Sophie: lkmpg-1.1.0-15mdv2010.0 noarch

lkmpg-1.1.0-15mdv2010.0.noarch.rpm

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>Chapter 10. Replacing Printks</H1
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>Table of Contents</B
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><A
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>Replacing <TT
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>printk</TT
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><A
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>Replacing <TT
CLASS="FUNCTION"
>printk</TT
></H1
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><P
>In <A
HREF="x49.htm#USINGX"
>the Section called <I
>Using X</I
> in Chapter 1</A
>, 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<A
NAME="AEN1156"
HREF="#FTN.AEN1156"
><SPAN
CLASS="footnote"
>[1]</SPAN
></A
> the command to load the
	module came from.</P
><A
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></A
><A
NAME="AEN1162"
></A
><A
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></A
><A
NAME="AEN1167"
></A
><P
>The way this is done is by using <TT
CLASS="VARNAME"
>current</TT
>, a pointer to the currently running task, to get the current
	task's <SPAN
CLASS="STRUCTNAME"
>tty</SPAN
> structure.  Then, we look inside that <SPAN
CLASS="STRUCTNAME"
>tty</SPAN
> structure to find a
	pointer to a string write function, which we use to write a string to the tty.</P
><A
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><DIV
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><A
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></A
><P
><B
>Example 10-1. print_string.c</B
></P
><PRE
CLASS="PROGRAMLISTING"
>/* 
 *  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 &#60;linux/kernel.h&#62;
#include &#60;linux/module.h&#62;
#include &#60;linux/init.h&#62;
#include &#60;linux/sched.h&#62;	/* For current */
#include &#60;linux/tty.h&#62;		/* For the tty declarations */
#include &#60;linux/version.h&#62;	/* For LINUX_VERSION_CODE */

MODULE_LICENSE("GPL");
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 
	 */
#if ( LINUX_VERSION_CODE &#60;= KERNEL_VERSION(2,6,5) )
	/* 
	 * The tty for the current task 
	 */
	my_tty = current-&#62;tty;
#else
	/* 
	 * The tty for the current task, for 2.6.6+ kernels 
	 */
	my_tty = current-&#62;signal-&#62;tty;
#endif

	/* 
	 * 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-&#62;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).  The 3rd parameter is
		 * a pointer to a string.  The 4th parameter is the length of
		 * the string.
		 */
		((my_tty-&#62;driver)-&#62;write) (my_tty,	/* The tty itself */
					   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.
		 */
		((my_tty-&#62;driver)-&#62;write) (my_tty, 0, "\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);</PRE
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><H3
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>Notes</H3
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><A
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><SPAN
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>[1]</SPAN
></A
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><P
><SPAN
CLASS="emphasis"
><I
CLASS="EMPHASIS"
>T</I
></SPAN
>ele<SPAN
CLASS="emphasis"
><I
CLASS="EMPHASIS"
>ty</I
></SPAN
>pe, 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.</P
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