2.6. Passing Command Line Arguments to a Module

Modules can take command line arguments, but not with the argc/argv you might be used to.

To allow arguments to be passed to your module, declare the variables that will take the values of the command line arguments as global and then use the MODULE_PARM() macro, (defined in linux/module.h) to set the mechanism up. At runtime, insmod will fill the variables with any command line arguments that are given. The variable declarations and macros should be placed at the beginning of the module for clarity. The example code should clear up my admittedly lousy explanation.

The MODULE_PARM() macro takes 2 arguments: the name of the variable and its type. The supported variable types are "b": single byte, "h": short int, "i": integer, "l": long int and "s": string. Strings should be declared as "char *" and insmod will allocate memory for them. You should always try to give the variables an initial default value. This is kernel code, and you should program defensively. For example:

    int myint = 3;
    char *mystr;

    MODULE_PARM (myint, "i");
    MODULE_PARM (mystr, "s");
	

Arrays are supported too. An integer value preceding the type in MODULE_PARM will indicate an array of some maximum length. Two numbers separated by a '-' will give the minimum and maximum number of values. For example, an array of shorts with at least 2 and no more than 4 values could be declared as:

    int myshortArray[4];
    MODULE_PARM (myintArray, "2-4i");
	

A good use for this is to have the module variable's default values set, like which IO port or IO memory to use. If the variables contain the default values, then perform autodetection (explained elsewhere). Otherwise, keep the current value. This will be made clear later on. For now, I just want to demonstrate passing arguments to a module.

/*  hello-5.c - Demonstrates command line argument passing to a module.
 */
#include <linux/module.h>
#include <linux/kernel.h>
#include <linux/init.h>
MODULE_LICENSE("GPL");
MODULE_AUTHOR("Peiter Jay Salzman");

static short int myshort = 1;
static int myint = 420;
static long int mylong = 9999;
static char *mystring = "blah";

MODULE_PARM (myshort, "h");
MODULE_PARM (myint, "i");
MODULE_PARM (mylong, "l");
MODULE_PARM (mystring, "s");


static int __init hello_5_init(void)
{
   printk(KERN_ALERT "Hello, world 5\n=============\n");
   printk(KERN_ALERT "myshort is a short integer: %hd\n", myshort);
   printk(KERN_ALERT "myint is an integer: %d\n", myint);
   printk(KERN_ALERT "mylong is a long integer: %ld\n", mylong);
   printk(KERN_ALERT "mystring is a string: %s\n", mystring);
   return 0;
}


static void __exit hello_5_exit(void)
{
   printk(KERN_ALERT "Goodbye, world 5\n");
}


module_init(hello_5_init);
module_exit(hello_5_exit);

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