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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

Please write comments if you find anything incorrect, or you want to share more information about the topic discussed above.



This article is attributed to GeeksforGeeks.org

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