Understanding “register” keyword in C

• Difficulty Level : Easy
• Last Updated : 21 Aug, 2019

Registers are faster than memory to access, so the variables which are most frequently used in a C program can be put in registers using register keyword. The keyword register hints to compiler that a given variable can be put in a register. It’s compiler’s choice to put it in a register or not. Generally, compilers themselves do optimizations and put the variables in register.

1) If you use & operator with a register variable then compiler may give an error or warning (depending upon the compiler you are using), because when we say a variable is a register, it may be stored in a register instead of memory and accessing address of a register is invalid. Try below program.

#include<stdio.h>

int main()

{

    register int i = 10;

    int* a = &i;

    printf("%d", *a);

    getchar();

    return 0;

}

2) register keyword can be used with pointer variables. Obviously, a register can have address of a memory location. There would not be any problem with the below program.

#include<stdio.h>

int main()

{

    int i = 10;

    register int* a = &i;

    printf("%d", *a);

    getchar();

    return 0;

}

3) Register is a storage class, and C doesn’t allow multiple storage class specifiers for a variable. So, register can not be used with static . Try below program.

#include<stdio.h>

int main()

{

    int i = 10;

    register static int* a = &i;

    printf("%d", *a);

    getchar();

    return 0;

}

4) Register can only be used within a block (local), it can not be used in the global scope (outside main).

#include <stdio.h>

// error (global scope)

register int x = 10;

int main()

{

    // works (inside a block)

    register int i = 10;

    printf("%d\n", i);

    printf("%d", x);

    return 0;

}

Compile Errors:

prog.c:3:14: error: register name not specified for 'x'register int x = 10;//error (global scope)^

5) There is no limit on number of register variables in a C program, but the point is compiler may put some variables in register and some not.