VxWorks Multiple Task Programming

Introduction

VxWorks, as a high-performance real-time operating system (RTOS), is widely utilized in embedded fields such as aerospace, industrial control, and automotive electronics. One of its core strengths lies in its robust support for multi-tasking, enabling developers to design applications where multiple tasks run concurrently, improving system efficiency and responsiveness. This article introduces the basics of multi-task programming in VxWorks, including task creation, management, and synchronization, with practical examples to guide developers.

What is Multi-Task Programming?

In VxWorks, a "task" is an independent unit of execution within a program, similar to a thread in other operating systems. Multi-task programming allows multiple tasks to share CPU resources under the VxWorks kernel’s scheduling mechanism. Each task has its own stack, priority, and state (e.g., running, ready, suspended), and the kernel manages their execution based on priority-based preemptive scheduling. This is particularly valuable in real-time systems where timing and resource allocation are critical.

Creating a Task in VxWorks


To create a task in VxWorks, you use the taskSpawn() function, which is the primary API for task creation. Below is an example of how to create a simple task:
#include <vxWorks.h>
#include <taskLib.h>

void myTask(void)
{
    while (1)
    {
        printf("My Task is running!\n");
        taskDelay(60); /* Delay for 60 ticks (1 second if tick rate is 60 Hz) */
    }
}

int main(void)
{
    /* Spawn the task */
    int taskId = taskSpawn("myTask",  /* Task name */
                           100,       /* Priority (0-255, lower is higher priority) */
                           0,         /* Options (e.g., VX_FP_TASK for floating-point) */
                           4096,      /* Stack size in bytes */
                           (FUNCPTR)myTask, /* Entry point of the task */
                           0, 0, 0, 0, 0, 0, 0, 0, 0, 0); /* Arguments (up to 10) */
    
    if (taskId == ERROR)
    {
        printf("Failed to create task!\n");
        return -1;
    }
    printf("Task created with ID: %d\n", taskId);
    return 0;
}

Explanation:

  • taskSpawn() takes parameters like the task name, priority, stack size, and function pointer to the task’s entry point.
  • The task myTask runs in an infinite loop, printing a message every second (assuming a 60 Hz system tick rate).
  • Error checking ensures the task is created successfully.

Task Management


Once tasks are created, VxWorks provides APIs to manage them:
  • taskSuspend(taskId): Suspends a task, halting its execution.
  • taskResume(taskId): Resumes a suspended task.
  • taskDelete(taskId): Terminates and removes a task from the system.
  • taskPrioritySet(taskId, newPriority): Changes a task’s priority dynamically.

For example, to suspend the task created above:


taskSuspend(taskId); /* Suspend the task */

Task Synchronization


In multi-task environments, tasks often need to coordinate to avoid conflicts (e.g., accessing shared resources). VxWorks offers synchronization primitives like semaphores and message queues:
  • Semaphores: Use semBCreate() for binary semaphores or semMCreate() for mutual exclusion (mutex) semaphores to protect shared resources.
  • Example:

SEM_ID semId = semBCreate(SEM_Q_FIFO, SEM_FULL); /* Create a binary semaphore */

void task1(void)
{
    semTake(semId, WAIT_FOREVER); /* Wait for semaphore */
    printf("Task 1 has the resource\n");
    semGive(semId); /* Release semaphore */
}

Practical Example: Two Tasks with Synchronization


Here’s a program with two tasks sharing a resource:
#include <vxWorks.h>
#include <taskLib.h>
#include <semLib.h>

SEM_ID semId;

void taskA(void)
{
    while (1)
    {
        semTake(semId, WAIT_FOREVER);
        printf("Task A is using the resource\n");
        taskDelay(60);
        semGive(semId);
    }
}

void taskB(void)
{
    while (1)
    {
        semTake(semId, WAIT_FOREVER);
        printf("Task B is using the resource\n");
        taskDelay(60);
        semGive(semId);
    }
}

int main(void)
{
    semId = semBCreate(SEM_Q_FIFO, SEM_FULL); /* Semaphore starts available */
    
    taskSpawn("taskA", 100, 0, 4096, (FUNCPTR)taskA, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0);
    taskSpawn("taskB", 101, 0, 4096, (FUNCPTR)taskB, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0);
    
    return 0;
}

Explanation:

  • Two tasks (taskA and taskB) alternate accessing a shared resource, controlled by a semaphore.
  • The semaphore ensures only one task prints its message at a time, preventing overlap.

Conclusion


Multi-task programming in VxWorks leverages its real-time capabilities to handle complex embedded applications efficiently. By mastering task creation, management, and synchronization, developers can build reliable systems that meet stringent timing requirements. This article provides a starting point—explore the VxWorks documentation for advanced topics like interrupt handling or task debugging.