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>Chapter 10. Replacing Printks</H1
><DIV
CLASS="SECT1"
><H1
CLASS="SECT1"
><A
NAME="AEN1117"
></A
>Replacing <TT
CLASS="FUNCTION"
>printk</TT
></H1
><A
NAME="AEN1120"
></A
><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="AEN1125"
HREF="#FTN.AEN1125"
><SPAN
CLASS="footnote"
>[1]</SPAN
></A
> the command to load the
module came from.</P
><A
NAME="AEN1129"
></A
><A
NAME="AEN1131"
></A
><A
NAME="AEN1134"
></A
><A
NAME="AEN1136"
></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
NAME="AEN1143"
></A
><DIV
CLASS="EXAMPLE"
><A
NAME="AEN1146"
></A
><P
><B
>Example 10-1. print_string.c</B
></P
><PRE
CLASS="PROGRAMLISTING"
>/* print_string.c - Send output to the tty you're running on, regardless of whether 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/sched.h> // For current
#include <linux/tty.h> // For the tty declarations
MODULE_LICENSE("GPL");
MODULE_AUTHOR("Peter Jay Salzman");
void print_string(char *str)
{
struct tty_struct *my_tty;
my_tty = current->tty; // The tty for the current task
/* If my_tty is NULL, the current task has no tty you can print to (this is possible,
* for example, 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 memory segment or the 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->driver).write)(
my_tty, // The tty itself
0, // We 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.
*
* BTW, this is why text files are different between Unix and MS Windows. In CP/M
* and its 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->driver).write)(my_tty, 0, "\015\012", 2);
}
}
int print_string_init(void)
{
print_string("The module has been inserted. Hello world!");
return 0;
}
void 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
CLASS="FOOTNOTES"
>Notes</H3
><TABLE
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><A
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><SPAN
CLASS="footnote"
>[1]</SPAN
></A
></TD
><TD
ALIGN="LEFT"
VALIGN="TOP"
WIDTH="95%"
><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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