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The Linux Kernel Module Programming Guide
Prev Chapter 10. Replacing Printks Next

10.1. Replacing printk

In Section, I said that X and kernel module programming don't mix. That's true for developing kernel modules, but in actual use, you want to be able to send messages to whichever tty[1] the command to load the module came from.

The way this is done is by using current, a pointer to the currently running task, to get the current task's tty structure. Then, we look inside that tty structure to find a pointer to a string write function, which we use to write a string to the tty.

Example 10-1. 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/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_AUTHOR("Peter Jay Salzman");

static void print_string(char *str)
        struct tty_struct *my_tty;

         * tty struct went into signal struct in 2.6.6 
         * The tty for the current task 
        my_tty = current->tty;
         * The tty for the current task, for 2.6.6+ kernels 
        my_tty = current->signal->tty;

         * 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
                ((my_tty->driver)->write) (my_tty,      /* The tty itself */
#if ( LINUX_VERSION_CODE <= KERNEL_VERSION(2,6,9) )         
                                           0,   /* Don't take the string 
                                                   from user space        */
                                           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) )         
                ((my_tty->driver)->write) (my_tty, 0, "\015\012", 2);
                ((my_tty->driver)->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!");




Teletype, originally a combination keyboard-printer used to communicate with a Unix system, and today an abstraction for the text stream used for a Unix program, whether it's a physical terminal, an xterm on an X display, a network connection used with telnet, etc.

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