Programming Embedded Systems, 2nd Edition

Task Mechanics

For the first Linux example, we reuse the Blinking LED program. This example shows how to create a task to toggle the LED at a constant rate.

The task blinkLedTask delays for 500 ms and then toggles the green LED. The task uses an infinite loop that calls the function usleep to suspend for the proper amount of time. The usleep function is passed the number of microseconds to suspend, which in this case is 50,000 ms.

After the delay function call, the blinkLedTask is blocked and put in the waiting state. The task is put in the ready state once the time has elapsed and then has been run by the scheduler. After the delay, the ledToggle function toggles the green LED.

#include <unistd.h>
#include "led.h"

/**********************************************************************
 *
 * Function:    blinkLedTask
 *
 * Description: This task handles toggling the green LED at a
 *              constant interval.
 *
 * Notes:       
 *
 * Returns:     None.
 *
 **********************************************************************/ 
void blinkLedTask(void *param)
{
    while (1)
    {
        /* Delay for 500 milliseconds. */
        usleep(50000);

        ledToggle();
    }
}

The main function’s job in this example is to create the LED task. The task is created by calling the function pthread_create. For this example, the default task attributes are used, and no parameters are passed to the task blinkLedTask.

The function pthread_join is used to suspend the main function until the blinkLedTask task terminates. In this case, the blinkLedTask task runs forever, ...