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LINUX DRIVES WEEDER I/O BOARDS

by Jack Dennon

Start ý Create the Device Driver ý Create the Device ý The Order of Things ý Load the Device Driver ý Call the Test Program ý Remove the Device Driver ý Sources and PDF

CREATE THE DEVICE

In any UNIX-like system such as Linux, attached peripheral devices are usually accessed through the file system. A directory entry that represents a peripheral device is said to point to a special file, also called a device file. Such an entry can be placed in any directory, but it is conventional to place device entries in the well-known directory /dev where programs can find them.

For example, to access a line printer, there will be one or more device files in this directory starting with lp. The command ls -l /dev/lp* will display entries such as:

crw-rw---- 1 root daemon 6, 0 Apr 27 1995 /dev/lp0
crw-rw---- 1 root daemon 6, 1 Apr 27 1995 /dev/lp1
crw-rw---- 1 root daemon 6, 2 Apr 27 1995 /dev/lp2

The c in the string crw-rw---- means that the device named /dev/lp0, for example, is a character special device, and the rest of the string shows that root and members of the daemon group have read and write access to this device. The line printer driver in this system has been assigned major number 6 and is intended to handle up to three attached printers that are to be selected via minor numbers 0, 1, and 2.

The kernel cares only about the device type c and the major device number. The minor numbers are passed to and used only by the device driver. The major number is the index into a kernel table of pointers to device drivers.

To examine the current contents of your kernelýs device table, you can use the command cat /proc/devices. You should see a display such as shown in Table 1. Here, you see that there is a device driver named lp, which occupies location 6 in the device driver table.