Chapter 10. Replacing Printks

Table of Contents


the Section called 使用 X带来的问题 in Chapter 1中, 我说过最好不要在X中进行内核模块编程。在真正的内核模块开发中的确是这样。 但在实际应用中,你想在任何加载模块的tty[1]终端中显示信息。

实现的方法是使用current指针,一个指向当前运行进程的指针,来获取当前任务的 tty终端的结构体。然后,我们找到在该tty结构体中 用来向tty写入字符信息的函数的指针。通过指针我们使用该函数来向终端写入信息。

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).  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,	/* 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->driver)->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!");




Teletype, 原先是一种用来和Unix系统交互的键盘和打印机结合起来的装置。现在,它只是一个用来同Unix或类似的系统交流文字流 的抽象的设备,而不管它具体是显示器,X中的xterm,还是一个通过telnet的网络连接。