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Chapter 9. Writing FreeBSD Device Drivers
Writing FreeBSD Device Drivers
Introduction Introdução
This chapter provides a brief introduction to writing device drivers for FreeBSD. A device in this context is a term used mostly for hardware-related stuff that belongs to the system, like disks, printers, or a graphics display with its keyboard. A device driver is the software component of the operating system that controls a specific device. There are also so-called pseudo-devices where a device driver emulates the behavior of a device in software without any particular underlying hardware. Device drivers can be compiled into the system statically or loaded on demand through the dynamic kernel linker facility `kld'.
Most devices in a UNIX(R)-like operating system are accessed through device-nodes, sometimes also called special files. These files are usually located under the directory [.filename]#/dev# in the filesystem hierarchy.
Device drivers can roughly be broken down into two categories; character and network device drivers.
Dynamic Kernel Linker Facility - KLD
The kld interface allows system administrators to dynamically add and remove functionality from a running system. This allows device driver writers to load their new changes into a running kernel without constantly rebooting to test changes.
The kld interface is used through:
`kldload` - loads a new kernel module
`kldunload` - unloads a kernel module
`kldstat` - lists loaded modules
Skeleton Layout of a kernel module
* KLD Skeleton
* Inspired by Andrew Reiter's Daemonnews article
#include <sys/types.h>
#include <sys/module.h>
#include <sys/systm.h> /* uprintf */
#include <sys/errno.h>
#include <sys/param.h> /* defines used in kernel.h */
#include <sys/kernel.h> /* types used in module initialization */
* Load handler that deals with the loading and unloading of a KLD.
static int
skel_loader(struct module *m, int what, void *arg)
int err = 0;
switch (what) {
case MOD_LOAD: /* kldload */
uprintf("Skeleton KLD loaded.\n");
uprintf("Skeleton KLD unloaded.\n");
/* Declare this module to the rest of the kernel */
static moduledata_t skel_mod = {