Understanding “register” keyword in C

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.

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