Can I send ALL my Ether to a Payable function? or should a little be saved to pay for Gas?

Question

I have this big doubt about how the EVM works,

Let's say I have an Ether balance of 1000000000000000000 in Wei (1 Ether) and I want to pass ALL of it to a payable function, let's call the function buy():

Contract.buy({ from: '0x0...', value: 1000000000000000000 });

Will that work? meaning; Will that in itself deduct GAS costs?

Or, on the contrary... I have to estimate Gas cost and deduct it myself from the Ether I'm sending? for example, If I estimate 50000 in Gas, I'd do it like this:

Contract.buy({ from: '0x0...', value: 999999999999950080, gas: 50000 });

Which approach is the correct one and Why?

Thanks in advance!

EDIT (Code in Swift):

    var options = Veev.options(gas: nil, from: addressFrom)
    let contract = web3.contract(contractABI, at: EthereumAddress(Rinkeby.address))
    let parameters = [EthereumAddress("0x0.."), amountToBuy] as [AnyObject]
    guard let estimatedGas = web3.contract(contractABI, at: EthereumAddress(Rinkeby.address))?.method("buy", parameters: parameters, options: options)?.estimateGas(options: nil) else { return }
    options.gas = estimatedGas
    let value: BigUInt = BigUInt(ether) - BigUInt(estimatedGas) * BigUInt(5000000000) - 1 // 5000000000 is gas Price
    options.value = value

Answer 1

The latter. The total ether required to make a transaction is amount + gasLimit * gasPrice (amount being how much ether you're sending). So you need to set aside enough ether to cover the maximum the transaction may cost for gas.

In your example of needing 50000 in gas, you'd want to send, say, 1 ether - 50000 * 1 gwei and specify a 1 gwei gas price.

Answer 2

Can I send ALL my Ether to a Payable function? or should a little be saved to pay for Gas?

Yes you can, by setting gasPrice is 0. But in real time we don't know how much time it will take to execute, It depends on situation. I seen in etherscan some of the transaction's value is 0.

Refer following link for the same: https://etherscan.io/tx/0x173fd250d63a62bb407f214ff924b49e1c90593572eab1a8c523783f2aa58ba3

In your example of you can estimate gas for buy() function using Contract. buy.estimateGas({ from: '0x0...', value: 1000000000000000000}) will tell how much gas will consume for a method. So no need to assume, but some dev's are complaining its not returning accurate value. Better to add 5-10% extra value.

Thats for safer side in above method lets assume its your estimated 55000 gas for exec buy(), then you need to send value to method is 1 ether - 55000 * 1 gwei