developer-notes.md raw

Developer Notes

Coding Style (General)

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, which is specified below. When writing patches, favor the new style over attempting to mimic the surrounding style, except for move-only commits.

Do not submit patches solely to modify the style of existing code.

Coding Style (C++)

src/.clang-format. You can use the provided clang-format-diff script tool to clean up patches automatically before submission. - Braces on new lines for classes, functions, methods. - Braces on the same line for everything else (including structs). - 4 space indentation (no tabs) for every block except namespaces. - No indentation for public/protected/private or for namespace. - No extra spaces inside parentheses; don't do ( this ). - No space after function names; one space after if, for and while. - If an if only has a single-statement then-clause, it can appear on the same line as the if, without braces. In every other case, braces are required, and the then and else clauses must appear correctly indented on a new line. - There's no hard limit on line width, but prefer to keep lines to <100 characters if doing so does not decrease readability. Break up long function declarations over multiple lines using the Clang Format AlignAfterOpenBracket style option.

required when doing so would need changes to significant pieces of existing code. - Variable (including function arguments) and namespace names are all lowercase and may use _ to separate words (snake_case). - Class member variables have a m_ prefix. - Global variables have a g_ prefix. - Constant names are all uppercase, and use _ to separate words. - Enumerator constants may be snake_case, or PascalCase. They should not be ALL_CAPS, according to the [C++ Core Guidelines](https://isocpp.github.io/CppCoreGuidelines/CppCoreGuidelines#renum-caps), to avoid clashing with macros. - Class names, function names, and method names are UpperCamelCase (PascalCase). Do not prefix class names with C. See [Internal interface naming style](#internal-interface-naming-style) for an exception to this convention.

- Test suite naming convention: The Boost test suite in file src/test/foo_tests.cpp should be named foo_tests. Test suite names must be unique.

- ++i is preferred over i++. - static_assert is preferred over assert where possible. Generally; compile-time checking is preferred over run-time checking. - Use a named cast or functional cast, not a C-Style cast. When casting between integer types, use functional casts such as int(x) or int{x} instead of (int) x. When casting between more complex types, use static_cast. Use reinterpret_cast and const_cast as appropriate. - Prefer `list initialization ({})` where possible. For example int x{0}; instead of int x = 0; or int x(0);

For function calls a namespace should be specified explicitly, unless such functions have been declared within it. Otherwise, argument-dependent lookup, also known as ADL, could be triggered that makes code harder to maintain and reason about:

#include <filesystem>

namespace fs {
class path : public std::filesystem::path
{
};
// The intention is to disallow this function.
bool exists(const fs::path& p) = delete;
} // namespace fs

int main()
{
    //fs::path p; // error
    std::filesystem::path p; // compiled
    exists(p); // ADL being used for unqualified name lookup
}

Block style example:

int g_count{0};

namespace foo {
class Class
{
    std::string m_name;

public:
    bool Function(const std::string& s, int n)
    {
        // Comment summarising what this section of code does
        for (int i = 0; i < n; ++i) {
            int total_sum{0};
            // When something fails, return early
            if (!Something()) return false;
            ...
            if (SomethingElse(i)) {
                total_sum += ComputeSomething(g_count);
            } else {
                DoSomething(m_name, total_sum);
            }
        }

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

Coding Style (C++ functions and methods)

in-out parameters, followed by any output parameters.

std::optional where helpful for returning values.

is initialized to on failure), easier to read, and often the same or better performance.

instead of a magic default value, if there is no real default). Non-optional input parameters should usually be values or const references, while non-optional in-out and output parameters should usually be references, as they cannot be null.

Coding Style (clang-tidy rules)

The clang-tidy tool is used to check some rules. Please refer to the [upstream documentation](https://clang.llvm.org/extra/clang-tidy/checks/list.html) about the details and rationale for each rule.

C++ Recursive Functions

Recursion is checked by clang-tidy and thus must be made explicit. Use NOLINTNEXTLINE(misc-no-recursion) to suppress the check.

C++ named arguments

When passing named arguments, use a format that clang-tidy understands. The argument names can otherwise not be verified by clang-tidy.

For example:

void function(Addrman& addrman, bool clear);

int main()
{
    function(g_addrman, /*clear=*/false);
}

Running clang-tidy

To run clang-tidy on Ubuntu/Debian, install the dependencies:

apt install clang-tidy clang

Configure with clang as the compiler with the below command that should create a compile_commands.json file within the build directory:

cmake -B build -DCMAKE_C_COMPILER=clang -DCMAKE_CXX_COMPILER=clang++ -DCMAKE_EXPORT_COMPILE_COMMANDS=ON

The output is denoised of errors from external dependencies.

