Page 220 - The Unofficial Guide to Lego Mindstorms Robots

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kill(pid[i]);
return 0;
}

void exec_helper(int (∗code_start) (int,char∗∗)) {
pid[task_index++] = execi(code_start, 0, NULL, 0, DEFAULT_STACK_SIZE);
}

int main() {
task_index = 0;

exec_helper(&avoid);
exec_helper(&seek_enlightenment);
exec_helper(&cruise);

exec_helper(&arbitrate);

execi(&stop_task, 0, NULL, 0, DEFAULT_STACK_SIZE);

tm_start();

return 0;
}

LightSeeker.c is a relatively large program, but it consists of easily understandable pieces. As before, I'll start at the bottom and
work backwards through the source code.

The main() function simply serves to start the other tasks in the program. A helper function, exec_helper(), is used to
start the three behavior tasks, avoid(), seek_enlightenment (), and cruise(). exec_helper() is also used to
start the arbitrate() task, which examines the output of the three behaviors and sends the appropriate command to the
motors. The exec_helper() function simply starts each task using execi() and stores the returned process ID in an
array. Back in main(), stop_task() is also started. When the Run button is pressed, stop_task() simply goes
through the process ID array that exec_helper() built and kills each process.

arbitrate() examines the output commands of each behavior. If the command is not COMMAND_NONE, the current motor
command is set from the behavior. The later behaviors, of course, will overwrite the motor command. The last behavior,
avoid(), is at the highest level. If it chooses to control the robot, Seek_enlightenment() and cruise() have
nothing to say about it.

To make it clearer what's going on while the robot is running, arbitrate() writes a character to the display that indicates
which behavior is currently active. A "c" on the right side of the display indicates that cruise() has control, an "s" stands
for seek_enlightenment(), and an "a" shows that the avoid() behavior is controlling the robot.