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How to get the amount of token sent to a smart contract via the fallback function in solidity

Let's say I have a token A and a smart contract B. A user sent some amount of A to to the contract B by direct transfer through his wallet. In the fallback function of the smart contract B, how will I get the amount of token A that was sent?

msg.value did'nt give me anything since it's not Eth that was sent.

Solution

  • When your contract receives ERC-20 tokens, the fallback() function is not invoked - nor any other function. Meaning, your contract does not get notified about incoming ERC-20 transfers unless you pull them with transferFrom().

    contract YourContract {
    function pullTokens() external {
        // need to have prior approval
        tokenContract.transferFrom(msg.sender, address(this), amount);
    }
}

    Note: Some other fungible token standards define these notification functions for recipient contracts - for example ERC-777 and its function tokensReceived().