SCO OpenServer

read(D2oddi)

read -- read data character-by-character

Synopsis

   void prefixread (dev_t dev);

Description

The read( ) entry point routine initiates and supervises the raw I/O data transfer from the character device to the user data area.

Arguments

dev: device number

Return values

None

Usage

When the device being accessed is a true character device (rather than a block device being accessed in raw mode through the cdevsw table), the transferred data is usually buffered by the driver. For example, using the tty structure to get a cblock, the ttread(D3oddi) and ttwrite(D3oddi) functions handle character driver buffering.

Drivers for low-speed character devices such as terminals and printers perform semantic processing of data. These drivers typically use the clist data structure and the TTY subsystem to perform buffering. These transfer data in and out of the user data area.

Drivers for high-speed character devices such as network interface boards generally set up their own buffering schemes.

Block devices

Block devices usually include a read( ) routine to support raw I/O for the device. The logic for these routines is:

Extract the minor device number for use by physio(D3oddi) when performing physical I/O.
Check the minor device number against the information in the scsi_dev_cfg(D4osdi) configuration table. If it is improper, use the cmn_err(D3oddi) function to issue an error message, set u.u_error to ENODEV, and return.
For disk devices, get the size of the device from a local driver array of disk sizes.
Call physck(D3oddi) to ensure that the I/O is correct for the device size.
Call physio(D3oddi). This routine calls dma_breakup(D3oddi) or pio_breakup(D3oddi) which then calls strategy(D2oddi) to perform the raw I/O.

Context and synchronization

User context. The driver can block and can do operations such as copyout(D3) that require access to the requesting process's user-level address space.

Hardware applicability

All

Version applicability

oddi: 1, 2, 2mp, 3, 3mp, 4, 4mp, 5, 5mp, 6, 6mp

SVR5 DDI compatibility

The read(D2) entry point routine provides similar functionality for DDI versions prior to DDI 8, although note that the syntax is different. DDI 8 drivers use the biostart(D2) entry point routine instead of read( ).

External dependencies

Drivers that contain a read( ) routine must have a r in the second column of the mdevice(F) file.

References

strategy(D2oddi), write(D2oddi)