Integer overflow and underflow in Blockchain / Solidity - Step-by-Step Execution

Concept Flow - Integer overflow and underflow

Start with integer value

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Perform arithmetic operation

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Check if result > max value?

Yes→Overflow occurs: wraps to min value

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Result is min value

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Check if result < min value?

Yes→Underflow occurs: wraps to max value

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Result is max value

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Result is within range

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Use result

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End

Start with an integer, do math, then check if it goes beyond max or min limits. If yes, overflow or underflow happens, wrapping the value around.

Execution Sample

Blockchain / Solidity

uint8 x = 255;
x = x + 1;
// x wraps to 0 due to overflow

Add 1 to the max value of an 8-bit unsigned integer, causing overflow and wrapping to 0.

Execution Table

Step	Variable x	Operation	Result	Overflow/Underflow
1	255	Initial value	255	No
2	255 + 1	Add 1	0	Overflow: wraps to 0
3	0	Final value	0	No

💡 After adding 1 to 255, overflow occurs and value wraps to 0, which is within range.

Variable Tracker

Variable	Start	After Step 2	Final
x	255	0	0

Key Moments - 2 Insights

Why does adding 1 to 255 result in 0 instead of 256?

Because the variable is an 8-bit unsigned integer with max 255, adding 1 causes overflow and wraps the value back to 0, as shown in execution_table step 2.

What happens if we subtract 1 from 0 in an unsigned 8-bit integer?

It causes underflow, wrapping the value to 255, the max value. This is similar to overflow but in the opposite direction.

Visual Quiz - 3 Questions

Look at the execution table, what is the value of x after adding 1 to 255?

A 0

B 256

C 255

D 1

💡 Hint

Check execution_table row 2 under 'Result' column.

At which step does overflow occur according to the execution table?

A Step 1

B Step 2

C Step 3

D No overflow occurs

💡 Hint

Look at the 'Overflow/Underflow' column in execution_table.

If x was a signed 8-bit integer with max 127, what would happen if x = 127 and we add 1?

A Value becomes 128 without issue

B Overflow occurs and value wraps to -128

C Underflow occurs and value wraps to 127

D Program throws an error

💡 Hint

Signed 8-bit integers overflow wraps from max positive to min negative, similar to unsigned wrapping shown in variable_tracker.

Concept Snapshot

Integer overflow and underflow happen when a number goes beyond its storage limits.
Unsigned integers wrap from max to min on overflow, and min to max on underflow.
Example: uint8 max is 255; adding 1 wraps to 0.
Signed integers wrap between positive max and negative min.
Always check for overflow/underflow in blockchain to avoid bugs.

Full Transcript

Integer overflow and underflow occur when a number exceeds its allowed range. For example, an 8-bit unsigned integer can hold values from 0 to 255. Adding 1 to 255 causes overflow, wrapping the value back to 0. Similarly, subtracting 1 from 0 causes underflow, wrapping to 255. This wrapping behavior is important in blockchain programming because it can cause unexpected bugs or vulnerabilities. The execution table shows step-by-step how the value changes and when overflow happens. Understanding this helps prevent errors in smart contracts and other blockchain code.