Global vs Static vs Const Variable in C with Example

In C, variables can be declared with various storage classes and qualifiers, such as global, static, and const. Let's go through each one with examples:

1. Global Variable:

A global variable is declared outside of any function and is accessible throughout the entire program.

#include <stdio.h>

// Declaration of a global variable
int global_variable = 42;

int main() {
    // Accessing the global variable
    printf("The value of global_variable is: %d\n", global_variable);

    return 0;
}

In this example, global_variable is accessible from any part of the program.

2. Static Variable:

A static variable has a scope limited to the block it is declared in, and it retains its value between function calls.

#include <stdio.h>

void example_function() {
    // Declaration of a static variable
    static int static_variable = 0;

    // Accessing and updating the static variable
    printf("The value of static_variable is: %d\n", static_variable);
    static_variable++;
}

int main() {
    // Calling the function multiple times
    example_function();
    example_function();
    example_function();

    return 0;
}

In this example, static_variable retains its value between function calls, and each time example_function is called, it increments the value.

3. Const Variable:

A const variable is a variable whose value cannot be modified during the program's execution.

#include <stdio.h>

int main() {
    // Declaration of a const variable
    const int const_variable = 42;

    // Accessing the const variable
    printf("The value of const_variable is: %d\n", const_variable);

    // Uncommenting the next line would result in a compilation error
    // const_variable = 50; // Error: assignment of read-only variable 'const_variable'

    return 0;
}

In this example, const_variable is declared as a constant, and any attempt to modify its value results in a compilation error.

Combining Global, Static, and Const:

#include <stdio.h>

// Declaration of a global static const variable
static const int GLOBAL_STATIC_CONST = 42;

int main() {
    // Accessing the global static const variable
    printf("The value of GLOBAL_STATIC_CONST is: %d\n", GLOBAL_STATIC_CONST);

    // Uncommenting the next line would result in a compilation error
    // GLOBAL_STATIC_CONST = 50; // Error: assignment of read-only variable 'GLOBAL_STATIC_CONST'

    return 0;
}

In this example, GLOBAL_STATIC_CONST is a global static const variable, combining the characteristics of being global (accessible throughout the program), static (limited scope to the translation unit), and const (value cannot be modified).