Extern storage class - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of extern variable usage across files

Consider two files:

File1.c:

int count = 5;

File2.c:

extern int count;
void printCount() { printf("%d", count); }

What will be the output when printCount() is called after linking both files?

C

/* File1.c */
int count = 5;

/* File2.c */
#include <stdio.h>
extern int count;
void printCount() { printf("%d", count); }

int main() {
    printCount();
    return 0;
}

A 0

B Runtime error due to uninitialized variable

C Compilation error due to missing definition

D 5

💡 Hint

Remember that extern declares a variable defined elsewhere.

The variable 'count' is defined in File1.c and declared as extern in File2.c. Linking both files allows printCount() to access the same variable, printing 5.

❓ Predict Output

intermediate

Value of extern variable after modification

Given the following code snippets:

File1.c:

int value = 10;

File2.c:

extern int value;
void changeValue() { value = 20; }

What is the value of value after calling changeValue()?

C

/* File1.c */
int value = 10;

/* File2.c */
#include <stdio.h>
extern int value;
void changeValue() { value = 20; }

int main() {
    changeValue();
    printf("%d", value);
    return 0;
}

A 10

B Undefined behavior

C 20

D Compilation error

💡 Hint

Extern allows access to the same variable across files.

The extern declaration links to the same variable 'value' defined in File1.c. Changing it in File2.c affects the original variable, so the printed value is 20.

❓ Predict Output

advanced

Output of extern variable with multiple definitions

What happens if you have these two files:

File1.c:

int num = 5;

File2.c:

int num = 10;
extern int num;

And you compile and link them together, then print num?

C

/* File1.c */
int num = 5;

/* File2.c */
#include <stdio.h>
int num = 10;
extern int num;

int main() {
    printf("%d", num);
    return 0;
}

A Linker error due to multiple definitions

B 5

C 10

D Runtime error

💡 Hint

Multiple definitions of the same global variable cause conflicts.

Defining 'num' in both files causes a multiple definition error during linking. Extern does not resolve this conflict if both define the variable.

❓ Predict Output

advanced

Behavior of extern with uninitialized variable

Consider these files:

File1.c:

extern int data;

File2.c:

int data;

What is the initial value of data when accessed in File1.c?

C

/* File1.c */
#include <stdio.h>
extern int data;

int main() {
    printf("%d", data);
    return 0;
}

/* File2.c */
int data;

A Runtime error

B 0

C Garbage value

D Compilation error

💡 Hint

Global variables without explicit initialization are zero-initialized.

Global variables like 'data' are automatically initialized to zero if not explicitly initialized. Extern links to the same variable.

🧠 Conceptual

expert

Understanding extern and static linkage

Which statement correctly explains the difference between extern and static storage classes in C?

A <code>extern</code> declares a variable defined elsewhere with external linkage; <code>static</code> limits variable visibility to the current file.

B <code>extern</code> creates a new variable local to the function; <code>static</code> shares the variable across multiple files.

C <code>extern</code> initializes a variable to zero; <code>static</code> requires explicit initialization.

D <code>extern</code> and <code>static</code> both make variables visible only inside the function they are declared.

💡 Hint

Think about variable visibility and linkage across files.

extern declares a variable defined in another file, giving it external linkage. static restricts the variable's visibility to the file it is declared in, giving it internal linkage.