FreeGas - Let contract owner pay the tx fee for you


Lity supports built-in random function after lity v1.2.7.

Built-it FreeGas Modifier

Lity provides a new modifier freegas. With freegas, developers can mark specific functions to be freegas, which means the transaction fee will be paid by the callee contract and caller user doesn’t need to care about the gas fee.

Because freegas will consume the contract balance for the transaction fee, Dapp developers should make sure that the deployed contract must have a fallback payable function and should send some money into the contract.

How Virtual Machine handles the freegas function

The blockchain could pass gasPrice=0 transactions to the VM to signal that the caller has requested a freegas function call. Learn more about free transactions The VM now performs the following actions.

  • If gasLimit < 5000000, it will just process it normally. The caller’s gas fee will be 0
  • If gasLimit >= 5000000, it will require the contract function to be freegas
    • The call consumes a max of (default gasPrice) * gasLimit amount of gas
    • the TX fails if the function is not freegas
    • the TX fails if the contract does not have enough fund to cover gas

The gas fee for caller is ALWAYS 0 in this case.

Example 1

pragma lity ^1.2.7;

contract FreeGas {
    uint x;
    function c(uint a) public freegas returns (uint) {
        x = a;
        return x;
    function () public payable {}