restrict keyword in C

• Difficulty Level : Medium
• Last Updated : 26 Jul, 2021

In the C programming language (after 99 standard), a new keyword is introduced known as restrict. 
 

• restrict keyword is mainly used in pointer declarations as a type qualifier for pointers.
• It doesn’t add any new functionality. It is only a way for programmer to inform about an optimization that compiler can make.
• When we use restrict with a pointer ptr, it tells the compiler that ptr is the only way to access the object pointed by it, in other words, there’s no other pointer pointing to the same object i.e. restrict keyword specifies that a particular pointer argument does not alias any other and the compiler doesn’t need to add any additional checks.
• If a programmer uses restrict keyword and violate the above condition, the result is undefined behavior.
• restrict is not supported by C++. It is a C only keyword.

• C

// C program to use restrict keyword.

#include <stdio.h>

// Note that the purpose of restrict is to

// show only syntax. It doesn't change anything

// in output (or logic). It is just a way for

// programmer to tell compiler about an

// optimization

void use(int* a, int* b, int* restrict c)

{

    *a += *c;

    // Since c is restrict, compiler will

    // not reload value at address c in

    // its assembly code. Therefore generated

    // assembly code is optimized

    *b += *c; 

}

int main(void)

{

    int a = 50, b = 60, c = 70;

    use(&a, &b, &c);

    printf("%d %d %d", a, b, c);

    return 0;

}

Output: 
 

120 130 70