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Explain volatile and restrict type qualifiers in C with an example
Type qualifiers add special attributes to existing data types in C programming language.
There are three type qualifiers in C language and volatile and restrict type qualifiers are explained below −
Volatile
A volatile type qualifier is used to tell the compiler that a variable is shared. That is, a variable may be referenced and changed by other programs (or) entities if it is declared as volatile.
For example, volatile int x;
Restrict
This is used only with pointers. It indicates that the pointer is only an initial way to access the deference data. It provides more help to the compiler for optimization.
Example Program
Following is the C program for volatile type qualifier −
int *ptr int a= 0; ptr = &a; ____ ____ ____ *ptr+=4; // Cannot be replaced with *ptr+=9 ____ ____ ____ *ptr+=5;
Here, the compiler cannot replace the two statements *ptr+=4 and *ptr+=5 by one statement *ptr+=9. Because, it is not clear if the variable ‘a’ can be accessed directly (or) through other pointers.
For example,
restrict int *ptr int a= 0; ptr = &a; ____ ____ ____ *ptr+=4; // Can be replaced with *ptr+=9 ____ ____ *ptr+=5; ____ ____
Here, the compiler can replace the two statements by one statement, *ptr+=9. Because, it is sure that variable cannot be accessed through any other resources.
Example
Following is the C program for the use of restrict keyword −
#include<stdio.h>
void keyword(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.
*b += *c;
}
int main(void){
int p = 10, q = 20,r=30;
keyword(&p, &q,&r);
printf("%d %d %d", p, q,r);
return 0;
}Output
When the above program is executed, it produces the following result −
40 50 30
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