register
This storage class declares register variables that have the same functionality as that of the auto variables. The only difference is that the compiler tries to store these variables in the register of the microprocessor if a free register is available. This makes the use of register variables to be much faster than that of the variables stored in the memory during the runtime of the program.
If a free registration is not available, these are then stored in the memory only. Usually, a few variables which are to be accessed very frequently in a program are declared with the register keyword which improves the running time of the program. An important and interesting point to be noted here is that we cannot obtain the address of a register variable using pointers.
Syntax
To specify the storage class for a variable, the following syntax is to be followed:
storage_class var_data_type var_name;
Example
Functions follow the same syntax as given above for variables. Have a look at the following C example for further clarification:
C
// A C program to demonstrate different storage // classes #include <stdio.h> // declaring the variable which is to be made extern // an initial value can also be initialized to x int x; void autoStorageClass() { printf ( "\nDemonstrating auto class\n\n" ); // declaring an auto variable (simply // writing "int a=32;" works as well) auto int a = 32; // printing the auto variable 'a' printf ( "Value of the variable 'a'" " declared as auto: %d\n" , a); printf ( "--------------------------------" ); } void registerStorageClass() { printf ( "\nDemonstrating register class\n\n" ); // declaring a register variable register char b = 'G' ; // printing the register variable 'b' printf ( "Value of the variable 'b'" " declared as register: %d\n" , b); printf ( "--------------------------------" ); } void externStorageClass() { printf ( "\nDemonstrating extern class\n\n" ); // telling the compiler that the variable // x is an extern variable and has been // defined elsewhere (above the main // function) extern int x; // printing the extern variables 'x' printf ( "Value of the variable 'x'" " declared as extern: %d\n" , x); // value of extern variable x modified x = 2; // printing the modified values of // extern variables 'x' printf ( "Modified value of the variable 'x'" " declared as extern: %d\n" , x); printf ( "--------------------------------" ); } void staticStorageClass() { int i = 0; printf ( "\nDemonstrating static class\n\n" ); // using a static variable 'y' printf ( "Declaring 'y' as static inside the loop.\n" "But this declaration will occur only" " once as 'y' is static.\n" "If not, then every time the value of 'y' " "will be the declared value 5" " as in the case of variable 'p'\n" ); printf ( "\nLoop started:\n" ); for (i = 1; i < 5; i++) { // Declaring the static variable 'y' static int y = 5; // Declare a non-static variable 'p' int p = 10; // Incrementing the value of y and p by 1 y++; p++; // printing value of y at each iteration printf ( "\nThe value of 'y', " "declared as static, in %d " "iteration is %d\n" , i, y); // printing value of p at each iteration printf ( "The value of non-static variable 'p', " "in %d iteration is %d\n" , i, p); } printf ( "\nLoop ended:\n" ); printf ( "--------------------------------" ); } int main() { printf ( "A program to demonstrate" " Storage Classes in C\n\n" ); // To demonstrate auto Storage Class autoStorageClass(); // To demonstrate register Storage Class registerStorageClass(); // To demonstrate extern Storage Class externStorageClass(); // To demonstrate static Storage Class staticStorageClass(); // exiting printf ( "\n\nStorage Classes demonstrated" ); return 0; } // This code is improved by RishabhPrabhu |
Output
A program to demonstrate Storage Classes in C Demonstrating auto class Value of the variable 'a' declared as auto: 32 -------------------------------- Demonstrating register class Value of the variable 'b' declared as register: 71 -------------------------------- Demonstrating extern class Value of the variable 'x' declared as extern: 0 Modified value of the variable 'x' declared as extern: 2 -------------------------------- Demonstrating static class Declaring 'y' as static inside the loop. But this declaration will occur only once as 'y' is static. If not, then every time the value of 'y' will be the declared value 5 as in the case of variable 'p' Loop started: The value of 'y', declared as static, in 1 iteration is 6 The value of non-static variable 'p', in 1 iteration is 11 The value of 'y', declared as static, in 2 iteration is 7 The value of non-static variable 'p', in 2 iteration is 11 The value of 'y', declared as static, in 3 iteration is 8 The value of non-static variable 'p', in 3 iteration is 11 The value of 'y', declared as static, in 4 iteration is 9 The value of non-static variable 'p', in 4 iteration is 11 Loop ended: --------------------------------
Related Article:
Storage Classes in C
C Storage Classes are used to describe the features of a variable/function. These features basically include the scope, visibility, and lifetime which help us to trace the existence of a particular variable during the runtime of a program.
C language uses 4 storage classes, namely: