what is Storage Class in c

C Programming Storage Class

Every variable and function in C programming has two properties: type and storage class. Type refers to the data type of variable whether it is character or integer or floating-point value etc.

There are 4 types of storage class:

1. automatic
2. external
3. static
4. register

Automatic storage class

Keyword for automatic variable

auto

Variables declared inside the function body are automatic by default. These variable are also known as local variables as they are local to the function and doesn't have meaning outside that function

Since, variable inside a function is automatic by default, keyword auto are rarely used.

External storage class

External variable can be accessed by any function. They are also known as global variables. Variables declared outside every function are external variables.

In case of large program, containing more than one file, if the global variable is declared in file 1 and that variable is used in file 2 then, compiler will show error. To solve this problem, keyword extern is used in file 2 to indicate that, the variable specified is global variable and declared in another file.

Example to demonstrate working of external variable

#include   
void Check();
int a=5;    
/* a is global variable because it is outside every function */
int main(){
    a+=4;
    Check();
    return 0;
}

void Check(){
   ++a;
/*  ----- Variable a is not declared in this function but, works in any function as they are global variable -------  */
   printf("a=%d\n",a);
}

Output

a=10

Register Storage Class

Keyword to declare register variable

register

Example of register variable

register int a;

Register variables are similar to automatic variable and exists inside that particular function only.

If the compiler encounters register variable, it tries to store variable in microprocessor's register rather than memory. Value stored in register are much faster than that of memory.

In case of larger program, variables that are used in loops and function parameters are declared register variables.

Since, there are limited number of register in processor and if it couldn't store the variable in register, it will automatically store it in memory.

Static Storage Class

The value of static variable persists until the end of the program. A variable can be declared static using keyword: static. For example:

static int i;

Here, i is a static variable.

Example to demonstrate the static variable

#include <stdio.h>
void Check();
int main(){
   Check();
   Check();
   Check();
}
void Check(){
    static int c=0;
    printf("%d\t",c);
    c+=5;
}

Output

0      5     10

During first function call, it will display 0. Then, during second function call, variable c will not be initialized to 0 again, as it is static variable. So, 5 is displayed in second function call and 10 in third call.

If variable c had been automatic variable, the output would have been:

0     0     0

A storage class defines the scope (visibility) and life time of variables and/or functions within a C Program.

There are following storage classes which can be used in a C Program

• auto
• register
• static
• extern

auto - Storage Class

auto is the default storage class for all local variables.

{ int Count; auto int Month; }

The example above defines two variables with the same storage class. auto can only be used within functions, i.e. local variables.

register - Storage Class

register is used to define local variables that should be stored in a register instead of RAM. This means that the variable has a maximum size equal to the register size (usually one word) and cant have the unary '&' operator applied to it (as it does not have a memory location).

{ register int Miles; }

Register should only be used for variables that require quick access - such as counters. It should also be noted that defining 'register' goes not mean that the variable will be stored in a register. It means that it MIGHT be stored in a register - depending on hardware and implimentation restrictions.

static - Storage Class

static is the default storage class for global variables. The two variables below (count and road) both have a static storage class.

static int Count; int Road; { printf("%d\n", Road); }

static variables can be 'seen' within all functions in this source file. At link time, the static variables defined here will not be seen by the object modules that are brought in.

static can also be defined within a function. If this is done the variable is initalised at run time but is not reinitalized when the function is called. This inside a function static variable retains its value during vairous calls.

void func(void); static count=10; /* Global variable - static is the default */ main() { while (count--) { func(); } } void func( void ) { static i = 5; i++; printf("i is %d and count is %d\n", i, count); } This will produce following result i is 6 and count is 9 i is 7 and count is 8 i is 8 and count is 7 i is 9 and count is 6 i is 10 and count is 5 i is 11 and count is 4 i is 12 and count is 3 i is 13 and count is 2 i is 14 and count is 1 i is 15 and count is 0

NOTE : Here keyword void means function does not return anything and it does not take any parameter. You can memoriese void as nothing. static variables are initialized to 0 automatically.

Definition vs Declaration : Before proceeding, let us understand the difference betweendefintion and declaration of a variable or function. Definition means where a variable or function is defined in realityand actual memory is allocated for variable or function. Declaration means just giving a reference of a variable and function. Through declaration we assure to the complier that this variable or function has been defined somewhere else in the program and will be provided at the time of linking. In the above examples char *func(void) has been put at the top which is a declaration of this function where as this function has been defined below to main()function.

There is one more very important use for 'static'. Consider this bit of code.

char *func(void); main() { char *Text1; Text1 = func(); } char *func(void) { char Text2[10]="martin"; return(Text2); }

Now, 'func' returns a pointer to the memory location where 'text2' starts BUT text2 has a storage class of 'auto' and will disappear when we exit the function and could be overwritten but something else. The answer is to specify

static char Text[10]="martin";

The storage assigned to 'text2' will remain reserved for the duration if the program.

extern - Storage Class

extern is used to give a reference of a global variable that is visible to ALL the program files. When you use 'extern' the variable cannot be initalized as all it does is point the variable name at a storage location that has been previously defined.

When you have multiple files and you define a global variable or function which will be used in other files also, then extern will be used in another file to give reference of defined variable or function. Just for understanding extern is used to decalre a global variable or function in another files.

File 1: main.c

int count=5; main() { write_extern(); }

File 2: write.c

void write_extern(void); extern int count; void write_extern(void) { printf("count is %i\n", count); }

Here extern keyword is being used to declare count in another file.

Now compile these two files as follows

gcc main.c write.c -o write

This fill produce write program which can be executed to produce result.

Count in 'main.c' will have a value of 5. If main.c changes the value of count - write.c will see the new value