Device Driver Tutorial

Defining the Module Linkage Structures

Two other module loading structures are required for every driver. The modlinkage(9S) module linkage structure is used by the _init(9E), _info(9E), and _fini(9E) routines to install, remove, and retrieve information from a module. The modldrv(9S) linkage structure for loadable drivers exports driver-specific information to the kernel. See the man pages for each structure to learn what each element is and what the value of each element should be.

The following code defines the modldrv(9S) and modlinkage(9S) structures for the driver shown in this chapter:

static struct modldrv md = {
    &mod_driverops,     /* Type of module. This is a driver. */
    "dummy driver",     /* Name of the module. */

static struct modlinkage ml = {

The first element in the modldrv(9S) structure is a pointer to a structure that tells the kernel what kind of module this is. Set this value to the address of the mod_driverops structure. The mod_driverops structure tells the kernel that the dummy.c module is a loadable driver module. The mod_driverops structure is declared in the modctl.h header file. You already included the modctl.h header file in your dummy.c file, so do not declare the mod_driverops structure in dummy.c. The mod_driverops structure is defined in the modctl.c source file.

The second element in the modldrv(9S) structure is a string that describes this module. Usually this string contains the name of this module and the version number of this module. The last element of the modldrv(9S) structure is a pointer to the dev_ops(9S) structure for this driver. You initialized the dev_ops(9S) structure for this driver in Defining the Device Operations Structure.

The first element in the modlinkage(9S) structure is the revision number of the loadable modules system. Set this value to MODREV_1. The next element of the modlinkage(9S) structure is the address of a null-terminated array of pointers to linkage structures. Driver modules have only one linkage structure. Enter the address of the md structure for the value of this element of the modlinkage(9S) structure. Enter the value NULL to terminate this list of linkage structures.