To run clang-tidy on all source files using the checks mentioned in the ./src/.clang-tidy file:

( cd ./src/ && run-clang-tidy -p ../build -j $(nproc) )

To run clang-tidy on one file:

( cd ./src/ && run-clang-tidy -p ../build -j $(nproc) ./path/to/single_file.cpp )

Optionally, append the run-clang-tidy command with the -quiet option to suppress printing of statistics and ignored warnings that can clutter the output. The -fix option also comes in handy to apply the fixes suggested by the tool but need to ensure that unrelated changes in the file are not committed.

To run clang-tidy on the changed source lines:

git diff | ( cd ./src/ && clang-tidy-diff -p2 -path ../build -j $(nproc) )

Coding Style (Doxygen-compatible comments)

Bitcoin Core uses Doxygen to generate its official documentation.

Use Doxygen-compatible comment blocks for functions, methods, and fields.

For example, to describe a function use:

/**
 * ... Description ...
 *
 * @param[in]  arg1 input description...
 * @param[in]  arg2 input description...
 * @param[out] arg3 output description...
 * @return Return cases...
 * @throws Error type and cases...
 * @pre  Pre-condition for function...
 * @post Post-condition for function...
 */
bool function(int arg1, const char *arg2, std::string& arg3)

A complete list of @xxx commands can be found at https://www.doxygen.nl/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, place the comment on the line(s) before it, using /** and */, //!, or ///:

//! Description before the member
int var;

Avoid trailing (inline) member comments like int var; //!< Description after the member.

- Rationale: Forgetting the < silently breaks Doxygen output.

Also OK:

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

Not picked up by Doxygen:

//
// ... Description ...
//

Also not picked up by Doxygen:

/*
 * ... Description ...
 */

A full list of comment syntaxes picked up by Doxygen can be found at https://www.doxygen.nl/manual/docblocks.html, but the above styles are favored.

Recommendations:

descriptions.

parameter descriptions.

about; it will build hyperlinks for these automatically. See https://www.doxygen.nl/manual/autolink.html for complete info.

previews (STRIP_CODE_COMMENTS = YES in Doxyfile.in). If you want a comment to be preserved, it must instead use // or /* */.

Generating Documentation

Assuming the build directory is named build, the documentation can be generated with cmake --build build --target docs. The resulting files will be located in build/doc/doxygen/html; open index.html in that directory to view the homepage.

Before building the docs target, you'll need to install these dependencies:

Linux: sudo apt install doxygen graphviz

MacOS: brew install doxygen graphviz

Coding Style (Python)

Refer to /test/functional/README.md#style-guidelines.

Development tips and tricks

Compiling for debugging

When using the default build configuration by running cmake -B build, the -DCMAKE_BUILD_TYPE is set to RelWithDebInfo. This option adds debug symbols but also performs some compiler optimizations that may make debugging trickier as the code may not correspond directly to the source.

If you need to build exclusively for debugging, set the -DCMAKE_BUILD_TYPE to Debug (i.e. -DCMAKE_BUILD_TYPE=Debug). You can always check the cmake build options of an existing build with ccmake build.

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.

Debug logging can be enabled on startup with the -debug and -loglevel configuration options and toggled while bitcoind is running with the logging RPC. For instance, launching bitcoind with -debug or -debug=1 will turn on all log categories and -loglevel=trace will turn on all log severity levels.

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

Signet, testnet, and regtest modes

If you are testing multi-machine code that needs to operate across the internet, you can run with either the -signet or the -testnet4 config option to test with "play bitcoins" on a test network.

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 test/functional/ for tests that run in -regtest mode.

DEBUG_LOCKORDER

Bitcoin Core is a multi-threaded application, and deadlocks or other multi-threading bugs can be very difficult to track down. The -DCMAKE_BUILD_TYPE=Debug build option adds -DDEBUG_LOCKORDER to the compiler flags. This inserts run-time checks to keep track of which locks are held and adds warnings to the debug.log file if inconsistencies are detected.

DEBUG_LOCKCONTENTION

Defining DEBUG_LOCKCONTENTION adds a "lock" logging category to the logging RPC that, when enabled, logs the location and duration of each lock contention to the debug.log file.

The -DCMAKE_BUILD_TYPE=Debug build option adds -DDEBUG_LOCKCONTENTION to the compiler flags. You may also enable it manually by building with -DDEBUG_LOCKCONTENTION added to your CPPFLAGS, i.e. -DAPPEND_CPPFLAGS="-DDEBUG_LOCKCONTENTION".

You can then use the -debug=lock configuration option at bitcoind startup or bitcoin-cli logging '["lock"]' at runtime to turn on lock contention logging. It can be toggled off again with bitcoin-cli logging [] '["lock"]'.

