Page 212 - The Unofficial Guide to Lego Mindstorms Robots

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tm_start();

return 0;
}

Let's start at the bottom and work our way up. The main() method uses execi() to start the two tasks:
display_task() and stop_task(). The process ID of the di splay task is saved away in the pid variable so that it can
b e stopped later. The last thing main() does is call tm_start() to start the task manager, which actually handles running
the tasks.

The stop_task() waits for the Run button to be pressed. When the button is pressed, stop_task() calls kill() to
st op the display process. What's that call to msleep() at the beginning of stop_task()? When legOS first boots on the
RCX, it's waiting for the Run bu tton to be pressed. As soon as it is, your program is started. It's very possible that
stop_task() will already be running b efore you have a chance to take your finger off the Run button. The call to
m sleep() simply delays a little while to g ive you a chance to release the Run button.

The display_task() is straightforward. It alternates between displaying "Hello" and "nurse" on the display, looping
forever until it is killed by stop_task().

Waiting (unistd.h)

For a le w t ):
c aner ay o wait for specific events, consider wait_event(

wakeup_t wait_event(wakeup_t (∗wakeup)(wakeup_t), wakeup_t dat a)
U se this function to wait for a specific event. The function pointed at b y wakeup is called, with data as a parameter, until it
returns a non-zero result. At this point, wait_event( ) will return.

It's not hard to use wait_event(), but it's certainly not obvious. It helps to look at some of the examples that come with
legOS. The following is a rewrite of the previous example that uses wait_ev ent() instead of a while loop:

#include "conio.h"
#include "direct-button.h"
#include "unistd.h"
#include "sys/tm.h"

pid_t pid;

int d isplay_task(int argc, char ∗∗argv) {
w hile(1) {
cputs("Hello");
lcd_refresh();
sleep (1);
cputs("nurse");
lcd_refresh();