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 the register keyword.  The keyword register hints to the compiler that a given variable can be put in a register. It’s the compiler’s choice to put it in a register or not. Generally, compilers themselves do optimizations and put the variables in a register.

Following are some interesting facts about the “register” keyword in C:

1. If you use & operator with a register variable then the 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 the address of a register is invalid.

Try the below program:

C

// C program that demonstrates accessing the address of a
// register is invalid
#include <stdio.h>

int main()
{
    // Declaring a register variable 'i' and initializing it
    // with 10
    register int i = 10;
    // Creating a pointer variable 'a' and assigning the
    // address of 'i' to it
    int* a = &i;
    printf("%d", *a);
    getchar();
    return 0;
}

Output

./Solution.c: In function 'main':
./Solution.c:6:5: error: address of register variable 'i' requested
     int* a = &i;
     ^

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

C

// C program that demonstrates register keyword can be used
// with pointer variables

#include <stdio.h>

int main()
{
    // Declaring an integer variable 'i' and initializing it
    // with 10
    int i = 10;
    // Declaring a register pointer variable 'a' and
    // assigning the address of 'i' to it
    register int* a = &i;
    printf("%d", *a);
    getchar();
    return 0;
}

Output

10

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

Try the below program:

C

// C program that demonstrates register can not be used with
// static

#include <stdio.h>

int main()
{
    // Declaring an integer variable 'i' and initializing it
    // with 10
    int i = 10;
    // ERROR: Attempting to use both register and static
    // storage classes for 'a'
    register static int* a = &i;
    printf("%d", *a);
    getchar();
    return 0;
}

Output

./Solution.c: In function 'main':
./Solution.c:6:5: error: multiple storage classes in declaration specifiers
     register static int* a = &i;
     ^

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

C

#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;
}

Output

./Solution.c:4:14: error: register name not specified for 'x'
 register int x = 10;
              ^

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

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