Extern storage class - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to declare an external variable.

C

extern int [1];

Drag options to blanks, or click blank then click option'

Aprintf

Bmain

Ccount

Dreturn

Attempts:

3 left

💡 Hint

Common Mistakes

Using a function name instead of a variable name.

Trying to declare a keyword as a variable.

The keyword extern declares a variable defined in another file or scope. Here, count is the external variable name.

2fill in blank

medium

Complete the code to define the external variable declared elsewhere.

C

int [1] = 10;

Drag options to blanks, or click blank then click option'

Acount

Bprintf

Cextern

Dmain

Attempts:

3 left

💡 Hint

Common Mistakes

Using extern keyword when defining the variable.

Using a different variable name than declared.

The variable count is defined here with an initial value. It matches the external declaration.

3fill in blank

hard

Fix the error in the code to correctly use the external variable.

C

extern int [1];

void printCount() {
    printf("Count is %d\n", [1]);
}

Drag options to blanks, or click blank then click option'

Amain

Bcount

Cprintf

Dreturn

Attempts:

3 left

💡 Hint

Common Mistakes

Using different names in declaration and usage.

Using function names instead of variable names.

The variable count must be used consistently as the external variable name.

4fill in blank

hard

Fill both blanks to declare and initialize an external variable correctly.

C

extern [1] [2];

// Defined in another file

Drag options to blanks, or click blank then click option'

Aint

Bfloat

Ccount

Dtotal

Attempts:

3 left

💡 Hint

Common Mistakes

Mixing data type and variable name order.

Using a float type when the variable is integer.

The external variable count is declared as type int.

5fill in blank

hard

Fill all three blanks to define and use an external variable properly.

C

int [1] = 5;

extern int [2];

void display() {
    printf("Value: %d\n", [3]);
}

Drag options to blanks, or click blank then click option'

Atotal

Bcount

Attempts:

3 left

💡 Hint

Common Mistakes

Using the same variable name for definition and external declaration.

Mismatching variable names in usage.

The variable total is defined, count is declared external and used in the function.

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