developer-notes.md

Developer Notes

Various coding styles have been used during the history of the codebase, and the result is not very consistent. However, we're now trying to converge to a single style, so please use it in new code. Old code will be converted gradually.

• Basic rules specified in src/.clang-format. Use a recent clang-format to format automatically using one of the [dev scripts] (/contrib/devtools/README.md#clang-formatpy).
  • Braces on new lines for namespaces, classes, functions, methods.
  • Braces on the same line for everything else.
  • 4 space indentation (no tabs) for every block except namespaces.
  • No indentation for public/protected/private or for namespaces.
  • No extra spaces inside parenthesis; don't do ( this )
  • No space after function names; one space after if, for and while.

Block style example:

namespace foo
{
class Class
{
    bool Function(char* psz, int n)
    {
        // Comment summarising what this section of code does
        for (int i = 0; i < n; i++) {
            // When something fails, return early
            if (!Something())
                return false;
            ...
        }

        // Success return is usually at the end
        return true;
    }
}
}

Doxygen comments

To facilitate the generation of documentation, use doxygen-compatible comment blocks for functions, methods and fields.

For example, to describe a function use:

/**
 * ... text ...
 * @param[in] arg1    A description
 * @param[in] arg2    Another argument description
 * @pre Precondition for function...
 */
bool function(int arg1, const char *arg2)

A complete list of @xxx commands can be found at http://www.stack.nl/~dimitri/doxygen/manual/commands.html. As Doxygen recognizes the comments by the delimiters (/** and */ in this case), you don't need to provide any commands for a comment to be valid; just a description text is fine.

To describe a class use the same construct above the class definition:

/**
 * Alerts are for notifying old versions if they become too obsolete and
 * need to upgrade. The message is displayed in the status bar.
 * @see GetWarnings()
 */
class CAlert
{

To describe a member or variable use:

int var; //!< Detailed description after the member

or

//! Description before the member
int var;

Also OK:

///
/// ... text ...
///
bool function2(int arg1, const char *arg2)

Not OK (used plenty in the current source, but not picked up):

//
// ... text ...
//

A full list of comment syntaxes picked up by doxygen can be found at http://www.stack.nl/~dimitri/doxygen/manual/docblocks.html, but if possible use one of the above styles.

Development tips and tricks

compiling for debugging

Run configure with the --enable-debug option, then make. Or run configure with CXXFLAGS="-g -ggdb -O0" or whatever debug flags you need.

debug.log

If the code is behaving strangely, take a look in the debug.log file in the data directory; error and debugging messages are written there.

The -debug=... command-line option controls debugging; running with just -debug or -debug=1 will turn on all categories (and give you a very large debug.log file).

The Qt code routes qDebug() output to debug.log under category "qt": run with -debug=qt to see it.

testnet and regtest modes

Run with the -testnet option to run with "play bitcoins" on the test network, if you are testing multi-machine code that needs to operate across the internet.

If you are testing something that can run on one machine, run with the -regtest option. In regression test mode, blocks can be created on-demand; see qa/rpc-tests/ for tests that run in -regtest mode.

DEBUG_LOCKORDER

Bitcoin Core is a multithreaded application, and deadlocks or other multithreading bugs can be very difficult to track down. Compiling with -DDEBUG_LOCKORDER (configure CXXFLAGS="-DDEBUG_LOCKORDER -g") inserts run-time checks to keep track of which locks are held, and adds warnings to the debug.log file if inconsistencies are detected.

Locking/mutex usage notes

The code is multi-threaded, and uses mutexes and the LOCK/TRY_LOCK macros to protect data structures.

Deadlocks due to inconsistent lock ordering (thread 1 locks cs_main and then cs_wallet, while thread 2 locks them in the opposite order: result, deadlock as each waits for the other to release its lock) are a problem. Compile with -DDEBUG_LOCKORDER to get lock order inconsistencies reported in the debug.log file.

Re-architecting the core code so there are better-defined interfaces between the various components is a goal, with any necessary locking done by the components (e.g. see the self-contained CKeyStore class and its cs_KeyStore lock for example).

Threads

• ThreadScriptCheck : Verifies block scripts.

• ThreadImport : Loads blocks from blk*.dat files or bootstrap.dat.

