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<H1><A NAME="SECTION00800000000000000000"> </A><A NAME="startup-param"> </A><A NAME="461"> </A>
<A NAME="462"> </A>
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Startup Parameters
</H1>
<P>
In many of the previous examples, we had to hard-wire something into
the kernel module, such as the file name for <TT>/proc</TT> files or the major
device number for the device so we can have <TT>ioctl</TT>'s to it. This goes
against the grain of the Unix, and Linux, philosophy which is to write
flexible program the user can customize.
<A NAME="465"> </A>
<P>
The way to tell a program, or a kernel module, something it needs before
it can start working is by command line parameters. In the case of kernel
modules, we don't get <TT>argc</TT> and <TT>argv</TT> -- instead, we get something
better. We can define global variables in the kernel module and <TT>insmod</TT>
will fill them for us.
<A NAME="469"> </A>
<A NAME="470"> </A>
<P>
In this kernel module, we define two of them: <TT>str1</TT> and <TT>str2</TT>. All
you need to do is compile the kernel module and then run
<TT>insmod str1=xxx str2=yyy</TT>.
When <TT>init_module</TT> is called, <TT>str1</TT> will point to the string
`<TT>xxx</TT>' and <TT>str2</TT> to the string `<TT>yyy</TT>'.
<A NAME="479"> </A>
<P>
In version 2.0 there is no type checking on these
arguments<A NAME="tex2html158"
HREF="footnode.html#foot480"><SUP>6.1</SUP></A>. If the first character of <TT>str1</TT> or <TT>str2</TT> is a
digit the kernel will fill the variable with the value of the integer,
rather than a pointer to the string. If a real life situation you have to
check for this.
<A NAME="483"> </A>
<P>
On the other hand, in version 2.2 you use the macro <TT>MACRO_PARM</TT> to
tell <TT>insmod</TT> that you expect a parameters, its name <EM>and its type</EM>.
This solves the type problem and allows kernel modules to receive strings
which begin with a digit, for example.
<A NAME="487"> </A>
<A NAME="488"> </A>
<P>
ex
<FONT SIZE="+1"><B>param.c</B></FONT>
<A NAME="493"> </A><A NAME="494"> </A>
<P>
<PRE>
/* param.c
*
* Receive command line parameters at module installation
*/
/* Copyright (C) 1998-99 by Ori Pomerantz */
/* The necessary header files */
/* Standard in kernel modules */
#include <linux/kernel.h> /* We're doing kernel work */
#include <linux/module.h> /* Specifically, a module */
/* Deal with CONFIG_MODVERSIONS */
#if CONFIG_MODVERSIONS==1
#define MODVERSIONS
#include <linux/modversions.h>
#endif
#include <stdio.h> /* I need NULL */
/* In 2.2.3 /usr/include/linux/version.h includes a
* macro for this, but 2.0.35 doesn't - so I add it
* here if necessary. */
#ifndef KERNEL_VERSION
#define KERNEL_VERSION(a,b,c) ((a)*65536+(b)*256+(c))
#endif
/* Emmanuel Papirakis:
*
* Prameter names are now (2.2) handled in a macro.
* The kernel doesn't resolve the symbol names
* like it seems to have once did.
*
* To pass parameters to a module, you have to use a macro
* defined in include/linux/modules.h (line 176).
* The macro takes two parameters. The parameter's name and
* it's type. The type is a letter in double quotes.
* For example, "i" should be an integer and "s" should
* be a string.
*/
char *str1, *str2;
#if LINUX_VERSION_CODE >= KERNEL_VERSION(2,2,0)
MODULE_PARM(str1, "s");
MODULE_PARM(str2, "s");
#endif
/* Initialize the module - show the parameters */
int init_module()
{
if (str1 == NULL || str2 == NULL) {
printk("Next time, do insmod param str1=<something>");
printk("str2=<something>\n");
} else
printk("Strings:%s and %s\n", str1, str2);
#if LINUX_VERSION_CODE >= KERNEL_VERSION(2,2,0)
printk("If you try to insmod this module twice,");
printk("(without rmmod'ing\n");
printk("it first), you might get the wrong");
printk("error message:\n");
printk("'symbol for parameters str1 not found'.\n");
#endif
return 0;
}
/* Cleanup */
void cleanup_module()
{
}
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