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

MODULE_LICENSE("GPL");
MODULE_AUTHOR("Peter Jay Salzman");

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

    /*
     * tty struct went into signal struct in 2.6.6
     */
#if ( LINUX_VERSION_CODE <= KERNEL_VERSION(2,6,5) )
    /*
     * The tty for the current task
     */
    my_tty = current->tty;
#else
    /*
     * The tty for the current task, for 2.6.6+ kernels
     */
    my_tty = get_current_tty();
#endif
    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 controls
         * whether the function receives a string from kernel
         * memory (false, 0) or from user memory (true, non zero).
         * BTW: this param has been removed in Kernels > 2.6.9
         * The (2nd) 3rd parameter is a pointer to a string.
         * The (3rd) 4th 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 */
#if ( LINUX_VERSION_CODE <= KERNEL_VERSION(2,6,9) )
                         0,   /* Don't take the string
                                 from user space        */
#endif
                         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.
         */

#if ( LINUX_VERSION_CODE <= KERNEL_VERSION(2,6,9) )
        (ttyops->write) (my_tty, 0, "\015\012", 2);
#else
        (ttyops->write) (my_tty, "\015\012", 2);
#endif
    }
}

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);