In this track: Execution traces and low-level calls
advanced · 4 MIN READ
Reentrancy across call frames
An external handoff can lead back into unfinished application logic.
LET’S PICTURE IT
A customer can return to a second service window before the first clerk finishes updating the record.
SEE IT FOR YOURSELF
A valid counter call, step by stepStep 1 / 4
This example begins with count zero and a request to call increment(). No real contract is deployed.
This example begins with count zero and a request to call increment(). No real contract is deployed.Execution follows the code’s input and permission rules. A website button is not the access-control mechanism.The function reads zero, calculates zero plus one, and writes one to persistent storage.The accepted state contains count one. Later reads can inspect it. Events are emitted only if the code implements them.
Animated local teaching model. No wallet, RPC or live blockchain data. Some labels and hashes are abbreviated for readability.
Mark the handoff point
An external call transfers control to other code. That code can call back into the original contract or another related contract before the first call returns.
Inspect unfinished state
If accounting still reflects the pre-action state, the callback may repeat an operation or observe an inconsistent condition. The danger depends on the reachable functions and shared state.
Protect the invariant
Apply a coherent state-transition design, suitable interaction ordering and guards where needed. A guard on one function may not cover every cross-function or cross-contract path.
Test the whole call sequence
A hostile mock should attempt relevant callbacks and inspect state at the boundary. A test that only calls the target function once cannot exercise reentry.
Follow the execution
A begins withdrawal
A calls B before consistent accounting
B calls back into A
A’s first frame has not finished
Review the invariant across both frames.
A local conceptual trace with explicit assumptions; this is not a trace of a live transaction.
QUICK CHECK · NO PRESSURE
Can reentry happen before the original external call returns?
The callback occurs within the same nested execution.