Interrupt service routines (ISRs) are critical portions of code that handle hardware interrupts in embedded systems. When working with pointers in ISRs, it is important to ensure their safety to avoid creating dangling pointers or encountering unexpected behavior. In this blog post, we will discuss some best practices for safely handling pointers in C++ ISRs.

1. Disable Interrupts

Before accessing or modifying any pointers in an ISR, it’s crucial to disable interrupts to prevent any race conditions or unexpected behavior. By disabling interrupts, you ensure exclusive access to the memory that pointers are referencing.

__disable_irq(); // Disable interrupts
// Code that handles pointers
__enable_irq(); // Enable interrupts

2. Use Volatile Pointers

In C++, the volatile keyword is used to indicate a variable whose value may change unexpectedly, such as when modified by an ISR. When working with pointers in ISRs, it is essential to use volatile pointers to ensure that the compiler doesn’t optimize away the pointer accesses or reorder the instructions.

volatile int* ptr; // Declare a volatile pointer

// Accessing pointer in ISR
void ISR() {
    *ptr = 10; // Assign a value to the pointer
}

3. Interrupt-Driven Pointers Updates

If the pointer itself needs to be updated within an ISR, it is essential to follow a safe update strategy. One approach is to use a flag variable to notify the main code that the pointer needs to be updated. This way, the pointer update occurs outside the ISR, ensuring synchronization and safe memory access.

volatile int* ptr;
volatile bool ptrUpdateFlag = false;

// Pointer update ISR
void pointerUpdateISR() {
    ptr = new int(42);
    ptrUpdateFlag = true; // Raise the update flag
}

// Main code outside ISR
void mainCode() {
    if (ptrUpdateFlag) {
        __disable_irq(); // Disable interrupts
        int* updatedPtr = ptr; // Create a local copy of the pointer
        ptrUpdateFlag = false; // Reset the update flag
        __enable_irq(); // Enable interrupts

        // Continue using the updated pointer
    }
}

Conclusion

Handling pointers in interrupt service routines requires careful consideration to maintain code safety and prevent dangling pointers. By following best practices such as disabling interrupts during pointer access, using volatile pointers, and implementing safe pointer update strategies, you can ensure reliable and predictable behavior in your C++ embedded systems.

#programming #C++