sizeof and memory budgeting in Embedded C - Step-by-Step Execution

Concept Flow - sizeof and memory budgeting

Declare variables

↓

Use sizeof operator

↓

Calculate memory needed

↓

Compare with budget

↓

Allocate

↓

Use memory safely

This flow shows how we declare variables, check their size with sizeof, compare with available memory, and decide if allocation is possible.

Execution Sample

Embedded C

int a;
char b[10];
int total_size = sizeof(a) + sizeof(b);
int memory_budget = 20;
if (total_size <= memory_budget) {
  // allocate memory
}

This code calculates total memory needed for variables and checks if it fits in the memory budget.

Execution Table

Step	Expression	Evaluation	Result	Action
1	sizeof(a)	size of int	4	Store size of int variable a
2	sizeof(b)	size of char[10]	10	Store size of array b
3	total_size = sizeof(a) + sizeof(b)	4 + 10	14	Calculate total memory needed
4	memory_budget	given	20	Memory available for allocation
5	total_size <= memory_budget	14 <= 20	true	Check if memory fits budget
6	if condition	true	execute allocation	Memory allocation allowed
7	End	-	-	Program continues safely

💡 Memory needed (14 bytes) fits within budget (20 bytes), so allocation proceeds.

Variable Tracker

Variable	Start	After Step 1	After Step 2	After Step 3	Final
a	declared	size=4 bytes	size=4 bytes	size=4 bytes	size=4 bytes
b	declared	size=unknown	size=10 bytes	size=10 bytes	size=10 bytes
total_size	undefined	undefined	undefined	14 bytes	14 bytes
memory_budget	20 bytes	20 bytes	20 bytes	20 bytes	20 bytes

Key Moments - 3 Insights

Why does sizeof(a) return 4 instead of the value stored in a?

sizeof returns the size in bytes of the variable's type, not its value. See execution_table step 1 where sizeof(a) evaluates to 4 bytes because 'a' is an int.

Why do we add sizeof(a) and sizeof(b) instead of adding their values?

We add sizes to find total memory needed, not variable values. execution_table step 3 shows adding 4 and 10 bytes to get total_size.

What happens if total_size is greater than memory_budget?

The condition in step 5 would be false, so allocation would not proceed. This prevents memory overflow.

Visual Quiz - 3 Questions

Look at the execution_table at step 3, what is the value of total_size?

A 14

B 10

C 4

D 20

💡 Hint

Check the 'Result' column at step 3 in execution_table.

At which step does the program decide if memory allocation is allowed?

A Step 2

B Step 6

C Step 5

D Step 4

💡 Hint

Look for the condition check in execution_table that compares total_size and memory_budget.

If memory_budget was 10 instead of 20, what would happen at step 5?

A Condition would be true, allocation allowed

B Condition would be false, allocation denied

C total_size would change to 10

D sizeof operator would return different values

💡 Hint

Compare total_size (14) with new memory_budget (10) in execution_table step 5.

Concept Snapshot

sizeof operator returns size in bytes of a variable or type.
Add sizes to budget memory before allocation.
Compare total size with available memory.
If total size fits budget, allocation proceeds safely.
Prevents memory overflow in embedded systems.

Full Transcript

This lesson shows how to use the sizeof operator in embedded C to find the memory size of variables. We add these sizes to calculate total memory needed. Then we compare this total with a memory budget to decide if allocation is safe. The execution table traces each step: getting sizes of variables, summing them, checking against budget, and deciding allocation. Key points include understanding sizeof returns size in bytes, not variable values, and why we add sizes for budgeting. The quiz tests understanding of these steps and outcomes.