Understanding "register" keyword in C

Last Updated : 16 Jun, 2026

The register storage class is used to suggest that a variable should be stored in a CPU register instead of main memory for faster access. Since registers are faster than memory, register variables are generally used for frequently accessed data.

  • The register keyword provides a hint to the compiler for optimization.
  • Modern compilers often perform register allocation automatically.

Address of a Register Variable Cannot Be Accessed

A register variable may be stored in a CPU register instead of memory. Therefore, using the address-of (&) operator with a register variable is not allowed.

C 
#include <stdio.h>

int main(){
    
    register int i = 10;

    int *ptr = &i;

    return 0;
}

Output

./Solution.c: In function 'main':
./Solution.c:7:5: error: address of register variable 'i' requested
    7 |     int *ptr = &i;
      |     ^~~

Register Can Be Used with Pointer Variables

A pointer variable itself can be declared as a register variable because the memory address stored by the pointer can reside in a CPU register.

C 
#include <stdio.h>

int main(){
    
    int x = 10;

    register int *ptr = &x;

    printf("%d", *ptr);

    return 0;
}

Output
10

Register Cannot Be Used with Static

The register and static keywords are both storage class specifiers. Since a variable can have only one storage class, they cannot be used together.

C 
#include <stdio.h>

int main(){
    
    register static int x = 10;

    return 0;
}


Output

./Solution.c: In function 'main':
./Solution.c:5:5: error: multiple storage classes in declaration specifiers
    5 |     register static int x = 10;
      |     ^~~~~~~~

Register Variables Cannot Be Declared Globally

The register storage class can only be applied to local variables declared inside a function or block. It cannot be used for global variables.

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

error: register name not specified for 'x'

There Is No Fixed Limit on Register Variables

C does not impose a limit on the number of register variables that can be declared in a program. However, the compiler decides which variables are actually stored in CPU registers based on the available registers and optimization settings.

  • Register variables provide faster access than ordinary local variables.
  • The compiler may ignore the register keyword if sufficient registers are not available.
  • The address of a register variable cannot be obtained using the & operator.
  • Register variables are local to a function or block.
  • Modern compilers usually handle register allocation automatically, making explicit use of register less common today.
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