10.1. Replacing printk

n 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);

Notes

[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.