Table of Contents
Restrict Keyword In C
In the C programming language, there’s a keyword called “restrict” introduced after the C99 standard. It’s used to help the compiler optimize code. When you use “restrict” with a pointer, it tells the compiler that this pointer is the only way to access the data it’s pointing to, with no other pointers pointing to the same data. This information allows the compiler to make code faster. If you use “restrict” and don’t follow this rule, the results could be unpredictable.
One important thing to know is that “restrict” is only for C, and you can’t use it in C++.
In essence, “restrict” helps the compiler make code faster by letting it know that a specific pointer is the only one accessing certain data.
Example of restrict
// 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 70FAQ-Restrict Keyword In C
Q1. What is a restrict qualifier?
Ans. The “restrict” qualifier in C is used with pointers to indicate that a specific pointer is the primary or sole means of accessing a particular object, and there should be no other pointers pointing to that same object. This informs the compiler to potentially optimize code accordingly by avoiding aliasing. It can improve code performance but must be used carefully to prevent undefined behavior. “Restrict” is a feature in C, not in C++.
Q2. What is char * restrict in C?
Ans. In C, “restrict” is considered a “type qualifier,” and it falls into the same category as “const” and “volatile.” Type qualifiers modify the behavior of data types. When using “restrict,” it’s applied to pointer types, which means you can write something like “T * restrict” to create a new type. For instance, “char const * restrict” is a valid type. “Restrict” is a way to indicate that certain pointers are the primary or sole means of accessing data, which can help the compiler optimize code. It’s an important tool in C for improving performance while working with pointers.
Q3. What is restrict keyword in C inline?
Ans. The “restrict” keyword in C is indeed a type qualifier that can be used with pointer variables. You can apply it to any declaration of a pointer variable or a function parameter. To use “restrict,” you insert it after the asterisk (*) and before the variable name in the pointer declaration. This keyword indicates to the compiler that the pointer is the primary or sole means of accessing a specific object, which can lead to potential performance optimizations by the compiler.
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