Double spend of bitcoin? How is that possible, please explain with simple words?

[Alina-malina]

I read in this resource https://bitcointalk.org/index.php?topic=327767. that it is possible to do double spend, it is all very interesting, but as i am new in bitcoin world, i would like to understand what is that, is it really something to be affraid of?

[Alina-malina]

I suggest the following solution preventing from "double spending". If the bitcoin client somehow can determine a double spending activity it should take that coin and return back to miners as a reward! I think it will be the best option for that. It is a big problem and may negative influence on bitcoin reputation. I hope developers will hear my words.

[CIYAM]

You cannot "double-spend" bitcoins - that's what the entire purpose of "confirmations" is about.

It is possible to end up with what you might think of as "double-spends" in the short term (but will never stay that way) although typically that would be before a tx is even confirmed at all (what are known as "0-confirmation" txs).

This is why you want to wait for X number of confirmations before considering a tx as "safe" (the value of X depending upon the value of the tx to you).

Understand that the "longest" link of blocks back to the genesis block (known as the "blockchain") contains exactly ZERO "double-spends" (otherwise Bitcoin would be completely broken).

Sometimes what are called "re-orgs" happen which can replace the latest blocks with a different (you could think of as "superior") set of blocks which is why for greater tx amounts you would be advised to wait for more confirmations (most standard clients consider 6 confirmations enough although maybe if it were a house you were selling you might want to insist upon say 100 confirmations to be more certain no re-org could result in the tx disappearing).

[Alina-malina]

OK thanks for explaination:)

[JTrain_51]

You will never see double spends with bitcoin
thats the hole point of confirmations is so we don't double spend

[Alina-malina]

So 6 confirmations usually is enough right? For example if I have 6 separate computers running bitcoin client and connected at the same ISP and when i transfer bitcoins, lets say, to Bob, is it possible to force those 5 computers to make confirmation for my transaction and that way make coin transfer faster?

<<bitcoin price on mtgox is 496 as I write this message>>.

[CIYAM]

So 6 confirmations usually is enough right? For example if I have 6 separate computers running bitcoin client and connected at the same ISP and when i transfer bitcoins, lets say, to Bob, is it possible to force those 5 computers to make confirmation for my transaction and that way make coin transfer faster?

Your computer(s) is/are irrelevant as only "miners" are involved in constructing new blocks which are the "confirmations".

So just be patient and wait - and for smaller amounts (especially in person) I myself am normally satisfied with just one or two confirmations.

Also try not to get too excited about the price - it tends to go up and down quite a bit when you decide to get on board the "Bitcoin ride".

[DeathAndTaxes]

So 6 confirmations usually is enough right? For example if I have 6 separate computers running bitcoin client and connected at the same ISP and when i transfer bitcoins, lets say, to Bob, is it possible to force those 5 computers to make confirmation for my transaction and that way make coin transfer faster?

That isn't how confirmations work.

A MINER includes tx in a block.  When your tx is included in the most recent block it has 1 confirmation.  The next block will build on top of the current block and then your block is 2 block deep = 2 confirmations, when your block is 6 blocks deep it is 6 confirmations.

The number of peers who validate a tx has nothing to do with confirmations.  If your tx is invalid then it will simply be dumped by all the nodes on the network.   0-confirm means the tx is valid but hasn't been included in a block yet.

[CIYAM]

0-confirm means the tx is valid but hasn't been included in a block yet.

And important to note that a "0 confirmation" tx could end up disappearing altogether despite being "valid" if it has not managed to gain sufficient priority after a certain amount of time (which generally occurs due to no fees being paid for the tx) - so I would recommend you always at the very least wait for 1 confirmation.

[whitemage]

Think of it as a boat and planes..

if you don't pay any transaction fee you would get the boat which take hours to reach.
If you pay some trasaction fee you would get plane instead and reach in 10 minute

[Alina-malina]

but how to pay for transaction fee?

[CIYAM]

but how to pay for transaction fee?

There should be a setting in your Bitcoin client for a "minimum transaction fee" - I would advise setting that to at least 0.0001 and perhaps to 0.0005 to get a higher than average priority (if an even higher fee is required the client should warn you and get you to accept adding that fee before the transaction is sent).

[Alina-malina]

I apreciate that, thank you very much for explaining about fee. I set it up!!