lkmpg/examples/print_string.c

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/*
* print_string.c - Send output to the tty we're running on, regardless if
* it is through X11, telnet, etc. We do this by printing the string to the
* tty associated with the current task.
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*/
#include <linux/init.h>
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#include <linux/kernel.h>
#include <linux/module.h>
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#include <linux/sched.h> /* For current */
#include <linux/tty.h> /* For the tty declarations */
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static void print_string(char *str)
{
/* The tty for the current task */
struct tty_struct *my_tty = get_current_tty();
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/* If my_tty is NULL, the current task has no tty you can print to (i.e.,
* if it is a daemon). If so, there is nothing we can do.
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*/
if (my_tty) {
const struct tty_operations *ttyops = my_tty->driver->ops;
/* my_tty->driver is a struct which holds the tty's functions,
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* one of which (write) is used to write strings to the tty.
* It can be used to take a string either from the user's or
* kernel's memory segment.
*
* The function's 1st parameter is the tty to write to, because the
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* same function would normally be used for all tty's of a certain
* type.
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* The 2nd parameter is a pointer to a string.
* The 3rd parameter is the length of the string.
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*
* As you will see below, sometimes it's necessary to use
* preprocessor stuff to create code that works for different
* kernel versions. The (naive) approach we've taken here does not
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* scale well. The right way to deal with this is described in
* section 2 of
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* linux/Documentation/SubmittingPatches
*/
(ttyops->write)(my_tty, /* The tty itself */
str, /* String */
strlen(str)); /* Length */
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/* ttys were originally hardware devices, which (usually) strictly
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* followed the ASCII standard. In ASCII, to move to a new line you
* need two characters, a carriage return and a line feed. On Unix,
* the ASCII line feed is used for both purposes - so we can not
* just use \n, because it would not have a carriage return and the
* next line will start at the column right after the line feed.
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*
* This is why text files are different between Unix and MS Windows.
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* In CP/M and derivatives, like MS-DOS and MS Windows, the ASCII
* standard was strictly adhered to, and therefore a newline requires
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* both a LF and a CR.
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*/
(ttyops->write)(my_tty, "\015\012", 2);
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}
}
static int __init print_string_init(void)
{
print_string("The module has been inserted. Hello world!");
return 0;
}
static void __exit print_string_exit(void)
{
print_string("The module has been removed. Farewell world!");
}
module_init(print_string_init);
module_exit(print_string_exit);
MODULE_LICENSE("GPL");