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3
Changelog.md
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@ -1,5 +1,8 @@
### 0.7.6 (unreleased)
Language Features:
* The fallback function can now also have a single ``calldata`` argument (equaling ``msg.data``) and return ``bytes memory`` (which will not be ABI-encoded but returned as-is).
Compiler Features:
* SMTChecker: Support named arguments in function calls.

27
docs/contracts/functions.rst
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@ -275,7 +275,10 @@ A contract can have at most one ``receive`` function, declared using
``receive() external payable { ... }``
(without the ``function`` keyword).
This function cannot have arguments, cannot return anything and must have
``external`` visibility and ``payable`` state mutability. It is executed on a
``external`` visibility and ``payable`` state mutability.
It can be virtual, can override and can have modifiers.
The receive function is executed on a
call to the contract with empty calldata. This is the function that is executed
on plain Ether transfers (e.g. via ``.send()`` or ``.transfer()``). If no such
function exists, but a payable :ref:`fallback function <fallback-function>`
@ -339,15 +342,22 @@ Below you can see an example of a Sink contract that uses function ``receive``.
Fallback Function
=================
A contract can have at most one ``fallback`` function, declared using ``fallback () external [payable]``
(without the ``function`` keyword).
This function cannot have arguments, cannot return anything and must have ``external`` visibility.
It is executed on a call to the contract if none of the other
A contract can have at most one ``fallback`` function, declared using either ``fallback () external [payable]``
or ``fallback (bytes calldata _input) external [payable] returns (bytes memory _output)``
(both without the ``function`` keyword).
This function must have ``external`` visibility. A fallback function can be virtual, can override
and can have modifiers.
The fallback function is executed on a call to the contract if none of the other
functions match the given function signature, or if no data was supplied at
all and there is no :ref:`receive Ether function <receive-ether-function>`.
The fallback function always receives data, but in order to also receive Ether
it must be marked ``payable``.
If the version with parameters is used, ``_input`` will contain the full data sent to the contract
(equal to ``msg.data``) and can return data in ``_output``. The returned data will not be
ABI-encoded. Instead it will be returned without modifications (not even padding).
In the worst case, if a payable fallback function is also used in
place of a receive function, it can only rely on 2300 gas being
available (see :ref:`receive Ether function <receive-ether-function>`
@ -364,12 +374,11 @@ operations as long as there is enough gas passed on to it.
to distinguish Ether transfers from interface confusions.
.. note::
Even though the fallback function cannot have arguments, one can still use ``msg.data`` to retrieve
any payload supplied with the call.
After having checked the first four bytes of ``msg.data``,
If you want to decode the input data, you can check the first four bytes
for the function selector and then
you can use ``abi.decode`` together with the array slice syntax to
decode ABI-encoded data:
``(c, d) = abi.decode(msg.data[4:], (uint256, uint256));``
``(c, d) = abi.decode(_input[4:], (uint256, uint256));``
Note that this should only be used as a last resort and
proper functions should be used instead.