Extern storage class

Introduction

The extern storage class tells the compiler that a variable or function is defined in another file or place. It helps share variables or functions across multiple files.

When you want to use a variable defined in another file.

When you want to share a global variable between different source files.

When you want to declare a function that is defined elsewhere.

When you want to avoid multiple definitions of the same variable.

When organizing large programs into multiple files.

Syntax

C

extern data_type variable_name;
extern return_type function_name(parameter_list);

The extern keyword only declares the variable or function; it does not allocate memory.

The actual variable or function must be defined exactly once elsewhere without extern.

Examples

This declares that an integer variable named count exists somewhere else.

C

extern int count;

This declares a function display defined in another file.

C

extern void display();

This declares an external character array named message.

C

extern char message[];

Sample Program

This example shows two parts: one file defines the variable number, and the other file declares it with extern to use it. When linked together, the program prints the value of number.

C

// File 1: define.c
int number = 10; // definition of variable

// File 2: use.c
#include <stdio.h>
extern int number; // declaration of external variable

int main() {
    printf("Number is %d\n", number);
    return 0;
}

Important Notes

Without extern, declaring the same variable in multiple files causes errors.

extern helps the linker find the actual variable or function during program build.

Use extern only for declarations, not definitions.

Summary

extern declares variables or functions defined elsewhere.

It allows sharing data across multiple files.

Helps avoid duplicate definitions and linker errors.

Practice

1. What is the main purpose of the extern storage class in C?

easy

A. To declare a variable or function defined in another file

B. To allocate memory for a variable inside a function

C. To create a constant variable

D. To define a variable local to a block

Solution

1. Step 1: Understand the role of extern

   The extern keyword tells the compiler that the variable or function is defined elsewhere, usually in another file.

2. Step 2: Compare with other storage classes

   Other options like local variables or constants do not use extern. It specifically helps share variables/functions across files.

3. Final Answer:

   To declare a variable or function defined in another file -> Option A

4. Quick Check:

   extern declares external variables [OK]

Hint: Remember: extern means 'defined outside this file' [OK]

Common Mistakes:

• Confusing extern with static
• Thinking extern allocates memory
• Using extern for local variables

2. Which of the following is the correct syntax to declare an external integer variable named count?

easy

A. int extern count;

B. extern int count;

C. extern int count = 0;

D. static extern int count;

Solution

1. Step 1: Recall correct extern declaration syntax

   The correct syntax places extern before the type: extern int count;

2. Step 2: Check invalid options

   int extern count; reverses order, which is invalid. extern int count = 0; initializes the extern variable, which is not allowed. static extern int count; combines static and extern, which conflicts.

3. Final Answer:

   extern int count; -> Option B

4. Quick Check:

   extern before type, no initialization [OK]

Hint: extern must come before type and no initialization [OK]

Common Mistakes:

• Initializing extern variables
• Placing extern after type
• Combining extern with static

3. Consider two files:
// file1.c
int count = 5;
// file2.c
extern int count;
printf("%d", count);
What will be the output when these files are compiled and run together?

medium

A. 5

B. 0

C. Compilation error

D. Undefined behavior

Solution

1. Step 1: Understand extern linking

   The variable count is defined in file1.c with value 5. The extern declaration in file2.c refers to the same variable.

2. Step 2: Predict output

   Since count is defined and initialized in file1.c, printing it in file2.c outputs 5.

3. Final Answer:

   5 -> Option A

4. Quick Check:

   extern links to defined variable value [OK]

Hint: Extern links to defined variable, prints its value [OK]

Common Mistakes:

• Assuming extern creates a new variable
• Expecting zero as default
• Ignoring multi-file linking

4. What is wrong with the following code snippet?

extern int value = 10;
int main() {
  printf("%d", value);
  return 0;
}

medium

A. No variable named value declared

B. Missing semicolon after extern declaration

C. extern variable cannot be initialized

D. printf syntax is incorrect

Solution

1. Step 1: Check extern initialization rules

   An extern declaration cannot initialize the variable. Initialization must be done in exactly one place without extern.

2. Step 2: Identify error in code

   The code tries to initialize value with 10 in the extern declaration, which causes a compilation error.

3. Final Answer:

   extern variable cannot be initialized -> Option C

4. Quick Check:

   extern declarations must not initialize [OK]

Hint: Never initialize extern variables where declared [OK]

Common Mistakes:

• Initializing extern variables
• Confusing extern with normal definitions
• Ignoring linker errors

5. You have two files:
// file1.c
int total = 100;
void update() { total += 50; }
// file2.c
extern int total;
void update();
int main() {
update();
printf("%d", total);
return 0;
}
What will be the output when compiled and run together?

hard

A. Compilation error due to missing extern

B. 100

C. Undefined behavior due to multiple definitions

D. 150

Solution

1. Step 1: Understand extern and function usage

   total is defined in file1.c and declared extern in file2.c. The function update() modifies total.

2. Step 2: Trace program execution

   Main calls update(), which adds 50 to total (100 + 50 = 150). Then it prints total, so output is 150.

3. Final Answer:

   150 -> Option D

4. Quick Check:

   Extern shares variable; function updates value [OK]

Hint: Extern shares variable; function changes value before print [OK]

Common Mistakes:

• Expecting original value without update
• Forgetting to declare extern
• Confusing declaration and definition