Writing Device Drivers

Loading and Unloading Drivers

Opening a special file (accessing the device) that is associated with a device driver causes that driver to be loaded. You can use the modload(1M) command to load the driver into memory, but modload does not call any routines in the module. The preferred method is to open the device.

Normally, the system automatically unloads device drivers when the device is no longer in use. During development, you might want to use modunload(1M) to unload the driver explicitly. In order for modunload to be successful, the device driver must be inactive. No outstanding references to the device should exist, such as through open(2) or mmap(2).

The modunload command takes a runtime-dependent module_id as an argument. To find the module_id, use grep to search the output of modinfo(1M) for the driver name in question. Check in the first column.

# modunload -i module-id

To unload all currently unloadable modules, specify module ID zero:

# modunload -i 0

In addition to being inactive, the driver must have working detach(9E) and _fini(9E) routines for modunload(1M) to succeed.