Questions tagged [blockchain]

The Ethereum blockchain is a distributed database based on the Ethereum whitepaper specification and stores both the full transaction list and the most recent state.

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Can I check whether any deployed contract has implemented some particular interface or not?

My concern is whether I can do interface detection with my own interface,,, Like suppose, i have implemented some interface say loyalty interface in my one contract and I have deployed that contract ,...
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can anyone explain why call, delegateCall, staticCall functions in solidity are called low level functions?

As i have read at many resources, that these functions are low level functions. I want to know, should i use these methods to interact with already deployed contracts or not ?
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Geth returns tx hash but blockchain

I have a Geth server on light mode, and implemented sending transactions based on web3.php project. Everything works well, except sending transactions. The code I am using sends from, to, and value ...
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Document version control

Version control systems are used to track record of changes to files. the files are contained in a repository or database, and it contains all the history versions of the files. My question is : Is ...
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Spending funds before the block is mined

Suppose I have an application that receives funds from users and instantly moves them into an associated cold storage account. If one of our nodes detects a transaction broadcast to one of our ...
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Can i know by any method whether the particular address is contract address or simple ethereum address?

Here is the problem I am facing. I am calling some function of already deployed contract using its contract address. Below are the lines of code :- function registerYourBrand(string memory brand, ...
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Can I change state of mapping variable in deployed contract from another contract?

I am calling addCoin and subCoin function of my AirToken contract and Hotel contract. This is function defination of addCoin and subCoin in AirToken contract and Hotel contract function addCoin(...
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Can anyone explain how bytes memory can be converted to address type in solidity

when I am using below function to convert bytes memory to address, this is working fine. Suppose input to the function is 0X0000000000000000000000004af3246b4fff356261136f113411cb187134D675 I am ...
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Ethereum Solidity Examples

I am pretty now the blockchain/solidity/ethereum world. I started in February, reading and diving into the world. Currently, I am trying to implement a simple game with ethereum and the help of smart ...
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Is it a good practice to store a lot of keys in a string array in smart contracts?

I am working on a project where blockchain will be used on a supply chain. One can create and transfer assets to another in the same. Whenever one is creating an asset or transferring any asset, ...
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ERC20 Anyone can call approve if knows ABI on contract and my address?

I think I misunderstand something. I'm using a Ganache for developing dApp. I built a default ERC-20 smart-contract. I've built it with truffle. And just for test I write some code in NodeJS: ...
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Why knownStates and pulledStates are always 0 on my full node?

It's the second time that I run a Geth in full node. The first one got synced fully, but the second one did not sync after a week. Both servers have the same configuration and infrastructure. Using ...
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How could I measure the total delay (time) to confirm the transaction?

First of all, I'm new to the Erthereum world, I need some help in measuring the total delay (time) require for confirming the transaction. I have a decentralized application over a Rinkeby network, ...
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Does a symbol have to be unique in ERC721? How to find out a unique symbol?

As per the ERC-721 contract on OpenZeppelin, you can create a token with this constructor: constructor(string _name, string _symbol) public {} Does the "_symbol" have to be unique across the ...
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What block will be set on pending transactions?

I have faced with different pending transactions in Ethereum. This is one of its strange ones which is pending after 9 days! Total amount which has sent is 1 USDT, and the fee is 0.0003 Ether. ...
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Call view function using web3.js 1.2.6

I am trying to call view only function, not sure whether this is correct procedure or not. Getting success in result but the result is in different format. Is this wrong ? const NODE_ADDRESS = ...
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Function call error in ethereum smart contract

I am trying to call a ethereum smart contract function but it gives me error. It works perfectly fine with remix JVM environment but throws error with web3 provider. I am just trying to call a ...
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Etherscan Forked Blocks' Reorg-Depth is Incorrect?

I suddenly found there is an reorg happened today whose depth is 3 on Etherscan.io here. The uncle blocks is at height 9919331 and the hash is ...
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How to deploy ethereum smart contract with parameters using web3 1.2.6?

I am trying to deploy smart contract using web3 1.2.6 , my smart contract accepts 2 argument in constructor : id , name. I dont know what i am doing wrong , but both approaches are not working. Error (...
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Is transaction history available as a part of the blockchain implementation, or do I have to implement saving token history myself

I am implementing a CryptoKitties style of ERC721 tokens, to be deployed on Ethereum. But I am not clear how to implement the transaction history of the individual token? For example all the ...
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Explicitly conversion of addres and payable

i can't understand this Explicit conversions to and from address are permitted for integers, integer literals, contact types and bytes20. However, Solidity prevents the conversions of address payable(...
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Connect an ethereum based blockchain to owncloud

My goal is to create a permissioned blockchain. There are 2 types of users, type 1 user can only read documents, type 2 users can read as well as write documents. I have created a blockchain using ...
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web3 filter function not working

I am trying to filter Transfer events from my local Ethereum node using web3 py. I am accessing my local ethereum node remotely using teamviewer and it was running and fully synced as well. My code is ...
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Can you force your smart contract to only process transactions in separate blocks

I am working on a smart contract and trying to come up with ways through which I can make sure that each transaction sent to my Ethereum smart contract is only processed as part of separate block, ...
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Miners are not working when running geth command in background in linux

I am running my private blockchain using geth on my ubuntu server with following cmd then its working fine geth --datadir ~/chaindata/ --networkid 45123 --verbosity 3 --rpc --port 30304 --rpcport ...
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'Contract' object has no attribute 'eventFilter' web3.py

I am trying to filter Transfer events from my local ethereum node using web3 py. I have created a venv in visual studio code. My code is as follows: def decodeLogswithfilter(t1, t2, abi1, addr): ...
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Ethereum in space?

