Extern storage class - Step-by-Step Execution

Concept Flow - Extern storage class

Declare variable with extern

↓

Variable defined elsewhere

↓

Linker connects declaration to definition

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Variable can be used in this file

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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.