/* * print_string.c - Send output to the tty we're running on, regardless if * it is through X11, telnet, etc. We do this by printing the string to the * tty associated with the current task. */ #include <linux/init.h> #include <linux/kernel.h> #include <linux/module.h> #include <linux/sched.h> /* For current */ #include <linux/tty.h> /* For the tty declarations */ static void print_string(char *str) { /* The tty for the current task */ struct tty_struct *my_tty = get_current_tty(); /* If my_tty is NULL, the current task has no tty you can print to (i.e., * if it is a daemon). If so, there is nothing we can do. */ if (my_tty) { const struct tty_operations *ttyops = my_tty->driver->ops; /* 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 is a pointer to a string. * The 3rd parameter is the length of the string. * * As you will see below, sometimes it's necessary to use * preprocessor stuff to create code that works for different * kernel versions. The (naive) approach we've taken here does not * scale well. The right way to deal with this is described in * section 2 of * linux/Documentation/SubmittingPatches */ (ttyops->write)(my_tty, /* The tty itself */ 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 not * just use \n, because it would not 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 requires * both a LF and a CR. */ (ttyops->write)(my_tty, "\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); MODULE_LICENSE("GPL");