print_string.c

/*
 * 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.
 */
#include <linux/init.h>
#include <linux/kernel.h>
#include <linux/module.h>
#include <linux/sched.h> /* For current */
#include <linux/tty.h> /* For the tty declarations */

static void print_string(char *str)
{
    /* The tty for the current task */
    struct tty_struct *my_tty = get_current_tty();

    /* 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.
     */
    if (my_tty) {
        const struct tty_operations *ttyops = my_tty->driver->ops;
        /* my_tty->driver is a struct which holds the tty's functions,
         * 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
         * same function would normally be used for all tty's of a certain
         * type.
         * The 2nd parameter is a pointer to a string.
         * The 3rd parameter is the length of the string.
         *
         * 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
         * scale well. The right way to deal with this is described in
         * section 2 of
         * linux/Documentation/SubmittingPatches
         */
        (ttyops->write)(my_tty, /* The tty itself */
                        str, /* String */
                        strlen(str)); /* Length */

        /* ttys were originally hardware devices, which (usually) strictly
         * 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.
         *
         * This is why text files are different between Unix and MS Windows.
         * In CP/M and derivatives, like MS-DOS and MS Windows, the ASCII
         * standard was strictly adhered to, and therefore a newline requires
         * both a LF and a CR.
         */
        (ttyops->write)(my_tty, "\015\012", 2);
    }
}

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");