From 88bce877c467982be049b4f6515974cec3ebd89e Mon Sep 17 00:00:00 2001 From: Alex Beregszaszi Date: Wed, 30 Aug 2017 02:31:54 +0100 Subject: Clarify some subtleties of the fallback function --- docs/contracts.rst | 21 ++++++++++++++++++--- 1 file changed, 18 insertions(+), 3 deletions(-) (limited to 'docs/contracts.rst') diff --git a/docs/contracts.rst b/docs/contracts.rst index aa3f8fa6..a1a44665 100644 --- a/docs/contracts.rst +++ b/docs/contracts.rst @@ -543,9 +543,12 @@ functions match the given function identifier (or if no data was supplied at all). Furthermore, this function is executed whenever the contract receives plain -Ether (without data). In such a context, there is usually very little gas available to -the function call (to be precise, 2300 gas), so it is important to make fallback functions as cheap as -possible. +Ether (without data). Additionally, in order to receive Ether, the fallback function +must be marked ``payable``. If no such function exists, the contract cannot receive +Ether through regular transactions. + +In such a context, there is usually very little gas available to the function call (to be precise, 2300 gas), +so it is important to make fallback functions as cheap as possible. In particular, the following operations will consume more gas than the stipend provided to a fallback function: @@ -556,6 +559,10 @@ In particular, the following operations will consume more gas than the stipend p Please ensure you test your fallback function thoroughly to ensure the execution cost is less than 2300 gas before deploying a contract. +.. note:: + Even though the fallback function cannot have arguments, one can still use ``msg.data`` to retrieve + any payload supplied with the call. + .. warning:: Contracts that receive Ether directly (without a function call, i.e. using ``send`` or ``transfer``) but do not define a fallback function @@ -563,6 +570,14 @@ Please ensure you test your fallback function thoroughly to ensure the execution before Solidity v0.4.0). So if you want your contract to receive Ether, you have to implement a fallback function. +.. warning:: + A contract without a payable fallback function can receive Ether as a recipient of a `coinbase transaction` (aka `miner block reward`) + or as a destination of a ``selfdestruct``. + + A contract cannot react to such Ether transfers and thus also cannot reject them. This is a design choice of the EVM and Solidity cannot work around it. + + It also means that ``this.balance`` can be higher than the sum of some manual accounting implemented in a contract (i.e. having a counter updated in the fallback function). + :: pragma solidity ^0.4.0; -- cgit