Extern storage class - Step-by-Step Execution

Concept Flow - Extern storage class

Declare variable with extern

↓

Variable defined elsewhere

↓

Linker connects declaration to definition

↓

Variable can be used in this file

↓

Program compiles and runs

Extern tells the compiler a variable is defined in another file, so it links them together for use.

Execution Sample

C

/* file1.c */
int count = 10;

/* file2.c */
#include <stdio.h>
extern int count;
void printCount() {
    printf("Count: %d\n", count);
}

Shows extern declaration in one file accessing a variable defined in another file.

Execution Table

Step	Action	Evaluation	Result
1	Compile file1.c	int count = 10;	Variable 'count' defined with value 10
2	Compile file2.c	extern int count;	Declaration of 'count' without definition
3	Link files	Link 'count' in file2.c to definition in file1.c	Both files share same 'count' variable
4	Run printCount()	printf("Count: %d\n", count);	Output: Count: 10
5	Program ends	No errors	Execution successful

💡 Program ends after printing count; extern links declaration to definition across files

Variable Tracker

Variable	file1.c (definition)	file2.c (extern declaration)	After linking	Final value
count	10	undefined (declared)	linked to 10	10

Key Moments - 3 Insights

Why does 'extern int count;' not allocate memory?

Because 'extern' only declares the variable exists elsewhere; memory is allocated where it is defined (see execution_table step 2 and 3).

What happens if the variable is not defined in any file?

Linker will give an error because extern expects a definition somewhere (see execution_table step 3).

Can you change the value of an extern variable?

Yes, since it refers to the actual variable defined elsewhere, changes affect the same memory (implied by step 4 output).

Visual Quiz - 3 Questions

Look at the execution_table, what is the value of 'count' after linking?

A undefined

B 10

C 0

D Linker error

💡 Hint

Check execution_table row 3 and variable_tracker final value for 'count'

At which step does the linker connect the extern declaration to the variable definition?

A Step 1

B Step 2

C Step 3

D Step 4

💡 Hint

Look at execution_table step describing linking action

If 'count' was not defined in file1.c, what would happen during linking?

A Linker error occurs

B Program runs normally

C Variable 'count' is set to 0 automatically

D Compiler error at file2.c

💡 Hint

Refer to key_moments about missing definition and execution_table step 3

Concept Snapshot

extern storage class in C:
- Declares variable exists elsewhere
- Does NOT allocate memory
- Used to share variables across files
- Linker connects extern to definition
- Allows access to same variable in multiple files

Full Transcript

The extern storage class in C tells the compiler that a variable is defined in another file. When you write 'extern int count;', you declare the variable without creating it. The actual variable must be defined elsewhere, like 'int count = 10;' in another file. During linking, the compiler connects the extern declaration to the real variable. This way, you can use the same variable across multiple files. If the variable is not defined anywhere, the linker will give an error. Using extern helps organize code by sharing variables without duplication.

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