In this FAQ we tried to give answers to the questions that are most frequently asked by those who are new to the world of cryptocurrency.<br
General Questions
What is bitcoin?
How do I get bitcoins?
Can I buy bitcoins through Paypal?
Where can I find the Bitcoin Users Forum?
How are new bitcoins created?
How many bitcoins are there now?
How many parts are bitcoins divided into?
What should I call bitcoin fractions?
How will the reward per block decrease when it reaches very small values?
How long will it take to create all the coins?
If no more coins are produced, will new blocks be created?
But if no more coins are created, what happens if bitcoins are lost? Wouldn’t that be a problem?
What happens if someone generates a new block chain or another digital currency makes Bitcoin go away?
Economics
What is bitcoin backed by? What backs it up?
What would happen if someone bought all existing bitcoins?
Would bitcoin cause a deflationary spiral?
Does bitcoin give an unfair benefit to first-timers?
Is bitcoin a pyramid scheme?
Is bitcoin a “bubble”?
I lost my “wallet” (wallet.dat file). Will I be able to recover my money?
Won’t the loss of wallets and the limited amount of bitcoins lead to high deflation, destroying Bitcoin?
Payment acceptance and transfer
Why do I have to wait 10 minutes before I can spend the money I received?
Do you have to wait 10 minutes to sell or buy anything for bitcoin?
I sent some bitcoins and they haven’t arrived yet! Where are they?
Why does my bitcoin address keep changing?
What is the fee for transfer?
Is it possible to expand the protocol so that it is possible to include both the payment amount and the message?
If I send BTCs to a non-existent address, will they be sent? Is there a check for the existence of the address?
What address do my payments come from? Can I influence it?
Where is the wallet physically located (wallet.dat file)?
What will happen if somebody sends me money and I am not online at that moment?
Network
Do I need to set up a firewall for my bitcoin client to work?
How does the search mechanism for other clients work?
Why don’t I have blocks loaded?
Can I download blocks from a third-party server and not by myself?
How long does it take the Bitcoin client to synchronize after the first installation? What does it do during this time?
If every transfer goes through the entire network, can the bitcoin network become very large?
What are the types of transactions in the Bitcoin network?
Why do I have only 8 connections?
Mining
What is mining?
Why was the “Generate Coins” option removed from the client?
Is mining used for any useful computations?
Isn’t that a waste of energy?
Why don’t we use calculations that would be useful for other purposes as well?
How does proof of correctness help the security of the bitcoin network?
What happens if two nodes generate a block at the same time?
How much can I earn?
Technical problems
When I try to start Bitcoin I get an error loading blkindex.dat, how do I fight it?
How to update Bitcoin wallet version from old to new?
Development
Does the Bitcoin project have a mailing list?
Does the Bitcoin project have a testing network?
Since what version did Bitcoin have encryption of the wallet?
Where can I get source codes of official Bitcoin client?
How do I build Bitcoin from source code?
General Questions
What is bitcoin? .
Bitcoin (BTC) is the monetary unit of the Bitcoin system. The abbreviation BTC (for example: 100 BTC) is used to denote the price or quantity. Bitcoin is intangible. This is just a number tied to a bitcoin address. For more information, see the article “What is Bitcoin?”
How do I get bitcoins?
There are many ways to acquire bitcoins:
- Accept bitcoins as payment for goods or services.
- There are several exchangers where you can exchange them for traditional currencies.
- Find a merchant and exchange them for cash.
- Create a new block (currently yields 12.5 bitcoins).
- Participate in the co-creation of the block.
For more information, see the article “How do I get BTC?”
Can I buy bitcoins through Paypal?
While it is possible to find someone who wants to sell you bitcoins via Paypal, most major exchangers and exchanges do not allow deposits via Paypal. Because it is not uncommon for someone to pay for a bitcoin purchase through Paypal, receive their bitcoins, and then fraudulently complain to Paypal that they did not receive their goods. In such cases, Paypal is very often on the side of the scammer, and therefore exchangers do not allow this method of funding the account.
Where can I find a bitcoin user forum?
You can find Russian-language support at https://forum.bits.media
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The largest multilingual forum is https://bitcointalk.org/
How are new bitcoins created? .
