restrict keyword in C

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 optimizations 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 and compiler doesn’t need to add any additional checks.
  • If a programmer uses restrict keyword and violate the above condition, result is undefined behavior.
  • restrict is not supported by C++. It is a C only keyword.
// 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

This article is contributed by Bishal Kumar Dubey. If you like GeeksforGeeks and would like to contribute, you can also write an article using contribute.geeksforgeeks.org or mail your article to contribute@geeksforgeeks.org. See your article appearing on the GeeksforGeeks main page and help other Geeks.

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