Storage Classes in C: Different Types of Storage Classes [With Examples]

Coding with C is highly centered upon using variables in every program. Those are the key aspects of C programming. Every variable in C has two properties; type and storage classes. Among them, the type refers to the data type of the variable, and storage classes in C determine the scope, lifetime, and visibility of the variable. Storage classes in C are used to find the lifetime, visibility, memory location, and initial value of a variable.

With that in mind, let us delve into storage class and the types of storage classes in C. 

In this blog post, we will have a detailed look at the storage classes in C, its types, and how its characteristics influence the output of the program with some programming examples. 

Storage Classes In C With Examples Of Use

Any storage class actually is representative of visibility and location of any variable. This tells users what portion of a code is accessible as a variable. Storage class for C can help describe the following things:

• A variable scope: Storage classes in C are widely used to determine the lifetime, visibility, or scope of existing variables and functions. These classes in C precede the modifying type of variables.
• Location for variable storage: While creating variables in C, it is assigned a specific memory address, referred to as the location of its storage in the system. Once a variable is given a value, this is the memory address it is stored in.
• Initialized value for variable: Once a variable is declared, it needs to be initialized first. Variable initialization is of two types: explicit and implicit. Explicitly initialized variables are ones that are given a value in the declaration statement, whereas when variables are given a value while processing, they are implicitly initialized.
• Variable lifetime: The amount of time a variable takes up a valid space in a computer’s storage is known as variable lifetime. It is also defined as the period between when a memory is assigned to hold one variable till the time it is freed. When the allocated variable is freed from the scope, its lifetime ends.
• Users accessing a variable: Refers to users or parts of a program that can access the variable. These can involve ‘extern’ variables, which one can define out of any function block, or ‘global’ variables, which one can define outside the scope of every existing variable.

Overall, a storage class stands representative of information about any variable. This is the best way to explain storage classes in c with examples.

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What Are Storage Classes In C?

Before delving into the various types, let us first address the question: What is a storage class in C? 

Storage classes in C allocate the storage area of a variable that will be retained in the memory. They are stored in the RAM of a system. Apart from the storage space, they determine the scope of a variable. Variables in the C program are stored in a physical location in the random memory, which is mainly the device’s memory and CPU registers.

Storage classes in C also define the lifetime of the variable and term it as ‘local’ or ‘global’. Storage classes are also useful to define the scope or visibility, and the initial value of the variable. There are primarily four storage classes in C, viz. automatic, register, static, and external. We will discuss each one by one further.

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How Storage Classes In C Are declared?

Storage classes in C define the scope, visibility, and lifetime of variables within a program.

The four storage classes in C are declared in a block or program with the storage class specifiers, auto, register, extern, static. There is one more storage class specifier, ‘typedef’ used in the syntactic form, and does not reserve storage. The specifiers instruct the compiler on storing the variables. The external storage class in C tell the compiler that variable defined is declared with external linkage. 

These are also referred to as the four primary storage class specifiers in C. Here is a detailed explanation of the four: 

• auto: Variables declared with the auto storage class in C are automatically created within a code block (usually within a function) and are destroyed when the block exits. This storage class is default for all local variables in C, so explicitly using “auto” is optional.
• register: The “register” storage class is used to suggest to the compiler that a variable be stored in a CPU register for faster access. However, it’s merely a suggestion, and the compiler might or might not comply. Like “auto” variables, “register” variables have block scope.
• static: Variables with the “static” storage class persist throughout the program’s execution. A static variable retains its value between function calls when declared inside a function. If declared outside a function, it becomes accessible only within the file where it’s declared, acting as a file-local variable.
• extern: The “extern” storage class is used to declare variables or functions defined in other files. It’s used to access global variables or functions defined in a separate file from where they’re being used. When used to declare a variable, it refers to a global variable defined elsewhere.

Difference Between Definition and Declaration of Storage Class in C

There is a key difference between defining and declaring a variable. Defining a variable is about allocating the memory for the variable and declaring it means initializing it with a value.

Declaring Storage Classes in C:

When the existence and properties of a variable, function, or identifier are announced to the compiler, it is called a declaration. It helps provide necessary information about the entity without allocating memory or specifying its value.

Declaration of storage classes in C might involve specifying the storage class of a variable with keywords like “static”, “extern”, “register”, and “auto.” This helps outline the variable’s characteristics, including scope, lifetime, and accessibility, without assigning memory or initializing its value.

Defining Storage Classes in C:

Definition involves providing the actual implementation or instantiation of the declared entity. For variables, it includes allocating memory, assigning an initial value, and specifying the characteristics declared earlier.

Defining a variable with a specific storage class involves assigning it memory as per the rules of the storage class and initializing its value if required. This is where the actual storage space for the variable is set aside based on its scope, lifetime, and other properties specified in the declaration.

