post: get_caller_address() == 0

_posts/2024-02-29-get_caller_address_zero.md

@@ -0,0 +1,14 @@
+---
+title: When is get_caller_address zero?
+date: 2024-02-29
+author: m
+tags: [starknet]
+---
+
+It's very rare for the caller address (as obtained by `get_caller_address()`) to be zero on Starknet. Let's examine when can this happen.
+
+The obvious case is when calling a view function. The call is not initiated by an account, so the caller is zero. But what about state changing functions?
+
+In this case, the caller address is zero when it's the StarknetOS executing the function. To the best of my knowledge, this happens only in the following scenarios: invoking `__validate__` and `__execute__` in an Account contract and [handling an L1 -> L2 message](https://book.cairo-lang.org/ch15-04-L1-L2-messaging.html?#sending-messages-from-ethereum-to-starknet) in an `#[l1_handler]` function. Note that these functions are public, so _anyone_ can invoke them, but when `get_caller_address()` is zero, you can be sure it's a legit execution initiated by the protocol.
+
+Thanks to [@gaetbout](https://twitter.com/gaetbout/), [@martiray](https://twitter.com/martriay) and [@_tserg](https://twitter.com/_tserg) for their input when researching this piece.