Receive and fallback functions in Blockchain / Solidity - Step-by-Step Execution

Concept Flow - Receive and fallback functions

Contract receives Ether

↓

Is msg.data empty?

No→Call fallback()

Yes↓

Call receive()

↓

Execute function body

↓

End

When a contract receives Ether, it checks if data is sent. If no data, receive() runs; if data or no receive(), fallback() runs.

Execution Sample

Blockchain / Solidity

contract Example {
  receive() external payable {
    // handle plain Ether
  }
  fallback() external payable {
    // handle calls with data or no receive
  }
}

This contract has receive() for plain Ether and fallback() for calls with data or when receive() is missing.

Execution Table

Step	msg.data	Function Called	Action	Output
1	empty	receive()	Accept Ether with no data	Ether received
2	non-empty	fallback()	Handle call with data	Fallback logic runs
3	empty	fallback()	No receive() defined, fallback handles Ether	Ether received via fallback
4	non-empty	fallback()	Fallback called for unknown function	Fallback logic runs
5	-	-	-	Execution ends

💡 Execution stops after receive() or fallback() handles the call

Variable Tracker

Variable	Start	After Step 1	After Step 2	After Step 3	After Step 4	Final
msg.data	varies	empty	non-empty	empty	non-empty	varies
Function Called	none	receive()	fallback()	fallback()	fallback()	none

Key Moments - 2 Insights

Why does fallback() run when msg.data is empty in step 3?

Because receive() is not defined in that case, fallback() handles the Ether receipt as shown in execution_table row 3.

What happens if msg.data is non-empty but receive() exists?

Fallback() runs because receive() only handles calls with empty msg.data, as shown in execution_table row 2.

Visual Quiz - 3 Questions

Look at the execution table, which function is called when msg.data is empty and receive() exists?

A receive()

B fallback()

C constructor()

D no function

💡 Hint

Check execution_table row 1 where msg.data is empty and receive() is called

At which step does fallback() handle Ether because receive() is missing?

A Step 1

B Step 3

C Step 2

D Step 4

💡 Hint

Look at execution_table row 3 where fallback() runs with empty msg.data

If msg.data is non-empty, which function always runs according to the table?

A receive()

B constructor()

C fallback()

D no function

💡 Hint

See execution_table rows 2 and 4 where msg.data is non-empty and fallback() runs

Concept Snapshot

receive() runs when contract gets Ether with empty msg.data
fallback() runs when msg.data is non-empty or receive() is missing
Both can be payable to accept Ether
Use receive() for plain Ether transfers
Use fallback() for calls with data or unknown functions

Full Transcript

When a contract receives Ether, it checks if the data sent (msg.data) is empty. If empty and receive() exists, receive() runs to accept Ether. If msg.data is not empty or receive() is missing, fallback() runs. This lets contracts handle plain Ether transfers and calls with data differently. The execution table shows steps where receive() or fallback() is called based on msg.data. Variables msg.data and Function Called change accordingly. Key moments clarify why fallback() runs sometimes even with empty msg.data. The visual quiz tests understanding of which function runs in different cases.