Table of Key Differences

There is a key difference between defining and declaring a variable. Defining a variable is about allocating the memory for the variable and declaring it means initializing it with a value.

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Syntax:

storage_class_specifier data_type variable_name;

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Special Case: When no storage class specifier is declared or defined in a program

There is at least one storage class specifier is given in the variable declaration. But, in case, there is no storage class specifier specified, the following rules are followed:

1. Variables declared within a function are considered as auto.

Local variables are defined with the help of an automatic/auto storage class. Unless explicitly specified, a variable in auto storage class is allocated by default.

An auto variable’s scope is limited only to a specific block. The access gets eliminated once the block loses control of the variable. Hence, only the block containing the declared auto variable can access it.

2. Functions declared within a function are considered as an extern.

Extern is a storage class stating that a variable is defined outside and not inside the same block where it has been used. An external linkage occurs when the value assigned to the variable is in a different block and can also be overwritten or changed in a separate block. Extern variables are basically global variables that are initialized with legal values at the location where they are declared for use elsewhere.

Normal global variables can be extern variables too. This can be achieved by placing the keyword ‘extern’ before declaring or defining it in any function or block. 

3. Variables and functions declared outside a function are considered static, with external linkage.

Despite being static, these variables and functions still have external linkage. External linkage means that they can be accessed by other parts of the program if they use the extern keyword to reference them. This enables other files in the program to access and use these variables or functions by explicitly stating their existence with the extern keyword.

When variables or functions are declared outside any function without an explicit storage class specifier, they are automatically treated as static (limited in visibility to the file in which they are declared) while still maintaining the ability for external parts of the program to access them by specifying extern.

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What are the Types of Storage Classes in C?

In C programming, storage classes play a crucial role in determining the scope, visibility, and lifetime of variables. There are many types of storage classes in C, with each class having a distinct characteristic that governs how variables are stored, accessed, and managed within a program. 

There are four storage classes in C, let’s have a look at them: 

1. Automatic Storage Classes in C

Every variable defined in a function or block belongs to automatic storage class by default if there is no storage class mentioned. The variables of a function or block belong to the automatic storage class are declared with the auto specifier. Variables under auto in C are local to the block where they are defined and get discarded outside the block.

The automatic storage class in C is implicit to the C programming language, and explicitly using the auto specifier is optional. Despite this, explicitly specifying auto for variables within a function or block reinforces their automatic storage behavior. This makes it clearer in the code but redundant due to the default behavior of local variables in C.

A Simple Program Showing Automatic Storage Classes:

#include <stdio.h>

int main( )

{

auto int i = 11;

{

auto int i = 22;

{

auto int i = 33;

printf ( “%d “, i);

}

printf ( “%d “, i);

}

printf( “%d”, i);

}

The output of the Program:

3 2 1

Explanation:

In the above program, there is three times the variable i is declared. Variables with the same name can be defined in different blocks. Thus, this program will compile and execute successfully without any error. The function ‘printf’ in the innermost block will print 3 and the variable i in this block will be destroyed after the block ends.

The next one, the second outer block prints 2 which is then succeeded by the outer block which prints 1. The automatic variables are initialized properly; else you will get undefined values as the compiler does not give them an initial value.

2. Register Storage Classes in C

The variables belonging to a register storage class are equivalent to auto in C but are stored in CPU registers and not in the memory, hence the name. They are the ones accessed frequently. The register specifier is used to declare the variable of the register storage class. Variables of a register storage class are local to the block where they are defined and destroyed when the block ends.

A Simple Program Showing Register Storage Classes:

#include <stdio.h>

int main()

{

register int i = 10;

int *p = &i; //error: address of register variable requested

printf(“Value of i: %d”, *p);

printf(“Address of i: %u”, p);

}

Explanation:

In the above program, the code tries to get the address of variable i into the pointer variable p but as i is declared as a register variable, the code won’t compile and will display the error ” Error: address of register variable requested”. 

Only certain types of variables are placed into registers. Register variables are not given an initial value by the compiler.

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3. Static Storage Classes in C

The visibility of static variables is zero outside their function or file, but their values are maintained between calls. The variables with static storage class are declared with the static specifier. Static variables are within a function or file. The static specifier works differently with local and global variables.

A static storage class will instruct a compiler for keeping the local variable existing during the span of a program in place of creating and then destroying the same every time the scope arises around it. Thus, making all local variables static will allow the maintenance of the values between all function calls.

A static modifier gets applied to all global variables. Once done, this causes that particular variable scope restriction to a file where the same is declared.

The static specifier in programming works differently with both local and global variables. Take a look below to understand the static storage class in c.

