Storage Classes in C: Types, Uses and Summary

Storage Classes in C play a pivotal role in determining the characteristics of variables and functions, encompassing essential aspects like visibility, scope, and lifetime. These classes serve as a roadmap to understanding the presence and behavior of specific variables throughout the runtime of a program.

• By categorizing variables and functions into different storage classes, C provides a flexible framework for managing memory and optimizing resource allocation.
• The features of a variable/function which includes visibility, scope, and lifetime are described by storage class C.
• It aids in tracking the presence of a specific variable throughout the program's execution.

Key Terms: Storage Class, Automatic Storage Class, External Storage Class, Static Storage Class, Register Storage Class.

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Types of Storage Classes in C

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There are 4 types of storage classes in C such as:

1. Automatic: Automatic is the default storage class where all the variables are declared inside a function or a block. These auto variables can be accessed within the block/function, not outside the block.
2. External: These are the variables that are defined elsewhere and not within the same block.
3. Static: These are the storage class that is used to declare static variables while writing programs in C language.
4. Register: These storage classes declare register variables with similar functionality to auto variables.

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Syntax

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To specify a variable for a storage class, the following is the syntax:

storage_class var_data_type var_name;

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Uses of storage class in C

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The storage classes in a C program have two properties such as storage class and type.

• Type: It refers to any given variable’s data type.
• Storage: It determines the variable’s lifetime, visibility, and also its scope.

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Summary of storage classes in C

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The table below shows the summary of storage classes in C:

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Automatic Storage Class

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These are also known as the auto storage class, serving as the default storage class for all locally scoped variables.

For example,

{

int mount;

auto int month;

}

The features of the Automatic Storage class are mentioned below:

• During the runtime, the allocation of memory for these variables occurs automatically.
• The scope for automatic variables is limited to a block where they are being defined.
• The visibility of these variables is limited to the block in which they are being defined.
• By default, the initialization of these variables is a garbage value.
• The memory which is assigned to an automatic variable is free when it exits from a block.

Read More: Introduction To Array​

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External Storage Class

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These are also known as extern storage classes, where reference is given to any global variable. While using the external class the variables cannot be initialized. An extern in any file would be used for providing the reference of the defined function or variable.

The most common use of the extern modifier is having two or more files that are shared over a similar global variable or function.

The features of the External Storage class are mentioned below:

• One cannot perform the initialization of an external variable with any given method or block but can perform the initialization of an external variable globally.
• The role of the external storage class is to indicate to the compiler that a variable described as "extern" has been declared.
• If a variable is declared as external to the compiler it will start searching for that variable.

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Static Storage Class

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This type of class gives instruction to a compiler to give local variables around during the program’s lifetime- instead of creating it.

The features of the Static Storage class are mentioned below:

• Static variables are defined as specifiers capable of retaining their assigned values between different function calls.
• These are visible to block or the function which has been defined.
• By default, the initial value of a static integral is 0 else it is null.
• In the case of static variables, static keywords are used.

Also Read:

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Register Storage Class

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The register storage class is defined for the local variable that is stored in the register which means that the maximum size of this variable is equal to that of the register size.

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Things to Remember

• Storage classes in C determine key attributes of variables, such as their visibility, scope, and lifetime, crucial for efficient memory management.
• The scope of a variable dictates where it can be accessed, while visibility refers to its accessibility within different parts of a program.
• By classifying variables into different storage classes—automatic, external, static, and register—C enables flexible memory allocation and resource optimization.
• The 'auto' storage class is the default, local to a function or block, with memory allocated and deallocated automatically upon entry and exit.
• The 'extern' class references global variables defined elsewhere, aiding code sharing across files, though initialization is limited.
• The 'Static' class maintains variable values between function calls, limiting visibility to the block or function in which they're defined.
• The 'Register' class denotes local variables stored in registers, potentially optimizing access times, albeit with limited availability due to register constraints.
• The default initial value for external integral value is going to be 0, else null.
• To declare a variable's storage class, use the syntax: storage_class data_type variable_name;