uwritec Kernel Service
Purpose
Retrieves a character from a buffer described by a uio structure.
Syntax
#include <sys/types.h>
#include <sys/errno.h>
#include <sys/uio.h>struct uio *uiop;
Parameter
| Item | Description |
|---|---|
| uiop | Points to a uio structure describing the buffer from which to read a character. |
Description
The uwritec kernel service reads a character from a buffer described by a uio structure. Device driver top half routines, especially character device drivers, frequently use the uwritec kernel service to transfer data out of a user area. The uio_resid and uio_iovcnt fields in the uio structure must be greater than 0 or an error is returned.
The uio_segflg field in the uio structure indicates whether the data is being read out of a user- or kernel-data area. This field also indicates whether the caller requires cross-memory operations and has provided the required cross-memory descriptors. The values for this flag are defined in the /usr/include/sys/uio.h file.
If the data is successfully read, the following fields in the uio structure are updated:
| Field | Description |
|---|---|
| uio_iov | Specifies the address of the current iovec element to use. |
| uio_xmem | Specifies the address of the current xmem element to use (used for cross-memory copy). |
| uio_iovcnt | Specifies the number of remaining iovec elements. |
| uio_iovdcnt | Specifies the number of iovec elements already processed. |
| uio_offset | Specifies the character offset on the device to which data is written. |
| uio_resid | Specifies the total number of characters remaining in the data area described by the uio structure. |
| iov_base | Specifies the address of the next available character in the data area described by the current iovec element. |
| iov_len | Specifies the length of the remaining data in the buffer described by the current iovec element. |
Execution Environment
The uwritec kernel service can be called from the process environment only.
Return Values
Upon successful completion, the uwritec service returns the character it was sent to retrieve.
| Item | Description |
|---|---|
| -1 | Indicates that
the buffer is empty or the user location is
not valid for one of these three reasons:
|