Simple Programs Showing Static Storage Classes with Local and Global Variables:

i. Local Variable

#include <stdio.h>

void staticDemo()

{

 static int i;

{

 static int i = 1;

  printf(“%d “, i);

i++;

}

  printf(“%d”, i);

  i++;

}

int main()

{

  staticDemo();

  staticDemo();

}

The output of the Program:

1 0

2 1 

Explanation:

When a local variable is defined by a static specifier, inside a function or block, permanent storage space is created in the compiler. The static local variable is visible to the function or block where it is specified and retains its value between the function calls. In the above program, the static variable i is defined at two places in two blocks inside the staticDemo()function. staticDemo() is called two in the main function. In the next call, the static variables retain their old values and need not be initialized again.

ii. Global Variable

#include <stdio.h>

static int gInt = 1;

static void staticDemo()

{

  static int i;

  printf(“%d “, i);

  i++;

  printf(“%d”, globalInt);

  globalInt++;

}

int main()

{

  staticDemo();

  staticDemo();

}

The output of the Program:

0 1

1 2

Explanation:

Static variables need to be initialized only once in a program and they are retained throughout the lifetime. They have a default initial value of zero.

When a global variable or function is defined by a static specifier, then that variable or function is known only to the file in which it is defined. For a global variable, other file routines cannot access and alter its contents as a static global variable has internal linkage. In the above program, the static global variable globalInt and a static function staticDemo(), are defined as static and they cannot be used outside the C file.

4. External Storage Classes in C

External storage class variables or functions are declared by the ‘extern’ specifier. When a variable is declared with extern specifier, no storage is allotted to the variable and it is assumed that it has been already defined elsewhere in the program. With an extern specifier, the variable is not initialized. The reason why extern is used to specify a variable in a program to declare it with external linkage. 

A Simple Program Showing External Storage Classes:

#include <stdio.h>

extern int i;

int main()

{

printf(“i: %d”, i);

}

int i = 1;

Explanation:

In the above C program, if extern int i is removed, there will be an error “Undeclared identifier ‘i’ because the variable i is defined after being used in printf. The extern specifier instructs the compiler that variable i has been defined and is declared here.

If you change extern int i; to extern int i = 5; you will get an error “Redefinition of ‘i'” because the extern specifier does not initialize a variable.

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A Quick Summary Of Various Storage Classes In C

Let us take a quick look at summing up the basic information about various storage classes in c.

• Any storage class in C can be used for representing any additional information about any variable.
• Storage class stands for the scope as well as the lifespan of the variable.
• Storage class also tells clearly who can access any variable and from what place.
• The four different storage classes in C program are auto, register, extern, and static.
• Storage class specifier for C language can be used for defining variables or functions as well as parameters.
• Auto can be used for any local variable that is defined within the block or function.
• Register gets used for storing a variable in the CPU registers instead of a memory location for ease of access.
• Static gets usage for global as well as local variables. Both have their use cases within the C program.
• Extern can be used for data sharing between multiple C files.

Advantages and disadvantages of C Storage Class

Let’s look at the advantages of The C Storage Class:

• Helps to determine the scope, visibility, lifetime of a variable.

Storage classes help define the scope of variables, indicating where in the program they can be accessed. They also control the visibility of variables, determining whether they can be accessed from other parts of the program or limited to specific functions or files.

• We can use more than one variable with the same name.

An advantage of storage classes in C is the ability to use identical variable names in different scopes without conflicts. This feature enhances code organization and readability by allowing distinct variables with the same name in separate scopes, streamlining maintenance, and facilitating localized operations without affecting other variables.

• Helps in Memory Optimization:

Certain storage classes, such as the register storage class, offer memory optimization by suggesting to the compiler that frequently used variables be stored in CPU registers. This optimization enhances access speed for critical variables, potentially boosting program performance, especially in scenarios where rapid data access is crucial.

Some disadvantages of The C Storage Class:

• We cannot use more than one storage class for a single variable.

One limitation of storage class in C is the inability to use multiple storage classes for a single variable. Each variable in C can only have one storage class specified. This constraint can be limiting when attempting to modify or adapt existing code, especially when a variable’s usage requires different storage class characteristics in different parts of the program. 

• Changing the program to meet different needs becomes difficult, as we have to keep in mind the scope and lifetime of the variable.

The fixed scope and lifetime defined by a specific storage class in C present challenges when adapting or extending code. Modifying the program to meet new requirements becomes intricate due to the predefined scope and lifetime of variables tied to their storage classes. This inflexibility complicates efforts to accommodate changing needs or incorporate new features, requiring meticulous management of variable declarations and storage classes. 

Final Words

Programming in C has garnered popularity in coding due to its versatility which allows programmers to use it in a myriad of applications, programs, and technologies. It is the backbone of operating systems and more complex-level software and is critical to programming languages and the software industry at large. As a foundational language, it is widely used for its simple and flexible nature, and its ability to function independently in machines.

This article details the concept of storage classes in C and tells you how its types differ from each other. When to use a particular storage class depends on the assignment and the scope, lifetime, and visibility of the variable you are dealing with.

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