restrict keyword in C

In the C programming language (after the C99 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 the programmer to inform about an optimization that the 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 violates the above condition, the result is undefined behavior.
• restrict is not supported by C++. It is a C-only keyword.

Example of restrict

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

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