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225
votes
7answers
153k views

What is meant by the term “gas”?

An Ethereum application that I want to use talks about needing "gas" to run. What is gas, and where do I get it?
172
votes
20answers
52k views

How can I securely generate a random number in my smart contract?

If I am writing a smart contract for some kind of (gambling) game, how can I generate a random number securely? What are the best practises and security trade-offs to be aware of?
163
votes
7answers
122k views

What are the Ethereum disk space needs?

How fast is Ethereum blockchain growing and how big is it likely to get in the future? Is it still about 1 GB per month? Are there any data pruning or compression algorithms in place or planned to be ...
162
votes
13answers
37k views

Upgradeable smart contracts

If the contract issuer wants to have a way to upgrade the contract code, so that account data and other things carry over, can Ethereum provide this? Also can this be done without changing the ...
151
votes
2answers
33k views

What is the difference between a transaction and a call?

What is the difference between a transaction and a call? In some interfaces, I can interact with contracts via calls or via transactions. What is the difference between the two, and can contracts also ...
144
votes
3answers
29k views

`external` vs `public` best practices

Apart from the public modifier Ethereum introduces the external one. Both may be called outside of the contract and inside (the later one by this.f() pattern). Moreover, according to the docs: ...
132
votes
1answer
50k views

What is Swarm and what is it used for?

What is Swarm and what is it used for? And what is Swarm's relationship with Whisper?
131
votes
2answers
27k views

Are there well-solved and simple storage patterns for Solidity?

Simple and appropriate data organization can challenge Solidity newcomers. It wants us to organize everything in ways many of us aren’t accustomed to. Are there well-solved general patterns for ...
130
votes
5answers
43k views

Why does Ethereum plan to move to Proof of Stake?

The initial version of Ethereum (Frontier) uses a Proof of Work based consensus algorithm, but I see that there are plans to implement a Proof of Stake based consensus algorithm called "Casper" in a ...
121
votes
5answers
87k views

How are ethereum addresses generated?

What criteria does a valid Ethereum address need to fulfill? Is it just a random number in hexadecimal? Or does it need to be derived in a specific way, according to some cryptographic algorithms? ...
119
votes
6answers
140k views

What is a DApp?

There are new terms in Ethereum like smart contract. What is a DApp and how is it different from smart contracts?
112
votes
8answers
52k views

What is an ABI and why is it needed to interact with contracts?

ABI is referenced in many places including the official Ethereum website. What is an ABI and why is it necessary to use?
104
votes
11answers
81k views

How would I explain Ethereum to a non-technical friend?

I've recently attempted to explain Ethereum to several non-techy friends. My explanation usually involves starting with a comparison to Bitcoin, but this quickly becomes complicated as I then have ...
102
votes
2answers
29k views

Comparison of the different TestNets

The Metamask client allows access to four test networks: Ropsten Kovan Rinkeby Goerli I'm curious: What are the similarities / differences between them? Which use cases are best for each network ...
101
votes
4answers
96k views

What is Geth's “fast” sync, and why is it faster?

One of the answers to this question suggested using Geth's --fast flag to help quickly synchronise the block data. How does the flag work, and how does using it speed up the synchronisation? Are we ...
99
votes
5answers
47k views

What is the difference between Swarm and IPFS?

IPFS seems a little slow on existing dapps. will Swarm load content more quickly? Also- is there a roadmap for swarm?
98
votes
7answers
26k views

How can a contract run itself at a later time?

In my blockchain application, I would like to have an event triggered at a later time, hours or perhaps even days after the application has most recently received a transaction from a user. How can I ...
96
votes
7answers
31k views

How can an Ethereum contract get data from a website?

What is the process/workflow behind the process where an Ethereum contract gets some data from a website?
95
votes
2answers
61k views

Which cryptographic hash function does Ethereum use?

Many sources refer to the use of the "SHA3" cryptographic hash function within Ethereum, but the Ethereum project began before the SHA3 standard was finalised, and there was some confusion/controversy ...
90
votes
6answers
32k views

How is the address of an Ethereum contract computed?

How is the address of an Ethereum contract computed? What use cases are there for knowing a contract's address in advance?
87
votes
6answers
63k views

Ethereum block architecture

Where can I find a description (preferably a diagram like below) showing the block structure in an Ethereum blockchain? Credits: Wikipedia
85
votes
12answers
35k views

Common useful JavaScript snippets for geth

In geth you can loadscript any .js file. I discovered this one on the wiki which I like a lot: function checkAllBalances() { var i =0; eth.accounts.forEach( function(e){ console.log(" eth....
85
votes
6answers
168k views

What is the total supply of Ether?

In Bitcoin, the total supply is capped at 21 million BTC. Is the total supply of Ether capped? How much will be mined before the Proof of Stake (POS) transition, and how will POS affect the issuance ...
79
votes
4answers
27k views

Difference between require and assert and the difference between revert and throw

I was looking at the docs and Im looking for clarification on the difference between require and assert and throw and revert. assert(bool condition): abort execution and revert state changes if ...
78
votes
5answers
134k views

What actually is a DAG?

I have tried googling it as well as finding it here. Many people here talk about it, but what actually is a DAG?
77
votes
3answers
26k views

What is an uncle/ommer block?

The Ethereum blockchain is described as containing "ommer" blocks, usually called "uncle" blocks by the general public. What is an ommer/uncle block, and why are they needed?
75
votes
9answers
15k views

Can contracts pay the gas instead of the message sender?

