Function modifiers in Blockchain / Solidity - Step-by-Step Execution

Concept Flow - Function modifiers

Call function

↓

Enter modifier code

↓

Run pre-function checks or code

↓

Execute function body

↓

Run post-function code if any

↓

Return result or finish

When a function with a modifier is called, the modifier code runs first, then the function body, then any post-code in the modifier.

Execution Sample

Blockchain / Solidity

modifier onlyOwner() {
    require(msg.sender == owner, "Not owner");
    _;
}

function changeOwner(address newOwner) public onlyOwner {
    owner = newOwner;
}

This code uses a modifier to check if the caller is the owner before changing ownership.

Execution Table

Step	Action	Evaluation	Result
1	Call changeOwner with msg.sender = 0x123	Enter onlyOwner modifier	Proceed to require check
2	Check require(msg.sender == owner)	msg.sender == 0x123, owner == 0xabc	False, revert with 'Not owner'
3	Call changeOwner with msg.sender = 0xabc	Enter onlyOwner modifier	Proceed to require check
4	Check require(msg.sender == owner)	msg.sender == 0xabc, owner == 0xabc	True, continue
5	Execute function body: owner = newOwner	newOwner = 0xdef	owner updated to 0xdef
6	Function ends	Return success	Ownership changed

💡 Execution stops if require condition fails; otherwise function completes normally.

Variable Tracker

Variable	Start	After Step 5	Final
owner	0xabc	0xdef	0xdef
msg.sender	varies	varies	varies

Key Moments - 2 Insights

Why does the function stop when msg.sender is not the owner?

Because the require statement in the modifier fails (see step 2 in execution_table), it reverts the transaction and stops execution before the function body runs.

What does the underscore (_) mean in the modifier?

The underscore marks where the function body runs inside the modifier. Before it runs, pre-checks happen; after it runs, post-code can run.

Visual Quiz - 3 Questions

Look at the execution_table, what happens at step 2 when msg.sender is not the owner?

A The function continues to execute

B The require check passes

C The transaction reverts with 'Not owner'

D The owner variable is updated

💡 Hint

Check the 'Evaluation' and 'Result' columns at step 2 in execution_table

At which step does the owner variable get updated?

A Step 4

B Step 5

C Step 2

D Step 6

💡 Hint

Look at the 'Action' and 'Result' columns in execution_table for when owner changes

If the underscore (_) was removed from the modifier, what would happen?

A The function body would not run

B The require check would be skipped

C The function would run twice

D The modifier would have no effect

💡 Hint

Recall that underscore (_) indicates where the function body runs inside the modifier

Concept Snapshot

Function modifiers run code before and/or after a function.
Use '_' inside modifier to mark function body location.
Common for access control like 'onlyOwner'.
If modifier code fails (e.g. require), function stops.
Modifiers help reuse checks and logic cleanly.

Full Transcript

Function modifiers in blockchain let you run extra code before or after a function runs. When you call a function with a modifier, the modifier code runs first. It can check conditions like if the caller is the owner. The underscore (_) in the modifier shows where the function's main code runs. If a check fails, like require(msg.sender == owner), the function stops immediately and reverts. This helps keep your code safe and clean by reusing checks in many functions.