• StartNode : Starts other threads.

• ThreadDNSAddressSeed : Loads addresses of peers from the DNS.

• ThreadMapPort : Universal plug-and-play startup/shutdown

• ThreadSocketHandler : Sends/Receives data from peers on port 8333.

• ThreadOpenAddedConnections : Opens network connections to added nodes.

• ThreadOpenConnections : Initiates new connections to peers.

• ThreadMessageHandler : Higher-level message handling (sending and receiving).

• DumpAddresses : Dumps IP addresses of nodes to peers.dat.

• ThreadFlushWalletDB : Close the wallet.dat file if it hasn't been used in 500ms.

• ThreadRPCServer : Remote procedure call handler, listens on port 8332 for connections and services them.

• BitcoinMiner : Generates bitcoins (if wallet is enabled).

• Shutdown : Does an orderly shutdown of everything.

Ignoring IDE/editor files

In closed-source environments in which everyone uses the same IDE it is common to add temporary files it produces to the project-wide .gitignore file.

However, in open source software such as Bitcoin Core, where everyone uses their own editors/IDE/tools, it is less common. Only you know what files your editor produces and this may change from version to version. The canonical way to do this is thus to create your local gitignore. Add this to ~/.gitconfig:

[core]
        excludesfile = /home/.../.gitignore_global

(alternatively, type the command git config --global core.excludesfile ~/.gitignore_global on a terminal)

Then put your favourite tool's temporary filenames in that file, e.g.

# NetBeans
nbproject/

Another option is to create a per-repository excludes file .git/info/exclude. These are not committed but apply only to one repository.

If a set of tools is used by the build system or scripts the repository (for example, lcov) it is perfectly acceptable to add its files to .gitignore and commit them.

Development guidelines

A few non-style-related recommendations for developers, as well as points to pay attention to for reviewers of Bitcoin Core code.

General Bitcoin Core

• New features should be exposed on RPC first, then can be made available in the GUI

  • Rationale: RPC allows for better automatic testing. The test suite for the GUI is very limited

• Make sure pull requests pass Travis CI before merging

  • Rationale: Makes sure that they pass thorough testing, and that the tester will keep passing on the master branch. Otherwise all new pull requests will start failing the tests, resulting in confusion and mayhem

  • Explanation: If the test suite is to be updated for a change, this has to be done first

Wallet

• Make sure that no crashes happen with run-time option -disablewallet.

  • Rationale: In RPC code that conditionally uses the wallet (such as validateaddress) it is easy to forget that global pointer pwalletMain can be NULL. See qa/rpc-tests/disablewallet.py for functional tests exercising the API with -disablewallet

• Include db_cxx.h (BerkeleyDB header) only when ENABLE_WALLET is set

  • Rationale: Otherwise compilation of the disable-wallet build will fail in environments without BerkeleyDB

General C++

• Assertions should not have side-effects

  • Rationale: Even though the source code is set to to refuse to compile with assertions disabled, having side-effects in assertions is unexpected and makes the code harder to understand

• If you use the .h, you must link the .cpp

  • Rationale: Include files define the interface for the code in implementation files. Including one but not linking the other is confusing. Please avoid that. Moving functions from the .h to the .cpp should not result in build errors

• Use the RAII (Resource Acquisition Is Initialization) paradigm where possible. For example by using unique_ptr for allocations in a function.

  • Rationale: This avoids memory and resource leaks, and ensures exception safety

C++ data structures

• Never use the std::map [] syntax when reading from a map, but instead use .find()

  • Rationale: [] does an insert (of the default element) if the item doesn't exist in the map yet. This has resulted in memory leaks in the past, as well as race conditions (expecting read-read behavior). Using [] is fine for writing to a map

• Do not compare an iterator from one data structure with an iterator of another data structure (even if of the same type)

  • Rationale: Behavior is undefined. In C++ parlor this means "may reformat the universe", in practice this has resulted in at least one hard-to-debug crash bug

• Watch out for out-of-bounds vector access. &vch[vch.size()] is illegal, including &vch[0] for an empty vector. Use vch.data() and vch.data() + vch.size() instead.

