Linux Device Drivers, Second Edition

Intermodule Communication

Very late in the pre-2.4.0 development series, the kernel developers added a new interface providing limited communication between modules. This intermodule scheme allows modules to register strings pointing to data of interest, which can be retrieved by other modules. We’ll look briefly at this interface, using a variation of our master and slave modules.

We use the same master module, but introduce a new slave module called inter. All inter does is to make a string and a function available under the name ime_string (ime means “intermodule example”) and ime_function; it looks, in its entirety, as follows:

static char *string = "inter says 'Hello World'";

void ime_function(const char *who)
{
    printk(KERN_INFO "inter: ime_function called by %s\n", who);
}

int ime_init(void)
{
    inter_module_register("ime_string", THIS_MODULE, string);
    inter_module_register("ime_function", THIS_MODULE, ime_function);
    return 0;
}

void ime_cleanup(void)
{
    inter_module_unregister("ime_string");
    inter_module_unregister("ime_function");
}

This code uses inter_module_register, which has this prototype:

void inter_module_register(const char *string, struct module *module,
                           const void *data);

string is the string other modules will use to find the data; module is a pointer to the module owning the data, which will almost always be THIS_MODULE; and data is a pointer to whatever data is to be shared. Note the use of a const pointer for the data; it is assumed that it will be exported ...