Signals in VxWorks: Asynchronous Notifications Between Tasks

🚀 Introduction

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Signals in VxWorks are a lightweight asynchronous communication mechanism.
They allow one task to notify another task (or itself) that an event has occurred.

Think of signals as software interrupts:

• Sent to tasks at any time.
• Trigger a signal handler (callback function).
• Great for asynchronous events like timers, I/O, or inter-task alerts.

🧩 Key Concepts of Signals

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• Signal Numbers → Each signal is identified by a number (SIGUSR1, SIGUSR2, etc.).
• Handlers → Tasks can register a function to handle signals.
• Delivery → A signal can be queued or replace existing ones depending on settings.

💻 Example: Using Signals in VxWorks

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We’ll demonstrate:

1. A signal handler function.
2. A task that waits for signals.
3. A task that sends signals to another.

Code Example

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#include <vxWorks.h>
#include <taskLib.h>
#include <signal.h>
#include <sigLib.h>
#include <stdio.h>
#include <unistd.h>

int targetTid;

// Signal handler
void mySignalHandler(int signo)
{
    printf("Task %d: Received signal %d\n", taskIdSelf(), signo);
}

// Task that registers a signal handler
void signalReceiverTask()
{
    struct sigaction sa;

    sa.sa_handler = (void (*)(int))mySignalHandler;
    sa.sa_flags = 0;
    sigemptyset(&sa.sa_mask);

    // Register handler for SIGUSR1
    sigaction(SIGUSR1, &sa, NULL);

    printf("Receiver task ready (TID=%d). Waiting for signals...\n", taskIdSelf());

    while (1)
    {
        taskDelay(200); // Idle loop
    }
}

// Task that sends signals
void signalSenderTask()
{
    while (1)
    {
        if (targetTid != 0)
        {
            kill(targetTid, SIGUSR1);
            printf("Sender: Sent SIGUSR1 to TID=%d\n", targetTid);
        }
        taskDelay(100); // ~1 second
    }
}

void usrAppInit(void)
{
    // Spawn receiver
    targetTid = taskSpawn("tRecv", 100, 0, 4000, (FUNCPTR)signalReceiverTask,
                          0,0,0,0,0,0,0,0,0,0);

    // Spawn sender
    taskSpawn("tSend", 110, 0, 4000, (FUNCPTR)signalSenderTask,
              0,0,0,0,0,0,0,0,0,0);
}

📝 Explanation of the Code

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1. Signal Handler Registration

   • sigaction() installs a handler for SIGUSR1.
   • Any time SIGUSR1 is sent, mySignalHandler() executes.

2. Receiver Task

   • Registers the handler and enters an idle loop.

3. Sender Task

   • Uses kill() to send SIGUSR1 to the receiver’s TID.

⚡ What You’ll See

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Expected output:

Receiver task ready (TID=1234). Waiting for signals...
Sender: Sent SIGUSR1 to TID=1234
Task 1234: Received signal 16
Sender: Sent SIGUSR1 to TID=1234
Task 1234: Received signal 16
...

🔍 Key Takeaways

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• Signals are lightweight asynchronous notifications in VxWorks.
• Use them for event-driven programming.
• Great for timers, I/O completions, or inter-task alerts.

✅ Wrap-Up

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In this tutorial, you learned:

• How to register and handle signals in VxWorks.
• How one task can send signals to another.
• How signals enable asynchronous communication.

In the next blog, we’ll explore Watchdog Timers in VxWorks: Monitoring Task Health.