From Good to Great: Elevating Your Embedded Systems Code with Pointers

Embedded systems are widely used in many industries, including automotive, aerospace, consumer electronics, and medical devices. These systems require software that is efficient, reliable, and optimized for the specific hardware platform. One key technique that can be used to achieve these goals is the use of pointers when writing C code for embedded systems.

Pointers are variables that store memory addresses, rather than values. They are a fundamental concept in C programming, and are essential for working with dynamic memory allocation, arrays, and structures. In embedded systems, pointers can be used to optimize code performance, reduce memory usage, and simplify complex data structures.

In this article, I will explore the power of using pointers in embedded systems, and provide code examples to illustrate their use.

Memory Optimization

One of the most important reasons to use pointers in embedded systems is to optimize memory usage. Embedded systems typically have limited memory resources, and every byte of memory counts. By using pointers, we can reduce the memory overhead of variables and data structures, and improve the performance of our code.

Consider the following example:

void foo(int a, int b, int c) 
? ? int sum = a + b + c;
? ? printf("The sum is: %d\n", sum);
}

int main() {
? ? foo(1, 2, 3);
? ? return 0;
}        

This code defines a function?foo()?that takes three integer arguments and calculates their sum. The?main()?function calls?foo()?with the values 1, 2, and 3. When we compile and run this code, we get the following output:

The sum is: 6        

However, this code is not optimal in terms of memory usage. When we call?foo(), the three integer arguments are pushed onto the stack, taking up a total of 12 bytes of memory. Inside the function, a new integer variable?sum?is created, taking up another 4 bytes of memory. This means that the total memory usage of this code is 16 bytes.

We can optimize this code by using pointers instead of passing the integer arguments by value. Here’s the modified code:

void foo(int *a, int *b, int *c, int *sum) 
? ? *sum = *a + *b + *c;
? ? printf("The sum is: %d\n", *sum);
}

int main() {
? ? int a = 1, b = 2, c = 3, sum;
? ? foo(&a, &b, &c, &sum);
? ? return 0;
}        

In this version of the code, we pass the integer arguments as pointers to?foo(). Inside the function, we dereference the pointers using the?*?operator to access the values of the integers. We also pass a pointer to an integer variable?sum, which will hold the result of the calculation.

When we compile and run this code, we get the same output as before:

The sum is: 6        

However, the memory usage of this code is much more efficient. Instead of pushing three integers onto the stack, we only push three pointers, which take up a total of 12 bytes of memory (assuming a 32-bit platform). Inside the function, we use the pointers to access the values of the integers, rather than creating new variables. This means that the total memory usage of this code is only 12 bytes, a 25% reduction compared to the previous version.

Of course, this example is a simple one, and the memory savings may not be significant in real-world applications. However, in larger programs with complex data structures and many function calls, the use of pointers can make a big difference in terms of memory usage and performance.

Pointer Arithmetic

Another powerful feature of pointers in C is pointer arithmetic. Pointer arithmetic allows us to perform arithmetic operations on pointers, such as adding or subtracting integers from the pointer value. This can be useful in embedded systems for working with arrays and other data structures.

Consider the following example:

int array[5] = {1, 2, 3, 4, 5}

int main() {
? ? int sum = 0;
? ? for (int i = 0; i < 5; i++) {
? ? ? ? sum += array[i];
? ? }
? ? printf("The sum is: %d\n", sum);
? ? return 0;
}        

This code defines an integer array?array?with five elements, and calculates the sum of its elements using a for loop. When we compile and run this code, we get the following output:

The sum is: 15        

However, this code is not optimal in terms of performance. Inside the loop, the program is accessing the elements of the array using the index?i. This requires the program to perform a multiplication and an addition operation for each access to the array element. This can be inefficient, especially if the array is large or the loop is executed frequently.

We can optimize this code by using pointer arithmetic to access the elements of the array. Here’s the modified code:

int array[5] = {1, 2, 3, 4, 5}

int main() {
? ? int sum = 0;
? ? int *p = array;
? ? for (int i = 0; i < 5; i++) {
? ? ? ? sum += *p++;
? ? }
? ? printf("The sum is: %d\n", sum);
? ? return 0;
}        

In this version of the code, we define a pointer?p?that points to the first element of the array?array. Inside the loop, we use the pointer?p?to access the elements of the array using pointer arithmetic. We use the?*?operator to dereference the pointer and access the value of the current element, and the?++?operator to increment the pointer to the next element.

When we compile and run this code, we get the same output as before:

The sum is: 15

However, the performance of this code is improved compared to the previous version. Instead of performing a multiplication and an addition operation for each access to the array element, we are using pointer arithmetic to increment the pointer to the next element. This can be much faster, especially if the array is large or the loop is executed frequently.

Complex Data Structures

Another area where pointers can be useful in embedded systems is in working with complex data structures, such as linked lists, trees, and graphs. These data structures are often used in applications such as real-time signal processing, control systems, and image and video processing.

Consider the following example:

typedef struct node 
int value;
struct node *next;
} node_t;


void print_list(node_t *head) {
node_t *current = head;
while (current != NULL) {
printf(“%d “, current->value);
current = current->next;
}
	printf(“\n”);
}

int main() {
? 	node_t *head = NULL;
	node_t *second = NULL;
	node_t *third = NULL;

	head = (node_t*) malloc(sizeof(node_t));
	second = (node_t*) malloc(sizeof(node_t));
	third = (node_t*) malloc(sizeof(node_t));

	head->value = 1;
	head->next = second;

	second->value = 2
	third->value = 3;
	third->next = NULL;

	print_list(head);

	free(head);
	free(second);
	free(third);

	return 0;
}        

This code defines a linked list data structure using a struct `node`. Each node contains an integer value and a pointer to the next node in the list. The code also defines a function `print_list()` that prints the values of all the nodes in the list. In the `main()` function, we create three nodes using the `malloc()` function to allocate memory dynamically.

We then set the values of the nodes and link them together to form a linked list. Finally, we call the `print_list()` function to print the values of the nodes, and free the dynamically allocated memory using the `free()` function.

This code demonstrates how pointers can be used to work with complex data structures in embedded systems. By using pointers to link the nodes of the linked list together, we can create a flexible and efficient data structure that can be easily modified and traversed.

It is worth noting that the use of pointers can also introduce some risks, such as memory leaks and pointer errors. Memory leaks occur when memory is allocated but not properly freed, which can lead to memory exhaustion and program crashes. Pointer errors can occur when pointers are used incorrectly, such as dereferencing a null pointer or accessing memory outside the bounds of an array.

To avoid these risks, it is important to follow best practices when using pointers in embedded systems. This includes properly initializing and freeing memory, checking for null pointers and array bounds, and using safe pointer operations such as memcpy() and memset().

Another important consideration when using pointers in embedded systems is the impact on code readability and maintainability. Pointers can make code more difficult to read and understand, especially for developers who are not familiar with the codebase. It is important to use pointers judiciously and document their use, to ensure that the code is clear and understandable.

In summary, the use of pointers is a powerful technique when writing C code for embedded systems. Pointers can be used to optimize memory usage, improve performance, and simplify complex data structures. However, the use of pointers also introduces risks such as memory leaks and pointer errors, and can impact code readability and maintainability. By following best practices and documenting their use, embedded systems developers can harness the power of pointers to create efficient and reliable software that meets the demanding requirements of real-world applications.