From 84a8c0c008acf80a70c60dbfc0f5c3666abcd676 Mon Sep 17 00:00:00 2001 From: chriseth Date: Tue, 31 May 2016 14:13:02 +0200 Subject: [PATCH] Document some pitfalls in connection with "send". --- docs/miscellaneous.rst | 18 ++++++++++++++++++ docs/types.rst | 6 ++++++ 2 files changed, 24 insertions(+) diff --git a/docs/miscellaneous.rst b/docs/miscellaneous.rst index b7a4bb7ac..504c4e0e9 100644 --- a/docs/miscellaneous.rst +++ b/docs/miscellaneous.rst @@ -148,6 +148,24 @@ Pitfalls Unfortunately, there are some subtleties the compiler does not yet warn you about. - In ``for (var i = 0; i < arrayName.length; i++) { ... }``, the type of ``i`` will be ``uint8``, because this is the smallest type that is required to hold the value ``0``. If the array has more than 255 elements, the loop will not terminate. +- If a contract receives Ether (without a function being called), the fallback function is executed. The contract can only rely + on the "gas stipend" (2300 gas) being available to it at that time. This stipend is not enough to access storage in any way. + To be sure that your contract can receive Ether in that way, check the gas requirements of the fallback function. +- If you want to send ether using ``address.send``, there are certain details to be aware of: + 1. If the recipient is a contract, it causes its fallback function to be executed which can in turn call back into the sending contract + 2. Sending Ether can fail due to the call depth going above 1024. Since the caller is in total control of the call + depth, they can force the transfer to fail, so make sure to always check the return value of ``send``. Better yet, + write your contract using a pattern where the recipient can withdraw Ether instead. + 2. Sending Ether can also fail because the recipient goes out of gas (either explicitly by using ``throw`` or + because the operation is just too expensive). If the return value of ``send`` is checked, this might provide a + means for the recipient to block progress in the sending contract. Again, the best practise here is to use + a "withdraw" pattern instead of a "send" pattern. +- Loops that do not have a fixed number of iterations, e.g. loops that depends on storage values, have to be used carefully: + Due to the block gas limit, transactions can only consume a certain amount of gas. Either explicitly or just due to + normal operation, the number of iterations in a loop can grow beyond the block gas limit, which can cause the complete + contract to be stalled at a certain point. This does not apply at full extent to ``constant`` functions that are only executed + to read data from the blockchain. Still, such functions may be called by other contracts as part of on-chain operations + and stall those. Please be explicit about such cases in the documentation of your contracts. ********** Cheatsheet diff --git a/docs/types.rst b/docs/types.rst index 2cd3654b7..6d4e928cc 100644 --- a/docs/types.rst +++ b/docs/types.rst @@ -86,6 +86,12 @@ and to send Ether (in units of wei) to an address using the ``send`` function: .. note:: If ``x`` is a contract address, its code (more specifically: its fallback function, if present) will be executed together with the ``send`` call (this is a limitation of the EVM and cannot be prevented). If that execution runs out of gas or fails in any way, the Ether transfer will be reverted. In this case, ``send`` returns ``false``. +.. warning:: + There are some dangers in using ``send``: The transfer fails if the call stack depth is at 1023 + (this can always be forced by the caller) and it also fails if the recipient runs out of gas. So in order + to make safe Ether transfers, always check the return value of ``send`` or even better: + Use a pattern where the recipient withdraws the money. + * ``call``, ``callcode`` and ``delegatecall`` Furthermore, to interface with contracts that do not adhere to the ABI,