File content as of revision 0:61ad4bb19246:

/** 10_rtos_3semaphores
 * In this example three semaphores are used to mantain the
 * monotonic execution order of three threads.
 * Each task is consumer of a semaphore and at the same time
 * producer of an another semaphore.
 * Watching the RGB LED you can see the order of execution.
 *
 * Hardware requirements:
 *  - FRDM-KL25Z board
 */

#include "mbed.h"
#include "rtos.h"

Semaphore s1(1);                            //allow task1 to run
Semaphore s2(0);                            //s2 has no token at start
Semaphore s3(0);                            //s3 has no token at start
DigitalOut led1(LED1);                      //Red LED
DigitalOut led2(LED2);                      //Green LED
DigitalOut led3(LED3);                      //Blue LED

void thread1(void const* args) {
    while (true) {
        s1.wait();
        led1 = 0;                           //Red LED ON
        Thread::wait(500+rand()%500);
        led1 = 1;                           //Red LED OFF
        s2.release();
    }
}

void thread2(void const* args) {
    while (true) {
        s2.wait();
        led2 = 0;                           //Green LED ON
        Thread::wait(500+rand()%500);
        led2 = 1;                           //Green LED OFF
        s3.release();
    }
}

void thread3(void const* args) {
    while (true) {
        s3.wait();
        led3 = 0;                           //Blue LED ON
        Thread::wait(500+rand()%500);
        led3 = 1;                           //Blue LED OFF
        s1.release();
    }
}

int main (void) {
    led1 = 1; led2 = 1; led3 = 1;           //Switch off all LEDs
    Thread t2(thread2);                     //Define and run thread2
    Thread t3(thread3);                     //Define and run thread3
    thread1(NULL);                          //Run thread1
}