In Section 1.2.1.2, 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 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); |
| [1] | 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. |