Global variables in programming are variables that are defined outside of any function and can be accessed from any part of the code within the same file or, in some cases, from other files as well. Unlike local variables that are created and destroyed within the scope of a function, global variables persist throughout the lifetime of the program. They are stored in the data segment of the program's memory and offer a way to share data between different functions without passing them as parameters.

To demonstrate how global variables work in C, I'll provide a sample program and then explain it in detail. After that, I'll illustrate a diagram showing where global variables live in memory.

#include <stdio.h>

int globalVar = 10; // Global variable

void display() {
    printf("Value of global variable: %d\n", globalVar);
}

int main() {
    printf("Value of global variable in main: %d\n", globalVar);
    display();
    return 0;
}

Explanation of the Code

• int globalVar = 10; - This is a global variable declaration. A global variable is defined outside of all functions, typically at the top of the program. It can be accessed from any function within the file.

• void display() - This is a function that prints the value of the global variable globalVar. Since globalVar is global, it doesn't need to be passed as an argument to the function.

• int main() - The main function of the program. It also accesses the global variable globalVar directly and calls the display function.

Global Variables in Memory

Global variables are stored in the data segment of the program's memory layout. The memory layout of a typical C program is divided into several segments:

1. Code Segment: Contains the compiled code of the program.

2. Data Segment: It is divided into two parts:

   • Initialized Data: This is where our global variable globalVar resides, along with other initialized global and static variables.

   • Uninitialized Data: Also known as the BSS segment, which contains uninitialized global and static variables.

3. Heap Segment: Used for dynamic memory allocation during program run-time.

4. Stack Segment: Used for storing local variables and function call details.

The global variable globalVar is placed within the Data Segment, highlighting how global variables are stored in memory.

The use of global variables in programming, particularly in C, is a topic that requires careful consideration due to both their advantages and disadvantages. Here's a breakdown of when to use them and why they are often advised against:

When to Use Global Variables

1. Shared Constants: If a value is constant and used across multiple functions, a global constant can be appropriate. For example, mathematical constants like PI.

2. Shared Across Multiple Functions: When a variable needs to be accessed by multiple functions and it's cumbersome or inefficient to pass it through function arguments.

3. State Representation: In certain cases, like in embedded systems or specific types of software architecture, global variables can be used to represent the state of the system.

4. Configuration Variables: Global variables can be used for configuration settings that need to be accessible throughout the program.

Why They Should Be Avoided

1. Maintainability Issues: Global variables can make a program hard to understand and maintain, as any part of the program can change their values.

2. Risk of Name Conflicts: Since global variables are accessible throughout the program, there's a higher risk of name conflicts, which can lead to bugs.

3. Testing and Debugging Difficulties: Functions that use global variables are more difficult to test and debug because their behavior can depend on the state of these globals, which may change anywhere in the program.

4. Concurrency Issues: In multithreaded applications, accessing global variables can lead to concurrency issues like race conditions unless properly synchronized, which adds complexity.

5. Limited Modularity and Scalability: Global variables imply a high degree of coupling between different parts of a program, making it harder to isolate modules or reuse code in different contexts.

6. Potential for Side Effects: Functions that use global variables can have side effects, making it difficult to predict the outcome of a function call, as it may depend on the state of the global variables.

Best Practices

• Use local variables where possible.

• If global variables are necessary, minimize their use and ensure they are well documented.

• Consider alternatives like passing variables to functions, using static variables within a function, or encapsulating them in structures or classes (in languages that support them).