Is it possible for a contract to pay the gas costs (or part of it) that result from the contract being called? Or does the sender of a message always pay the resulting gas costs no matter what?
74
votes
6answers
274k views

How do I buy Ethereum with USD?

I am a complete noob at investing. I have never bought stock or invested in anything before but I have $300 I would like to turn into Ethereum. Could someone please tell me step by step how to ...
73
votes
9answers
93k views

Help with very slow mist sync

I have seen other posts in reddit where users experienced slow sync times. I am using windows 10 and version 3.7 (I saw on gitter that other users on windows 10 had trouble with version 3.8 crashing). ...
69
votes
4answers
26k views

Difference between CALL, CALLCODE and DELEGATECALL

CALL and CALLCODE take the same number of operands (in the execution stack). For the exception flag being pushed on top of the stack: 0 means exception, 1 means successful execution. CALL is easy to ...
68
votes
8answers
55k views

How can I check if an Ethereum address is valid?

I've read many times that you should never input an address by hand unless you want to accidentally send Ether into no-mans-land. I'd like to know what those checksums might be. Is there a way to tell ...
68
votes
5answers
51k views

What's the difference between proof of stake and proof of work?

The Ethereum frontier network currently uses a proof of work (PoW) consensus algorithm, while a future version of the network plans to utilise a proof of stake (PoS) algorithm instead. What's the ...
67
votes
1answer
11k views

Why is the average block time 17 seconds?

The network stats show a constant average blocktime of around 17 seconds. Whenever you check it is around 16~18 seconds. Reading the blog post by vitalik buterin it states the block time should be 12 ...
66
votes
2answers
24k views

Solidity: What is the difference between `view` and `constant`?

Solidity 0.4.16 introduced the view and constant function modifiers. The documentation says: constant for functions: Same as view. Does this mean view is just an alias for constant? If so, why do ...
65
votes
2answers
21k views

What's the difference between 'msg.sender' and 'tx.origin'?

If either are used in the body of a Solidity function that may change the state of a contract (write call), what's the primary difference and/or benefit of using one over the other. For example: ...
65
votes
2answers
17k views

ELI5 How does a Merkle-Patricia-trie tree work?

I understand that Merkle tree are Hashes of Hashes, they have the advantage that you can verify only a subtree. But what about Patricia? What does a trie mean? And how is it used in Ethereum?
65
votes
2answers
50k views

What is the relationship between Mist and Ethereum Wallet?

In https://github.com/ethereum/mist/releases 0.3.6 the files are named Mist. Starting in 0.3.7 the files are named Ethereum-Wallet. Will files named Mist take over in the future, or will there be 2 ...
63
votes
2answers
32k views

What does each genesis.json parameter mean?

I'm trying to setup genesis.json for my private network. What does each parameter mean and what value can I set in genesis.json? nonce: any data? timestamp: any time? parentHash: any hash? extraData: ...
62
votes
7answers
162k views

What is the recommended way to safely store Ether?

There are several options for storing Ether : Accounts Mist/Geth accounts. Paper wallets. Icebox Air gapped offline signing of transactions Wallet Mist Multi-signature. In short there are a lot ...
61
votes
2answers
21k views

What does the indexed keyword do?

What does the "indexed" keyword do in the below line of code? I'm guessing it just tells the event object that the following input should be logged? Can we use it other places ie outside of events? ...
60
votes
2answers
22k views

What OPCODES are available for the Ethereum EVM?

The Ethereum virtual machine has a large number of operation codes and base level instruction sets. Is a complete listing available?
58
votes
6answers
84k views

What number of confirmations is considered secure in Ethereum?

In Bitcoin, 6 confirmations are considered secure (very low probability of transaction/block reversal). What is the current number in Ethereum?
57
votes
3answers
14k views

Why can't contracts make API calls?

The model that contracts can only accept and process data, instead of also being able to retrieve data from the Internet, seems limiting (even if not, it's less direct). If a concern is that data or ...
57
votes
6answers
199k views

How to import a plain private key into geth or Mist?

This is such a basic question, but I can not figure out how to import a plain, unencrypted private key into geth or mist? I tried geth account import 7[......]bla but it seems this command is always ...
57
votes
5answers
16k views

How can you decompile a smart contract?

On the blockchain I can inspect the code of a contract, and see the EVM opcodes. Is there a way to decompile this and convert it back to (Solidity) source code?
55
votes
7answers
8k views

How can I verify that a contract on the blockchain matches the source code?

Given the (Solidity) source code of a smart contract, is there a way to deterministically compile the code and compare it with the code on the blockchain? I want to verify that the contract does what ...
54
votes
4answers
137k views

For pool mining, what exactly is a share?

Can someone please explain exactly what a share is in the context of pool mining? I have a superficial understanding of testing random nonces to find a hash under the current difficulty. I also ...
53
votes
13answers
24k views

How do I set up a private ethereum network?

I'd like to set up a local (private) blockchain that isn't connected to the wider internet so that I can deploy some test contracts. How do I go about doing that?
53
votes
4answers
31k views

What proof of work function does Ethereum use?

The Ethereum frontier network currently uses a proof of work (PoW) based consensus algorithm. But which specific PoW function does that algorithm use, and why was it chosen?
52
votes
4answers
40k views

<address>.send vs <address>.transfer best practice usage?

I noticed that address's offer two member functions that perform the transfer of ether to the specified address. send returns false upon failure and transfer raises an exception. In the Solidity ...

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