New coins are generated by a node in the network every time it finds a solution to a mathematical problem (i.e.. creates a new block), which is difficult to perform and can demonstrate proof of his work. The reward for solving the block is automatically adjusted so that in the first 4 years of the bitcoin network will be created 10 500 000 BTC. The amount is halved every 4 years, i.e.. 5,250,000 over the next four years, 2,625,000 over the next four years, and so on. Thus, the total number of coins will be 21,000,000 BTC.
Blocks must be created, on average, every 10 minutes. As the number of people trying to create these blocks changes, the difficulty of creating new coins will change to maintain this value. Difficulty changes every 2016 blocks. The probability that someone will create a block is based on the ratio of the speed of the system being used, to the cumulative calculation speed of all other systems generating blocks in the network.
How many bitcoins are there now? .
The number of blocks multiplied by the number of coins in a block equals the sum of existing coins. The number of coins in a block is 50 BTC for the first 210,000 blocks, 25 BTC for the next 210,000 blocks, then 12.5 BTC and so on. Reward for a found block drops by 2 times every 210,000 blocks. If you do not trust the sites that display the current number of issued bitcoins, you can calculate by yourself, knowing the number of blocks and the emission reduction algorithm.
How many pieces does a bitcoin have?
Technically, bitcoin can be divided up to 8 decimal places using existing data structures, so 0.00000001 BTC is the smallest amount at the moment. Ideas about securing even smaller pieces of bitcoin may be relevant in the future, should the need arise.
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What should I call bitcoin shares? .
There is a lot of discussion about naming bitcoin shares at the moment. Major candidates:
- 1 BTC = 1 bitcoin
- 0.01 BTC = 1 cBTC = 1 bitcoin (also known as bitcent)
- 0.001 BTC = 1 mBTC = 1 milli-bitcoin (also called mbit (pronounced “embit”) or millibit)
- 0.000 001 BTC = 1 µBTC = 1 micro-bitcoin (also called yubit or microbit)
The abbreviations described above are accepted abbreviations in the SI system for thousandths, millionths, and billionths. There are many arguments against the private case of 0.01 BTC, because it is unlikely to represent anything meaningful while the bitcoin economy is growing (and it is not equal to 0.01 dollars or euros, of course). Also, you should not call 0.01 BTC a cent, a penny, or a kopeck. It’s the world’s currency. The only exception is Satoshi, the smallest bitcoin at the moment.
- 0.000 000 01 BTC = 1 Satoshi
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in honor of Satoshi Nakamoto, a pseudonym for the inventor of bitcoin.
How will the reward per block decrease when it reaches very low values?
The reward after 0.00000001 BTC will become 0. No more coins will be created. The calculation is done as a bitwise shift to the right of a 64-bit integer number, which means that it is divided by 2 and rounded down. Integer equals the value in BTC * 100 000 000. This is how the BTC are stored in your account in the bitcoin client. Keep in mind that with existing rules, it will take about 100 years before there are problems in bitcoin partitioning.
How long will it take to create all the coins? .
The last block to create coins will be block #6,929,999. It will be created around the year 2140. Then the total number of coins in circulation will be 20,999,999.9769 BTC. Even if the division accuracy is increased from the current 8 decimal places, the total amount of BTC in circulation will always be just under 21 million (assuming that everything else remains unchanged). For example, with 16 decimal places, the total number of coins would be 20,999,999,999999999496 BTC.
If no more coins are produced, will new blocks be created? .
Absolutely! Even before the end of coin creation, the use of transfer commissions will become the main income for block creation. When coin creation is over, there will be a need to maintain the ability to use bitcoin, and then the full reward for block creation will be transfer fees. Therefore, even after block number 6,929,999, more and more blocks will be created for transfers.
But if no more coins are created, what happens if bitcoins are lost? Wouldn’t that be a problem?
According to the law of supply and demand, the fewer bitcoins available, the higher the demand for the remaining ones, which will therefore have a higher value. So, if bitcoins are lost, the remaining bitcoins will increase in value to compensate for the loss, i.e.. the number of bitcoins needed to buy something decreases. This is a deflationary economic model. As the average transaction size decreases, transfers are likely to operate in milli- or micro-bitcoins. The bitcoin protocol uses a base block of a hundred million bitcoins (“satoshi”), but unused bits are also available in the protocol. Later on, they can be used to designate even smaller parts.
What happens if someone generates a new blockchain or another digital currency makes Bitcoin go away?