Is there a list of missions that have flown to space that have broadcasted the Ethereum network to ground? I'm interested to know the feasibility of this project and would like to start by surveying ...
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Provable API for calling ETH-USD price in a contract

This contract is hosted at https://github.com/provable-things/ethereum-examples/blob/master/solidity/truffle-examples/kraken-price-ticker/contracts/KrakenPriceTicker.sol I have made the constructor ...
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TypeError: Operator != not compatible with types string storage ref and int_const 0

Im using blockchain technology for the first time, for a college project. Im trying to run a smart contract from Github (in Powershell windows 10). Im unable to compile the contract due the following ...
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Im getting the following error: ParserError: Expected primary expression. struct ManagingOrder.Parking memory new_Parking = Parking({

Im using blockchain technology for the first time, for a college project. Im trying to run a smart contract from Github. After debugging the errors shown while compiling the smart contract (in ...
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Web3 is not connecting to metamask. TypeError: Null object returned to account

I am currently following the tutorial in Dapp University YouTube channel where I am coding my own ERC-20 token https://www.youtube.com/watch?v=XdKv5uwEk5A&t=21611s. At 5:51:00 onwards you can see ...
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Potential Concurrent Engines for Ethereum?

I have been reading some papers on the topic of concurrency on blockchains, smart contract systems to be more specific. Most of the research in the last 3 years uses Ethereum as a reference of the ...
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Want to Access remote dai smart contract in another smart contract

I want to transfer Dai tokens from account1 to account2. My account1 has 50 DAI tokens. Kovan Dai contract address given by makerdao is: 0x4F96Fe3b7A6Cf9725f59d353F723c1bDb64CA6Aa. What I am doing ...
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Syncing keeps behind the blockchain

I am trying to run a geth node just to learn what I can do with it. I started geth with fast syncing. The problem is that it never syncs and always lags behind ~100 blocks. My datadir is on my HDD and ...
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Getting HTTP/1.1 403 Forbidden when i want to get account balance

Some months ago, I was able to get the balance of my account using this code (I run the code in Unity 2018 and get the code from tutorial in github. I modified some parts of the code). This is the ...
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External node request using Geth

when I try to use Web3 provider in Remix it gives me this error can any one tell me how to do it thank you
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Duplicate data in “returnValues” field of event emitted in solidity script

I am using quoram distribution through Azure blockchain service.. we are emitting few events according to our logic and these events are being used to get specific data from the transaction.. though ...
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38 views

How to find the source code of my contract which is not yet verified

I had created a token using eth based blockchain via remix.ethereum.org But I need to verify the source code ,but I am not having the backup copy of source code How can I see the source code of the ...
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74 views

Error: Returned error: gas required exceeds allowance (89459648255865) or always failing transaction

I am running a contract on a private network in geth, and using web3js. I have the following error when I run a transaction: web3.min.js:1 Uncaught (in promise) Error: Returned error: gas required ...
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ParserError pragma solidity ^0.4.21

thank you so much for your help I really appreciated this contract give me error in the line with (length) "ParserError: Expected primary expression. returns candidate.length" pragma solidity ^0....
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mapp values do not upadte when calling other functions after populating the map

I am using react as my front end to call functions from smart contract . this is the front end where i call the functions. However, after calling the first function which populates the map I then ...
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How to deploy blockchain app in heroku

I want to deploy my blockchain app in heroku and i just want to use ganache as my blockchain network. is that possible?
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Is it possible to use ganache blockchain when I deploy my web app in heroku? or should I deploy the contract first to a test network?

const web3 = new Web3( new Web3.providers.HttpProvider( "https://kovan.infura.io/v3/INFURA_KEY" ) ); const abi = JSON.parse( '[ { "constant": false, "inputs": [ { "internalType": "string", "...
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Value of emitted events in App.js

Actually in my App.js I have called my contract function like this: [ this.state.swapcontract.methods.sendFunds(address).send({ from: this.state.account , value:amount}) ] The function is called ...
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solidity declarationerror identifier not found or not unique for voting

I have this code for a voting system but when I compile it give me error in here ( boolean if_voted; ) can any one help me please. thank you this is the code : pragma solidity >=0.4.22 <0.7....
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Question about transaction confirmation

My question is that when a miner broadcast some transactions and other nodes don't think these transactions are valid, what will happen? Fork?
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How to read blockheaders of Ethereum Blockchain?

TO be specific, how to read block headers of Rinkeby on Ropsten blockchain? Since both the blockchains are public test networks, reading data of one blockchain on another should be possible using ...
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Is it possible to create this with Ethereum?

I am building a mobile application and I want to create a cryptocurrency around it. My problem is that I am new to crypto and to be honest I am not even sure if it’s possible what I want to do. To ...
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when iam runing npm run dev it showing me this error, how to solve this [closed]

[![shiva@shiva-VirtualBox:~/sta$ npm run dev StarNotary@1.0.0 dev /home/shiva/sta lite-server ** browser-sync config ** { injectChanges: false, files: [ './**/*.{html,htm,css,js}' ], ...
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ipfs with ethereum

Module not found: Can't resolve 'ipfs-api' in 'C:\Users.......\src' I have used this code //using the infura.io node, otherwise ipfs requires you to run a //daemon on your own computer/server. const ...

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