lkmpg/examples/print_string.c

/*
 *  print_string.c - Send output to the tty we're running on, regardless if it's
 *  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 */

MODULE_LICENSE("GPL");

static void print_string(char *str)
{
    struct tty_struct *my_tty;
    const struct tty_operations *ttyops;

    /*
     * The tty for the current task, for 2.6.6+ kernels
     */
    my_tty = get_current_tty();
    ttyops = my_tty->driver->ops;

    /*
     * If my_tty is NULL, the current task has no tty you can print to
     * (ie, if it's a daemon).  If so, there's nothing we can do.
     */
    if (my_tty != NULL) {
        /*
         * 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't just use \n, because it wouldn't 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 requirs 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);