Call stack behavior - Step-by-Step Execution

Concept Flow - Call stack behavior

Program starts

↓

Call main()

↓

Call function A

↓

Call function B

↓

Function B returns

↓

Function A returns

↓

main() returns

↓

Program ends

This flow shows how functions are called and returned, stacking and unstacking on the call stack.

Execution Sample

C

#include <stdio.h>

void B() {
    printf("In B\n");
}

void A() {
    B();
    printf("Back in A\n");
}

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

This code calls function A from main, which calls function B, then returns back step-by-step.

Execution Table

Step	Action	Call Stack	Output
1	Program starts, main() called	main
2	main() calls A()	main -> A
3	A() calls B()	main -> A -> B
4	B() executes printf	main -> A -> B	In B
5	B() returns	main -> A
6	A() executes printf	main -> A	Back in A
7	A() returns	main
8	main() returns
9	Program ends

💡 All functions returned, call stack is empty, program ends.

Variable Tracker

Call Stack	Start	After Step 2	After Step 3	After Step 5	After Step 7	Final
Stack Frames		main -> A	main -> A -> B	main -> A	main

Key Moments - 3 Insights

Why does the call stack grow when a function is called?

Each function call adds a new frame to the stack to keep track of where to return and local variables, as shown in steps 2 and 3 of the execution_table.

When does the call stack shrink?

The stack shrinks when a function finishes and returns, removing its frame, as seen in steps 5, 7, and 8.

Why is the output 'In B' printed before 'Back in A'?

Because B() runs first and prints before returning to A(), which then prints, matching the call stack order in steps 4 and 6.

Visual Quiz - 3 Questions

Look at the execution_table at step 3, what is the call stack?

A main -> B

B main -> A -> B

C main -> A

D A -> B

💡 Hint

Check the 'Call Stack' column at step 3 in the execution_table.

At which step does function B() return and its frame is removed from the stack?

A Step 6

B Step 4

C Step 5

D Step 7

💡 Hint

Look for 'B() returns' in the 'Action' column of the execution_table.

If function A() did not call B(), how would the call stack at step 3 change?

A It would be main -> A

B It would be main only

C It would be main -> A -> B

D It would be empty

💡 Hint

Refer to the call stack growth logic in the key_moments and execution_table.

Concept Snapshot

Call stack stores function calls in order.
Each call adds a frame on top.
Return removes the top frame.
Stack shows current active functions.
Output order follows call and return sequence.

Full Transcript

This example shows how the call stack works in C. When main() calls A(), a frame for A is added on top of main's frame. Then A() calls B(), adding B's frame on top. B() runs and prints 'In B', then returns, removing its frame. Control goes back to A(), which prints 'Back in A' and returns, removing its frame. Finally, main() returns and the program ends. The call stack grows with each call and shrinks with each return, managing the order of execution and local data.