> For the complete documentation index, see [llms.txt](https://docs.multichain.org/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://docs.multichain.org/developer-guide/anycall-v6/anyfallback.md).

# anyFallback

If the `anyExec` contract execution failed on Chain B, it can call `_fallback` function to send messages back to Chain A. 

&#x20;`anyExecute` <mark style="color:red;">**DAPP receiver contract(Chain B failed)**</mark>**&#x20;-> anyCall**(Chain B) **->** *SMPC Network* **-> anyExec**(Chain A) **->** `anyExecute` **(**<mark style="color:green;">**DAPP sender contract(Chain A)**</mark>**)**

**->**`anyFallback`

**Format of the fallback function needs to be**:&#x20;

`function anyFallback(address _to, bytes calldata _data) external;`

### Logic

When `anyExecute` fails on Chain B, the anycall contract would issue another call back to chain A with data including the `anyFallback` selector.

Hence the `anyExecute` execute normal function or fallback function based on the selector.

```
emit LogAnyCall(
                _from,
                _fallback,
                abi.encodeWithSelector(IApp.anyFallback.selector, _to, _data),
                address(0),
                _ctx.fromChainID,
                0, // pay fee on dest chain
                _appID,
                nonce);
```

### Example

Example Contract Code:<https://ftmscan.com/address/0x2b6fbe2a08491130dcc2fe13f0e60b3540d92a55#code>

1. Chain A First call: <https://ftmscan.com/tx/0xE45DCF2F196982D5F2B4F84AD0926B9D1A439F62B9E950AE5BBA0460B775C389>

&#x20; 2\. Chain B issued callback: <https://bscscan.com/tx/0x3ccb1f91ad66348ec5dd8676d1dd59372fd37bca4e202982b582cd8d4e5c582c#eventlog>

&#x20; 3\. anyFallback executing: <https://ftmscan.com/tx/0x215002fb282c630dada2ebeb56ea605ae6c24df8a52db46b977e9c081893d061#eventlog>

`anyExecute` function:

When executing, anyExecute first look at the selector of the calldata passed in to determine if it should execute normal flow or fallback function.

```
function anyExecute(bytes calldata data)
        external
        
        returns (bool success, bytes memory result)
    {
        bytes4 selector = bytes4(data[:4]);
        if (selector == this.anyExecute.selector) {
            (
                string memory message
            ) = abi.decode(
                data[4:],
                (string)
            );

            if (compareStrings(message,"fail")){
                return (false, "fail on purpose");
            }

            emit NewMsg(message);
        } else if (selector == this.anyFallback.selector) {

            // original data with selector would be passed here if thats the case
            (address _to, bytes memory _data) = abi.decode(data[4:], (address, bytes));
            this.anyFallback(_to, _data);
        } else {
            return (false, "unknown selector");
        }
        return (true, "");
    }
```


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