Once created, smart contracts on the Ethereum blockchain cannot be modified or updated. The article discusses how smart contracts can be "self-destructed" to remove them from the blockchain. It explains that a selfdestruct function sends the contract's funds to a designated address and clears its data. While this eliminates bugs or stops execution, funds sent to a self-destructed contract will be lost. The article concludes by advising self-destructing contracts when no longer needed to stop execution and potentially lower gas costs.

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ARTICLES
SELF DESTRUCTING SMART CONTRACTS
IN ETHEREUM
DONN FELKER
POSTED: JUNE 20, 2018 • 1 COMMENT

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Once a contract is created on the Ethereum blockchain it cannot be modi ed.
Said another way – it’s written in stone.
UPDATING YOUR CODE IS PROBLEMATIC …
ACTUALLY IMPOSSIBLE
Updating a smart contact can be a problem if you need to change the code that runs the smart
contract because the contact code is … well … written in stone – in the Ethereum blockchain. If
you need to make a change to your code, you need to deploy a new version of your smart
contract.
Therein lies the rub … your old code is still out there running.
For example, here’s some actual code that is on the Ethereum Testnet blockchain (the test
version of the Ethereum blockchain):

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View on Etherscan
Soo.. why can’t you update the code?
You can’t update your code because each subsequent block on the blockchain contains the
hash of the previous block(s) and changing this block to contain new code would mean we’d
have to recalculate all the subsequent blocks in the blockchain and that’s not something that’s
possible as this is exactly one of the things that the Ethereum blockchain prevents (changing
history).
So … we can’t update our code. Good news though – we can remove it, and its data.
HOW TO “SELFDESTRUCT” A CONTRACT

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When the time comes to stop having your contract executing its code or if you simply want to
shut it down, and get rid of it you can have a smart contract self-destruct with the selfdestruct
operation .
Here’s an example of what selfdestruct would look like in a Solidity smart contract function:
function close() public onlyOwner { //onlyOwner is custom modifier
selfdestruct(owner); // `owner` is the owners address
}
The selfdestruct operation above (which is an OPCODE at the EVM level) sends all of the
current smart contract balance to a destination address – in this case to the owners address,
which is stored in the owner variable.
At the same time, the contract’s data is cleared, freeing up space in the Ethereum blockchain
and potentially lowering your gas price.
Important note: If you’re going self-destruct a contract AND you still need the data for whatever
reason, you will need to query the data from the contract and store it somewhere (o ine, into
another contract, etc).
BENEFITS OF USING SELFDESTRUCT
Bene t Reasoning
Stop Execution
If you no longer want the contract running, you can selfdestruct it to “turn it
o ”.
Eliminate Bugs (Similar to above). You can kill o smart contracts that are buggy.
Lower Gas
Price
It costs less gas to selfdestruct a contract.
Why is the gas price lower?
1

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Self-destructing a smart contract is cheaper than simply performing a similar operation such as
address.send(this.balance) . It has been reported that this OPCODE uses negative gas because
the operation frees up space on the blockchain by clearing the contracts data. This negative gas
can o set the total gas cost. Example, if you’re cleaning up a bunch of data rst and then
performing the selfdestruct operation, your gas cost can be negative. (source)
SELFDESTRUCT GOTCHAS
Gotcha Reasoning
No Undo Once the contract is self destructed, there’s no coming back. Game over.
Losing
ETH
If you send a transaction or funds to a self destructed contract your funds will be
lost.
Yes, that’s right! To re-iterate:
If you send a transaction or funds to a self-destructed contract you will lose your
funds.
If you’ve developed a DApp that has the smart contract address stored in the DApp, you will
need to make sure that you change or remove that address any time you self-destruct a
contract so that you do not accidentally send a transaction or funds to it.
CONCLUSION
Whilst the Ethereum blockchain is immutable by default, there are ways to deploy new code or
kill o old smart contracts. If you want to stop the execution of the smart contract, simply call
the selfdestruct operation in your contract (make sure its secured behind a secure modi er).
Have a question? Please leave a comment below.

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DONN FELKER
Donn Felker is the founder of Caster.IO. He's is a best selling author in mobile
development and he also co-hosts the Fragmented Podcast - a Podcast for
Android Developers. Most recently he's been diving deep into blockchain with
a focus on Ethereum and Solidity. Follow him on Twitter, Instagram or visit his
site.
Looking to get started in Ethereum development? Watch our course on building an Ethereum
DApp.
A big thank you goes out to Will Button and Ravindra Kumar for reviewing this article.
1. Interesting Fact – This OPCODE used to be called suicide , but the developers of Solidity
felt that while this term was technically valid, it was important to recognize this as a
mental health condition and decided to change it to selfdestruct . Bravo Solidity Team!
You can see the actual Ethereum Improvement Proposal for the change, with the
reasoning here – EIPs/eip-6.md at master · ethereum/EIPs · GitHub
FILED UNDER: BLOCKCHAIN TAGGED WITH: SOLIDITY

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