Bitcoin’s blockchain cannot be easily rewritten, a consequence of one of bitcoin’s central security mechanisms. The system accepts the chain of blocks, which is the longest and most complex, that is, the one for which more work was spent. And since each successive block is derived from the previous one, to create a new chain, it would require more computation than has been invested in generating bitcoin coins in the history of its existence.. Otherwise such a chain will not be accepted. And as bitcoin grows and develops, it becomes more and more difficult to overtake all the power involved in it over time. This does not negate the possibility of a double-spending attack, but its possibilities are also quite limited.
A much more realistic threat could be the displacement of bitcoin by another virtual currency. As bitcoin evolves, more flaws may surface that other new electronic currencies may no longer have. However, this does not mean that bitcoin will immediately become a thing of the past.
Economy
What is bitcoin secured by? What supports it?”.
Bitcoins have value because they are accepted as payment. When we say that currency is backed by gold, we mean that there is a place where you can exchange currency for gold. In a sense, we can say that bitcoin is secured by the price that the seller has set for the product, i.e.. The seller’s promise to exchange the goods for a certain amount of currency. The main misconception is that bitcoins derive their value from the cost of electricity needed to create them. Cost does not equal value – hiring 1,000 people to dig a large hole in the ground can be expensive, but is not essential. While scarcity is an important requirement for useful currency, currency alone cannot make anything of value. For example, your fingerprints are in short supply, but that doesn’t mean they have a cost.
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What would happen if someone bought all the existing bitcoins?
What if someone bought up all the gold in the world? Well, you could try to buy it, but the seller would raise the price of gold until the buyer ran out of money. Not all bitcoins are for sale. Just as in the case of gold, no one can buy bitcoins that are not for sale.
Will bitcoin cause a deflationary spiral?
See the article Deflationary spiral.
Does bitcoin unfairly benefit first-timers? .
First participants have a large amount of bitcoin at the moment because they took the risk and invested resources in an untested technology. By doing so, they have helped bitcoin become what it is now and are helping it become what it should become in the future (hopefully a widely distributed decentralized digital currency). Therefore, it is only fair that they reap the benefits of their successful investment.
In any case, any bitcoin created will change hands dozens of times as a medium of exchange, so the profit made from the initial distribution will be small compared to the overall level of commercial activity in the bitcoin network.
Bitcoin is a pyramid scheme?
In a pyramid scheme, founders convince investors that they will make a profit. Bitcoin offers no such guarantee. There is no central legal entity, only individuals building the economy. In a financial pyramid scheme, the early participants can only profit from those who came later than them. But bitcoin has possible win-win decouplings. The early entrants profit from a rising rate. Late entrants profit from the stability and widespread acceptance of the digital currency. The fact that the first entrants are more profitable prevents the conclusion of a financial pyramid. Apple stock was not a financial pyramid, although the first investors got rich.
Bitcoin is a bubble? .
Yes, just like the euro and the dollar. They only have value when exchanged for something, and no value when used. If everyone suddenly stopped accepting your dollars, euros, or bitcoins, the bubble would burst and their value would drop to zero. But this is unlikely to happen: even in Somalia, where the government disappeared 20 years ago, Somali shillings are still accepted as payment.
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I lost my “wallet” (wallet.dat file). Will I be able to get my money back?
No. Lost money is lost forever. They will not (and cannot) be restored or re-generated.
Will the loss of wallets and limited bitcoins lead to high deflation, destroying Bitcoin?
Unlike most currencies, bitcoin is likely to undergo gradual deflation over time, given its limited issuance of 21 million. coins. This number has been known since the beginning of the project, and their rate of generation also remains predictable.
In addition, the bitcoin system has a danger that does not threaten other currencies: if a bitcoin user loses his wallet, the money in it is gone forever, unless the wallet is found again. Missing from general circulation, including. That is, as users lose their wallets, the number of btc will not just not increase or stay the same, but slowly decrease.
As a result, according to the laws of supply and demand, the value of coins is expected to rise steadily. But no one really knows exactly what will happen to the currency, which is subject to constant deflation. However, bitcoin has a good solution to this situation: it can divide up to very small amounts. That way, even if people lose most of their bitcoins, the system should still function normally. No one knows exactly what will happen as coins are lost from use, but it doesn’t seem to be as dire a threat as many believe.
Receiving and transmitting payments
Why do I have to wait 10 minutes before I can spend the money I received?
10 minutes is the average time to find a block. Depending on luck, finding a block can take more or less than 10 minutes.
