External Variables in c

External variables in C are variables that are declared outside of any function and are accessible to multiple source files within a program. They allow sharing data between different parts of a program, such as functions in separate source files. External variables are declared using the extern keyword to indicate that their storage is defined elsewhere in the program. Here’s a detailed explanation of external variables in C:

Characteristics of External Variables:

1. Declaration:
   • External variables are declared outside of any function, typically at the beginning of a source file or in a header file.
   • They are declared using the extern keyword to indicate that their storage is defined elsewhere.
2. Scope:
   • External variables have file scope, meaning they are accessible throughout the source file in which they are declared.
   • They are not visible outside their source file by default unless explicitly declared as extern in other source files.
3. Lifetime:
   • External variables have a lifetime that extends throughout the program’s execution.
   • They are allocated and deallocated once during the program’s lifetime.
4. Memory Allocation:
   • Memory for external variables is typically allocated in the data segment of memory.
   • The memory is allocated and initialized when the program starts, and deallocated when the program terminates.

Example of External Variables:

File1.c:

#include <stdio.h>

extern int num; // Declaration of external variable

int main() {

    printf(“Value of num in File1: %d\n”, num); // Accessing external variable

    return 0;

}

File2.c:

int num = 10; // Definition of external variable

In this example:

• num is an external variable declared in File1.c with the extern keyword.
• The definition of num is provided in File2.c, where it is assigned the value 10.
• File1.c can access and use the value of num because it’s declared as extern.
• The linker resolves references to num by associating them with the definition in File2.c.

Use Cases of External Variables:

1. Sharing Data Between Source Files:
   • External variables allow sharing data between functions defined in separate source files.
2. Global Configuration Settings:
   • External variables can be used to store global configuration settings that are accessed by multiple parts of a program.
3. Maintaining State:
   • External variables can be used to maintain state information across multiple function calls within a program.
4. Shared Resources:
   • External variables are useful for sharing resources such as file handles, buffers, and counters across different parts of a program.

External variables provide a mechanism for sharing data between different parts of a program, facilitating modularity, code reuse, and flexibility in program design. However, their use should be judicious to avoid potential issues related to data integrity and encapsulation.