restrict Keyword in C

The restrict keyword is a type qualifier that was introduced in the C99 standard. It is used to tell the compiler that a pointer is the only reference or access point to the memory it points to, allowing the compiler to make optimizations based on that information.

Let's take a look at an example:

#include <stdio.h>

// Function that specifies its parameters as restricts
void add(int *restrict arr1, int *restrict arr2,
         int *restrict res, int n) {
    for (int i = 0; i < n; i++)
        res[i] = arr1[i] + arr2[i];
}

int main() {
    int arr1[] = {1, 2, 3, 4, 5};
    int arr2[] = {10, 20, 30, 40, 50};
    int res[5];
    int n = 5;

  	// Calling function add
    add(arr1, arr2, res, n);

    for (int i = 0; i < n; i++)
        printf("%d ", res[i]);

    return 0;
}

11 22 33 44 55 

Explanation: In this example, the restrict keyword indicates that the pointers arr1, arr2, and result do not overlap, allowing the compiler to optimize memory access and improve performance. Since no other pointer modifies the arrays, the compiler can safely optimize the loop in the add_arrays function.

Syntax of restrict

The syntax for declaring a pointer with the restrict keyword is as follows:

type* restrict name;

• type: The data type of the pointer (e.g., int, char, float).
• restrict: The keyword that marks the pointer as restricted.
• name: The name of the pointer variable.

Example of restrict

#include <stdio.h>

void multiply_arrays(int *restrict arr1, int *restrict arr2, int *restrict result, int size) {
  
    // Loop through the arrays and multiply corresponding elements
    for (int i = 0; i < size; i++) {
      
        // Multiply and store the result in the result array
        result[i] = arr1[i] * arr2[i];  
    }
}

int main() {
  
    // Initialize two arrays with values
    int arr1[] = {1, 2, 3};
    int arr2[] = {4, 5, 6};
    int result[3];

    // Call the multiply_arrays function to multiply elements of arr1 and arr2
    multiply_arrays(arr1, arr2, result, 3);

    // Print the resulting array
    for (int i = 0; i < 3; i++) {
        printf("%d ", result[i]);
    }

    return 0;
}

4 10 18 

Explanation: In this example, the restrict keyword tells the compiler that the arrays arr1, arr2, and result do not overlap, allowing for optimized memory access. This helps the compiler make better performance optimizations, especially when dealing with large arrays.

Need of Restrict Keyword

The restrict keyword helps the compiler optimize memory access, making the code run faster by allowing reordering or parallelizing memory operations. It also improves code clarity by clearly indicating that a pointer won't point to the same memory as another, making the code easier to understand. It's especially helpful in performance-critical applications like scientific computing or graphics, where large data sets are manipulated.

When Should You Use restrict

The usage of restrict should be specific to the cases given below:

• Performance-Critical Code: Use restrict in performance-sensitive areas like loops or large data processing, as it helps the compiler optimize memory access for faster execution.
• Parallel Computing: In multithreaded or SIMD computing, restrict helps optimize memory access and prepares the code for parallel execution.
• Large Arrays or Data Structures: When handling large arrays or complex data structures, restrict improves memory access efficiency.