thehub/support/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.

• 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 method(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;
    }
}
}

// Globally scoped functions start with a Capital.
void Function(const std::string &value)
{
}

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

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.

hub.log

If the code is behaving strangely, take a look in the hub.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 hub.log file).

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

testnet, testnet4, scalenet and regtest modes

Run with the -testnet option to run with "play bitcoins" on the v3 test network, if you are testing multi-machine code that needs to operate across the internet. The testnet v3 network has gotten quite large and it is recommended that you use testnet v4 (for compatibility and feature testing) or scalenet (for scaling testing).

Run with the -testnet4 option to run with "play bitcoins" on the v4 test network, if you are testing multi-machine code that needs to operate across the internet for compatibility or non-scaling related features. This chain has a hard limit of 2 MB blocks and is intended to be kept lightweight and quick to sync.

Run with the -scalenet option to run with "play bitcoins" on the scaling test network if you are testing multi-machine code that needs to operate across the internet for the purposes of testing high-throughput activity. This chain has an initial hard cap on blocksize of 256 MB and will be reset back to block 10,000 every 6 months or so.

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

Flowee the Hub 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 hub.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.

The LOCK/TRY_LOCK are a legacy idea from a past era and essentially just wrap boost::recursive_mutex and a scoped_lock. Please use actual mutexes in new code, or even better, use atomics and write lock-free code.

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 hub.log file.

NOTE: order inconsistencies-checks are broken and assume all locks are global variables. Its best to ignore the warnings.

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 code.

General C++

• Assertions should not have side-effects

  • Rationale: stuff breaks because asserts are not compiled in release mode.

• 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_pointer 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 vector out-of-bounds exceptions. &vch[0] is illegal for an empty vector, &vch[vch.size()] is always illegal. Use begin_ptr(vch) and end_ptr(vch) to get the begin and end pointer instead (defined in serialize.h)

• 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.

• 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

• 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, 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