Blocks (displayed as “confirmation” in the interface of the client program) are a way of reaching an agreement on who owns how many bitcoins. Once the block is located, everyone agrees that you now own your received coins, so you can spend them. Before the block with your transfers is found, some nodes in the network believe otherwise, as someone may be trying to cheat the system by trying to cancel the transfer. The more confirmations of the transaction, the less chance of cancellation. Only 6 blocks or 1 hour is enough to make cancellation almost impossible. This is much better than credit cards, for which you can make refunds even after three months after the initial transaction!
Why exactly ten minutes? This is the compromise chosen by Satoshi between the distribution time of new blocks in large networks and the amount of work wasted due to chain splitting. If these words don’t make any sense to you, don’t worry. Reading the technical documentation should make things clearer.
Do you have to wait 10 minutes to sell or buy anything for bitcoin? .
No, it is reasonable to sell things without waiting for confirmation, but only if the transaction is not high value.
When people ask this question they tend to think of uses such as supermarkets or snack machines. Transfers without confirmations still appear in the interface of the client program, but you cannot spend the bitcoins received. However, you can reason about the risk associated with the assumption that you can spend it in the future. In general, selling things that are pretty cheap (like snacks, digital downloads, etc.) transfers without confirmations won’t be a problem if you’re working with a well-coordinated network node.
I sent some bitcoins and they haven’t arrived yet! Where are they?
Don’t panic, there are a number of reasons why your bitcoins may not show up and many ways to diagnose them. First of all, check the current block number in the bottom right corner of your client and compare it to this. If these numbers differ by more than 1 or 2, you need to wait until the block with your translation is loaded. If not, it is possible that your transfer has not yet been included in the. You can check for delayed transfers online by going here and searching for your address. If a translation is on that list, you have to wait for it to be included in the block before it will be displayed in the client. Keep in mind that if the transfer is based on coins that have recently been received, the transfer will take longer. For example, someone just sent you a coin and you decided to send it immediately, then your transfer will take longer, although you can speed up the transfer with a 0.01 BTC fee. Transfers with zero fees can take hours or even days.
Why does my bitcoin address keep changing? .
Every time a transfer arrives at the address listed under “Your address”, the customer changes it to the new address. Thus, the client recommends that you use a new address for each transfer in order to increase anonymity. All of your old addresses can still be used: you can see them under Settings -> Your Receive Addresses.
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What is the transfer fee?
Some transfers may require a transfer fee for faster confirmation. The transfer fee is processed and received by the creator of the block. The last version of the client evaluates the necessity of the commission and its amount.
The commission is added to the payment amount. For example if you send 1.234 BTC and a client asks for 0.0005 BTC commission then your balance will be deducted 1.2345 BTC and the recipient will receive 1.234 BTC.
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As the commission is connected to the amount of data that is transferred and not to the amount of bitcoins sent, the commission can seem both very low (0.0005 BTC for 1,000 BTC transfer) and very high (0.005 BTC for 0.02 BTC transfer, or about 25%). If you receive small amounts (e.g., small payments from joint mining), the commission on sending will be higher.
Is it possible to extend the protocol to include both the payment amount and the message?
At the moment, the ability to pass external data in transactions already exists (via complex scripting mechanisms). However:
- there will be a fee of 0.01 BTC for each kilobyte (as it is now arranged with normal transactions).
- ECDSA (the public key algorithm that Bitcoin uses) does not support encryption, so you will not be able to encrypt the message.
- This feature is unlikely to be used in Bitcoin itself. That’s not what the blockchain was intended for.
If I send BTCs to a non-existent address, will they be sent? Is there a check to see if the address exists?
They will be sent, there is no check. The recipient will be the lucky owner of the key to that address in the indefinite future.
Which address are my payments coming from? Can I influence it?”.
In the official client at the moment (Bitcoin 0.9) there is no ability to choose a sending address. The address is selected from the oldest transactions. Since this could potentially decrease anonymity, there is a solution in the form of a patch to the official client and even patched binaries, see the developer page
Where is the wallet physically located (wallet.dat file)?
The wallet.dat file can be in different locations, depending on your operating system. On Windows it is %APPDATA%\Bitcoin, in more detail:
- Windows XP – drive:/Documents and Settings/User_name/Application Data/Bitcoin
- Windows Vista, 7, 2008 – Disk:/Users/Username/AppData/Roaming/Bitcoin
On Linux you can find it at ~/.bitcoin/, and on Mac OSX at /Users/user_name/Application Support/Bitcoin/
What happens if someone sends me money and I’m not online at the time?
