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I want to return the gas used to the user when he uses a payable function. I used tx.gasprice but it always returns 1.

How can I get the actual gas used value (not the maximum or user set values, but really the gas used value for the transaction, in my case, a function call)?

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3 Answers 3

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30

You can measure the gas used in a block of code by calculating the difference between gasleft() at the start and gasleft() at the end:

function test() returns (uint256 gasUsed)
{
    uint256 startGas = gasleft();

    // ...some code here...

    gasUsed = startGas - gasleft();
}

This does not yet take into account the constant cost of a transaction, which as far as I know is usually approximately 21000.

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4
  • Is it possible to get the like "startgas" within solidity and without web3 or external calls?
    – dkb
    May 12, 2018 at 18:50
  • 1
    This actually a good idea and this is available in the solidity environment, no need for web3.
    – Jaime
    May 12, 2018 at 18:58
  • @Jaime what I wrote was just an example. For sure I had some other functions that should work with gas. But I didn't want to talk detailed about it.
    – dkb
    May 12, 2018 at 19:04
  • 1
    @Jaime im working through a similar implementation thats not giving me the expected results: ethereum.stackexchange.com/questions/111741/…
    – lopezdp
    Oct 18, 2021 at 14:48
6

tx.gasPrice is the price per 1 gas, not the gas paid. There is no way to get this information from the variables in the environment in solidity. The solution is to have a frontend app that sends this info to your contract.

Hope this help.

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3
  • It helps but the problem is, I want to work with that value within solidity itself, so use it in functions. Is that in no way possible?
    – dkb
    May 12, 2018 at 18:27
  • No, as far as I know. The only option is to use web3 and send this info to your contract. Then you can read the value it as any other variable.
    – Jaime
    May 12, 2018 at 18:52
  • the answer from @Jesse Busman is what you need
    – Jaime
    May 12, 2018 at 18:59
1

use getTransactionReceipt: doc

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2
  • Hello, I meant in solidity. Not web3. Is that possible?
    – dkb
    May 12, 2018 at 18:19
  • 2
    This link is not the answer. This is web3js, he asked for solidity
    – Itération 122442
    May 13, 2018 at 8:29

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