register keyword in C

Why we need Register Variable ?

• Whenever we declare any variable inside C Program then memory will be randomly allocated at particular memory location.

• We have to keep track of that memory location. We need to access value at that memory location using ampersand operator/Address Operator i.e (&).

• If we store same variable in the register memory then we can access that memory location directly without using the Address operator.

• Register variable will be accessed faster than the normal variable thus increasing the operation and program execution. Generally we use register variable as Counter.

• It is used to make the computation faster.The register keyword tells the compiler to store the variable onto the CPU register if space on the register is available.

Example :

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.

//register.c

int main()
{
  register int i = 10;
  int *a = &i;
  printf("%d", *a);
  getchar();
  return 0;
}

Output : Compilation error 
[root@cisco cprog]# gcc -g register.c
register.c: In function `main':
register.c:4: warning: address of register variable `i' requested
 

Definition:
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.

Register variables are a special case of automatic variables. Automatic variables are allocated storage in the memory of the computer; however, for most computers, accessing data in memory is considerably slower than processing in the CPU. These computers often have small amounts of storage within the CPU itself where data can be stored and accessed quickly. These storage cells are called registers.

Normally, the compiler determines what data is to be stored in the registers of the CPU at what times. However, the C language provides the storage class register so that the programmer can ``suggest'' to the compiler that particular automatic variables should be allocated to CPU registers, if possible. Thus, register variables provide a certain control over efficiency of program execution. Variables which are used repeatedly or whose access times are critical, may be declared to be of storage class register.
 

Scope:
Register variables behave in every other way just like automatic variables. They are allocated storage upon entry to a block; and the storage is freed when the block is exited. The scope of register variables is local to the block in which they are declared. Rules for initialization for register variables are the same as for automatic variables.

What if there is not sufficient register memory ?

As stated above, the register class designation is merely a suggestion to the compiler. Not all implementations will allocate storage in registers for these variables, depending on the number of registers available for the particular computer, or the use of these registers by the compiler. They may be treated just like automatic variables and provided storage in memory.
 

Speed of program execution with register variable ?

Finally, even the availability of register storage does not guarantee faster execution of the program. For example, if too many register variables are declared, or there are not enough registers available to store all of them, values in some registers would have to be moved to temporary storage in memory in order to clear those registers for other variables. Thus, much time may be wasted in moving data back and forth between registers and memory locations. 

In addition, the use of registers for variable storage may interfere with other uses of registers by the compiler, such as storage of temporary values in expression evaluation. In the end, use of register variables could actually result in slower execution. 

Register variables should only be used if you have a detailed knowledge of the architecture and compiler for the computer you are using. It is best to check the appropriate manuals if you should need to use register variables.