Any transfer to the “correct” address must be successful. You don’t need to be connected to the Bitcoin network to receive money. Once you connect (or create an address), the money sent to it is available to you.
Go to
Do I need to set up a firewall to run the bitcoin client? .
The client usually connects to other nodes on TCP port 8333. You need to allow outgoing TCP connections on port 8333 if you want your bitcoin client to connect to more nodes. The client will also try to connect to IRC (TCP port 6667) to connect to other hosts.
If you want to limit your firewall rules to multiple ISPs and/or don’t want to allow IRC connections, you can find stable nodes in the backup node list. If your ISP blocks general IRC ports, note that lfnet also listens on port 7777. Connecting to this alternate port currently requires either recompiling the client or changing the routing rules. For example, on Linux, you can bypass the blocking of port 6667 as follows:
echo 173.246.103.92 irc.lfnet.org >> /etc/hosts
iptables -t nat -A OUTPUT -p tcp –dest 173.246.103.92 –dport 6667 -j DNAT –to-destination :7777 -m
comment –comment “bitcoind irc connection”
How does the search for other clients work?
The client finds other clients first of all using dns seed and by connecting to the IRC server (#bitcoin channel on irc.lfnet.org). If you cannot connect to an IRC server (such as when connecting via TOR), the built-in host list will be used to look for other hosts.
Why aren’t my blocks loading? .
Add bitcoin.exe to the list of your antivirus. Running bitcoin.exe with -rescan key can also help, it will recheck all chains.
Can I download blocks from a third-party server instead of myself?
You can find an archive of blocks on third-party servers, but it’s not recommended for paranoid people or just those who have enough time.
How long does it take the Bitcoin client to sync after the first installation? What does it do in the meantime?
The official bitcoin.org client downloads the whole blockchain during synchronization and checks for itself that all the rules of the bitcoin system have been correctly followed. This check requires many accesses to the hard disk, so the total synchronization time depends on the speed of the disk and, to a lesser extent, on the speed of the processor. This can take anywhere from a couple of hours on a fast SSD drive to several days on slow hard drives. So leaving your computer to sync blocks, make sure that your computer’s power-saving settings don’t turn off its hard drive. Until the client downloads all the last blocks, your last payments will not be seen by the client and displayed in the wallet.
If you don’t want to wait for blocks to download at all, you can install another lightweight client such as Multibit or Electrum. Although these clients are theoretically worse in terms of security.
If every transfer goes through the entire network, could the bitcoin network become very large? .
The bitcoin protocol allows you to write lightweight client programs that can use the bitcoin network without downloading the entire transfer history. Since traffic is constantly increasing, such customers will be developed. Network nodes with full history may at some point become a separate service. With a few software changes, full-fledged bitcoin network nodes can easily keep up with VISA and MasterCard using fairly modest hardware (a couple of modern servers). It’s worth noting that the MasterCard network is designed the same way as the bitcoin network – as a peer-to-peer broadcast network.
What are the types of transactions in the Bitcoin network?
There are three types of transactions:
- Generating coins, the so-called coinbase transaction
- Reallocating coins between key owners, a transaction with “inputs” and “outputs.
- Text messages
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Read more in the article “Bitcoin Transactions”.
Why do I only have 8 connections?
See Do I need to set up a firewall to make my Bitcoin client work?
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Mining
What is mining?
Mining or mining is the process of expending computing power in order to find blocks and thus create new bitcoins. The essence of mining is to find such a number that the hash sum from it satisfies the requirements of the current complexity for “sealing the block”, and the finder of this number is paid a certain amount, at the moment it is 12.5 BTC. For more information on mining, see the Mining FAQ.
Why has the “Generate Coins” option been removed from the client?
In the early days of bitcoin, it was very easy to find new blocks with a regular processor. With the increase in the number of miners, the complexity of creation has increased so much that, the time to find a block by the processor has increased to several years. The only cost-effective way to mine is to use modern high-performance graphics cards, FPGAs or ASIC miners with special software and/or join a pool of co-mining. Because CPU mining has become essentially useless, the option has been removed from the GUI.
Is mining being used for any useful computations?
The computations performed by mining are internal to the bitcoin network and are not associated with any other distributed computing projects. They serve to secure the bitcoin network, which is useful in itself.
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Isn’t that a waste of energy?
