Lifetime and scope comparison - Step-by-Step Execution

Concept Flow - Lifetime and scope comparison

Start Program

↓

Declare Variables

↓

Enter Function

↓

Check Variable Scope

↓

Local

↓

Lifetime & Scope Behavior

↓

End Program

This flow shows how variables are declared and how their lifetime and scope differ depending on whether they are local, global, or static.

Execution Sample

C

#include <stdio.h>
int global_var = 10;

void func() {
    int local_var = 5;
    static int static_var = 0;
    static_var++;
    printf("%d %d %d\n", global_var, local_var, static_var);
}

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

This code shows global, local, and static variables and how their values change or reset across function calls.

Execution Table

Step	Variable	Value	Scope	Lifetime	Action	Output
1	global_var	10	Global	Whole program	Initialized before main
2	func() call 1				Enter func
3	local_var	5	Local to func	During func call	Initialized
4	static_var	0	Static local	Whole program	Initialized once
5	static_var	1	Static local	Whole program	Incremented
6	Output				Print values	10 5 1
7	func() call 1				Exit func, local_var destroyed
8	func() call 2				Enter func again
9	local_var	5	Local to func	During func call	Initialized again
10	static_var	1	Static local	Whole program	Retains previous value
11	static_var	2	Static local	Whole program	Incremented
12	Output				Print values	10 5 2
13	func() call 2				Exit func, local_var destroyed
14	Program End				main returns, program ends

💡 Program ends after main returns; local variables destroyed after each func call; static and global variables persist.

Variable Tracker

Variable	Start	After func call 1	After func call 2	Final
global_var	10	10	10	10
local_var	N/A	5 (exists during call)	5 (exists during call)	N/A (destroyed after calls)
static_var	0	1	2	2

Key Moments - 3 Insights

Why does static_var keep its value between function calls but local_var does not?

Because static_var has static lifetime and is initialized only once, it keeps its value across calls. local_var is created and destroyed each time func runs, so it resets (see execution_table rows 4-7 and 9-13).

Does global_var change during the program execution?

No, global_var is initialized once before main and remains unchanged throughout (see execution_table rows 1, 6, and 12).

When exactly is local_var created and destroyed?

local_var is created at the start of each func call and destroyed when func returns (see execution_table rows 3 and 7 for first call, 9 and 13 for second).

Visual Quiz - 3 Questions

Look at the execution_table at step 5, what is the value of static_var?

A 5

B 0

C 1

D 10

💡 Hint

Check the 'Value' column for static_var at step 5 in execution_table.

At which step does local_var get destroyed after the first func call?

A Step 3

B Step 7

C Step 6

D Step 9

💡 Hint

Look for the action mentioning local_var destruction after func call 1 in execution_table.

If static_var was not static, what would be its value at step 11?

A 1

B 2

C 0

D Undefined

💡 Hint

Consider that non-static local variables are reinitialized each call (see variable_tracker for static_var changes).

Concept Snapshot

Lifetime and Scope in C:
- Global variables: scope whole program, lifetime entire run
- Local variables: scope inside function/block, lifetime during call
- Static local variables: scope local, lifetime entire program
- Static variables keep value between calls
- Local variables reset each call

Full Transcript

This example compares lifetime and scope of variables in C. Global variables exist and keep their value throughout the program. Local variables exist only during a function call and are destroyed after. Static local variables behave like locals in scope but keep their value between calls because their lifetime is the whole program. The execution table shows how static_var increments across calls, local_var resets, and global_var stays constant. Key moments clarify why static_var retains value and when local_var is created and destroyed. The quiz tests understanding of these concepts by referencing specific steps and variable values.