Storage Class in c Language

Storage Class in C Language :

here we are sharing the storage classes in c language. variables can be put into some combination of scope, lifetime and visibility by using some keywords and placing them appropriately and this is called a storage class of the variable.

Broadly speaking there are only two storage classes. Auto and static. Other storage classes are extern and register.

1. Static Variables

Depending In the context where the static declaration is used, it can give two different meanings.

1. If declared at global scope it means that the variable has file scope.

E.g.1

static myprintf( ); // only file scope

declares that myprintf function is having file scope and is not accessible to other files.

E.g. 2.

int a; // global scope

static int b; // file scope

2. If declared within a block it means that the variable is having a static lifetime but with local scope. As a whole, the static keyword helps in limiting the variables to file scope such that collision of names at the time of linkage is avoided. In other words, static limits variables to internal linkage. A static variable (with local or block scope) can be initialized with the address of any external or static item, but not with the address of another auto item. Because the address of an auto item is not a constant since auto items are created in stacks and erased as the function exits.

2. Extern

‘extern’ is used for two main purposes; it can be used for:

_ for specifying global linkage and

_ forward declarations,

The first use of extern is to explicitly specify that the variable is a candidate for external linkage.

extern int fun(); specifies that the function fun is defined somewhere else and that will be known only at link-time.

extern int i; // declare i is defined somewhere else

...

int i = 10;

// i resolves to this variable i.

// This is resolved by the linker and the programmer may have

// used it for forward reference.

3. Auto

Auto keyword specifies that the variable is of an automatic lifetime. It is not instructive to the compiler but for you to just remember that it is of automatic type. Practically it serves no purpose (of course you know that the variables declared inside functions are automatic and told explicitly that the variable is auto doesn’t help you anyway).

4. Register

You can specify a local variable as register if you think that variable may be accessed frequently. The compiler will try to put that variable in a microprocessor register if one is available. Else this keyword is ignored and treated as if it is declared as auto. Declaring such frequently used variables to be placed in registers may only lead to small performance improvement. Modern compilers will easily find out the variables that will be frequently accessed and will place them accordingly. This is a deprecated feature

(so will be omitted in future versions of C) and so is not recommended to use. It is illegal to apply ‘&’ operator to a register variable. Why because of both the pointer that stores the address of the variable and the value in that the address has to be in the memory for applying & and * operators. Then only these operations remain meaningful and valid. However, register variables may not be inactive memory when the & operator applied and so C prohibits applying this operator on register variables. Register variables can be used to improve program efficiency. Looping variables are good candidates for such register optimizations,

for( i=0 ; i < INT_MAX; i++)

arrayAccess[i];

In such cases, the efficiency of execution certainly increases. If a memory the register is not available for placing it in memory it may at-least place the variable in cache memory that is still an improvement in execution speed. Anyway, using register variables is found to speed up many programs by a factor

of two.

Scope of a variable -:

v In C, a variable can be declared anywhere in the program but before using them.

v The area of the program within which a variable is accessible, known as its scope.

v A variable can be accessed within the block where it is declared.

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Sunday 16 February 2020

Storage Class in c Language