The expenditure of energy to create a free monetary system is hardly a waste of. In addition, the services required to run widespread money systems, such as banks and credit companies, also consume energy, perhaps even more than bitcoin.
Why don’t we use calculations that would be useful for other purposes as well? </span
In order to secure the bitcoin network, calculations must have some very specific features that are incompatible with calculations for other purposes.
How does proving the security of the bitcoin network work correctly help?
To give a general idea of the mining process, let’s imagine the following:
Payload =
Random number = 1
Hash = SHA2( SHA2(Payload + Random number) )
The work done by the miner consists of trying random numbers until the hash function equals the value, which has the rare property of being less than a certain target value. (In other words: the hash “starts with a certain number of zeros.”) As you can see, the mining process itself does not compute anything special. It simply tries to find a random number, which – in combination with the payload – yields a hash with specific properties.
The advantage of using such a mechanism is that it is very easy to check the result: given the payload and the specific random number, only one call to the hash function is needed to make sure that the hash has the desired properties. Since there is no other known way to find a hash other than a simple enumeration, this can be used as a “proof of work” in which someone has invested a huge amount of computing power to find the right random number for a given payload.
This function is used in the bitcoin network to provide various aspects of. An attacker who wants to introduce a malicious payload into a network will need to make the necessary proof of work before it is accepted. And as long as honest miners have the bulk of the processing power in their hands, they can always get ahead of the attacker.
What happens if two nodes generate a block at the same time? .
It happens occasionally, the block will be accepted the block after which the next one was found first.
I mean: two nodes send a “win” at the same time.
That is: two nodes send their “winning” blocks at the same time. Some nodes learn first about the block from the first node (“block A”), others about the block from the second node (“block B”). Each part of the network will continue to hash from this block, trying to generate the next block. If the machine that generates the next block (“block C”) uses block B, then it is block B that will be considered the “winner,” and the nodes using block A will move to block C. (of course, this can be frustrating for someone who has generated block A, because. he hoped to get coins for it, but his block became wrong as a result of “winning” Block B) .
How much can I earn? .
Use your calculator to calculate. If you don’t know the speed of the device, look in this table.
Technical issues
When I try to start Bitcoin, I get an error loading blkindex.dat, how do I fight it?
This can happen when Bitcoin does not shut down correctly. Don’t forget to back up the wallet.dat file, since it too can get corrupted if you terminate the app in an emergency.
Try the following steps:
- Close the Bitcoin program (check in the manager), make a backup of the wallet.dat file (usually in %APPDATA%\Bitcoin)
- delete all files in %APPDATA%\Bitcoin folder except for wallet.dat, remove the Bitcoin program
- install the latest version of Bitcoin available and do not downgrade again
- start Bitcoin and wait for all blocks to download
- check balance and sent/received transfers
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How do I upgrade my Bitcoin wallet from the old to the new version?
- Close Bitcoin program (check in manager), back up wallet.dat file (usually in %APPDATA%\Bitcoin)
- Erase everything in %APPDATA%\Bitcoin except blocks(blk0001.dat and blkindex.dat) and wallet(wallet.dat).
- Uninstall Bitcoin like any other program using Control Panel if it was installed via installer, or delete software folder if it was downloaded and unpacked from archive
- install new version of Bitcoin by downloading installer from official bitcoin.org website
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Development
Does the Bitcoin project have a mailing list?
Yes, link.
Does the Bitcoin project have a network to test? .
Take a look at this post: link
When did Bitcoin get wallet encryption?
Wallet encryption appeared in Bitcoin version 0.4.0, previous version – 0.3.24 had no encryption and keys in wallet.dat file were kept in clear format. Also in version 0.4.0 Berkeley DB version (bdb version 4.8) was updated, so in case of a downgrade to earlier versions, using the working folder of the new wallet will lead to program errors. To be able to downgrade, back up your entire working folder with the wallet before upgrading to Bitcoin 0.4.0 and higher. Backward compatibility is supported, i.e.. A version 0.4.0 installed on top of 0.3.24 will see all the downloaded blocks and funds in your account.
Where can I get the source codes of the official Bitcoin client?
Here is a link to the source code of the official Bitcoin client.
How do I build Bitcoin from source? .
Topic on building version 0.3: link, Notes on building Bitcoin on Linux (pdf)
FAQ is being constantly updated, if you have questions that can be useful to others – don’t be lazy, post them in the forum and they will be published.