure. I/O to such a device is constrained by the maximum offset value. When performing I/O on a device on which the concept of position has no relevance, the driver may preserve the uio_offset or uio_loffset, perform the I/O operation, then restore the uio_offset or uio_loffset to the field's initial value. I/O performed to a device in this manner is not constrained.