• Vector bounds checking is only enabled in debug mode. Do not rely on it

• Make sure that constructors initialize all fields. If this is skipped for a good reason (i.e., optimization on the critical path), add an explicit comment about this

  • Rationale: Ensure determinism by avoiding accidental use of uninitialized values. Also, static analyzers balk about this.

• Use explicitly signed or unsigned chars, or even better uint8_t and int8_t. Do not use bare char unless it is to pass to a third-party API. This type can be signed or unsigned depending on the architecture, which can lead to interoperability problems or dangerous conditions such as out-of-bounds array accesses

• Prefer explicit constructions over implicit ones that rely on 'magical' C++ behavior

  • Rationale: Easier to understand what is happening, thus easier to spot mistakes, even for those that are not language lawyers

Strings and formatting

• Be careful of LogPrint versus LogPrintf. LogPrint takes a category argument, LogPrintf does not.

  • Rationale: Confusion of these can result in runtime exceptions due to formatting mismatch, and it is easy to get wrong because of subtly similar naming

• Use std::string, avoid C string manipulation functions

  • Rationale: C++ string handling is marginally safer, less scope for buffer overflows and surprises with \0 characters. Also some C string manipulations tend to act differently depending on platform, or even the user locale

• Use ParseInt32, ParseInt64, ParseUInt32, ParseUInt64, ParseDouble from utilstrencodings.h for number parsing

  • Rationale: These functions do overflow checking, and avoid pesky locale issues

• For strprintf, LogPrint, LogPrintf formatting characters don't need size specifiers

  • Rationale: Bitcoin Core uses tinyformat, which is type safe. Leave them out to avoid confusion

Threads and synchronization

• Build and run tests with -DDEBUG_LOCKORDER to verify that no potential deadlocks are introduced. As of 0.12, this is defined by default when configuring with --enable-debug

• When using LOCK/TRY_LOCK be aware that the lock exists in the context of the current scope, so surround the statement and the code that needs the lock with braces

  OK:

{
    TRY_LOCK(cs_vNodes, lockNodes);
    ...
}

Wrong:

TRY_LOCK(cs_vNodes, lockNodes);
{
    ...
}

Source code organization

• Implementation code should go into the .cpp file and not the .h, unless necessary due to template usage or when performance due to inlining is critical

  • Rationale: Shorter and simpler header files are easier to read, and reduce compile time

• Don't import anything into the global namespace (using namespace ...). Use fully specified types such as std::string.

  • Rationale: Avoids symbol conflicts

GUI

• Do not display or manipulate dialogs in model code (classes *Model)

  • Rationale: Model classes pass through events and data from the core, they should not interact with the user. That's where View classes come in. The converse also holds: try to not directly access core data structures from Views.

Git and github tips

• For resolving merge/rebase conflicts, it can be useful to enable diff3 style using git config merge.conflictstyle diff3. Instead of

    <<<
    yours
    ===
    theirs
    >>>
  

  you will see

    <<<
    yours
    |||
    original
    ===
    theirs
    >>>
  

  This may make it much clearer what caused the conflict. In this style, you can often just look at what changed between original and theirs, and mechanically apply that to yours (or the other way around).

• When reviewing patches which change indentation in C++ files, use git diff -w and git show -w. This makes the diff algorithm ignore whitespace changes. This feature is also available on github.com, by adding ?w=1 at the end of any URL which shows a diff.

• When reviewing patches that change symbol names in many places, use git diff --word-diff. This will instead of showing the patch as deleted/added lines, show deleted/added words.

• When reviewing patches that move code around, try using git diff --patience commit~:old/file.cpp commit:new/file/name.cpp, and ignoring everything except the moved body of code which should show up as neither + or - lines. In case it was not a pure move, this may even work when combined with the -w or --word-diff options described above.

• When looking at other's pull requests, it may make sense to add the following section to your .git/config file:

    [remote "upstream-pull"]
            fetch = +refs/pull/*:refs/remotes/upstream-pull/*
            url = [email protected]:bitcoin/bitcoin.git
  

  This will add an upstream-pull remote to your git repository, which can be fetched using git fetch --all or git fetch upstream-pull. Afterwards, you can use upstream-pull/NUMBER/head in arguments to git show, git checkout and anywhere a commit id would be acceptable to see the changes from pull request NUMBER.