Assertions and Checks

The util file src/util/check.h offers helpers to protect against coding and internal logic bugs. They must never be used to validate user, network or any other input.

violation would mean that it is not safe to continue program execution. The code is always compiled with assertions enabled. - For example, a nullptr dereference or any other logic bug in validation code means the program code is faulty and must terminate immediately.

failure, it will throw an exception, which can be caught to recover from the error. - For example, a nullptr dereference or any other logic bug in RPC code means that the RPC code is faulty and cannot be executed. However, the logic bug can be shown to the user and the program can continue to run.

safely continue even if the assumption is violated. In debug builds it behaves like Assert/assert to notify developers and testers about nonfatal errors. In production it doesn't warn or log anything, though the expression is always evaluated. However, if the compiler can prove that an expression inside Assume is side-effect-free, it may optimize the call away, skipping its evaluation in production. This enables a lower-cost way of making explicit statements about the code, aiding review. - For example it can be assumed that a variable is only initialized once, but a failed assumption does not result in a fatal bug. A failed assumption may or may not result in a slightly degraded user experience, but it is safe to continue program execution.

Compiling for test coverage

Using LCOV

LCOV can be used to generate a test coverage report based upon ctest execution. LCOV must be installed on your system (e.g. the lcov package on Debian/Ubuntu).

To enable LCOV report generation during test runs:

cmake -B build -DCMAKE_BUILD_TYPE=Coverage
cmake --build build
cmake -P build/Coverage.cmake

# A coverage report will now be accessible at `./build/test_bitcoin.coverage/index.html`,
# which covers unit tests, and `./build/total.coverage/index.html`, which covers
# unit and functional tests.

Additional LCOV options can be specified using LCOV_OPTS, but may be dependent on the version of LCOV. For example, when using LCOV 2.x, branch coverage can be enabled by setting LCOV_OPTS="--rc branch_coverage=1":

cmake -DLCOV_OPTS="--rc branch_coverage=1" -P build/Coverage.cmake

To enable test parallelism:

cmake -DJOBS=$(nproc) -P build/Coverage.cmake

Using LLVM/Clang toolchain

The following generates a coverage report for unit tests and functional tests.

Configure the build with the following flags:

Consider building with a clean state using rm -rf build 
# MacOS may instead require `-DCMAKE_C_COMPILER="$(brew --prefix llvm)/bin/clang" -DCMAKE_CXX_COMPILER="$(brew --prefix llvm)/bin/clang++"`
cmake -B build -DCMAKE_C_COMPILER="clang" \
   -DCMAKE_CXX_COMPILER="clang++" \
   -DAPPEND_CFLAGS="-fprofile-instr-generate -fcoverage-mapping" \
   -DAPPEND_CXXFLAGS="-fprofile-instr-generate -fcoverage-mapping" \
   -DAPPEND_LDFLAGS="-fprofile-instr-generate -fcoverage-mapping"
cmake --build build # Append "-j N" here for N parallel jobs.

Generating the raw profile data based on ctest and functional tests execution:

# Create directory for raw profile data
mkdir -p build/raw_profile_data

# Run tests to generate profiles
LLVM_PROFILE_FILE="$(pwd)/build/raw_profile_data/%m_%p.profraw" ctest --test-dir build # Append "-j N" here for N parallel jobs.
LLVM_PROFILE_FILE="$(pwd)/build/raw_profile_data/%m_%p.profraw" build/test/functional/test_runner.py # Append "-j N" here for N parallel jobs

# Merge all the raw profile data into a single file
find build/raw_profile_data -name "*.profraw" > build/raw_profile_data_files.txt
llvm-profdata merge -f build/raw_profile_data_files.txt -o build/coverage.profdata
Note: The "counter mismatch" warning can be safely ignored, though it can be resolved by updating to Clang 19. The warning occurs due to version mismatches but doesn't affect the coverage report generation.

Generating the coverage report:

llvm-cov show \
    --object=build/bin/test_bitcoin \
    --object=build/bin/bitcoind \
    -Xdemangler=llvm-cxxfilt \
    --instr-profile=build/coverage.profdata \
    --ignore-filename-regex="src/crc32c/|src/leveldb/|src/minisketch/|src/secp256k1/|src/test/" \
    --format=html \
    --show-instantiation-summary \
    --show-line-counts-or-regions \
    --show-expansions \
    --output-dir=build/coverage_report \
    --project-title="Bitcoin Core Coverage Report"
Note: The "functions have mismatched data" warning can be safely ignored, the coverage report will still be generated correctly despite this warning. This warning occurs due to profdata mismatch created during the merge process for shared libraries.

The generated coverage report can be accessed at build/coverage_report/index.html.

Compiling for Fuzz Coverage

cmake -B build \
   -DCMAKE_C_COMPILER="clang" \
   -DCMAKE_CXX_COMPILER="clang++" \
   -DCMAKE_C_FLAGS="-fprofile-instr-generate -fcoverage-mapping" \
   -DCMAKE_CXX_FLAGS="-fprofile-instr-generate -fcoverage-mapping" \
   -DBUILD_FOR_FUZZING=ON
cmake --build build # Append "-j N" here for N parallel jobs.

Running fuzz tests with one or more targets

# For single target run with the target of choice
LLVM_PROFILE_FILE="$(pwd)/build/raw_profile_data/txorphan.profraw" ./build/test/fuzz/test_runner.py ../qa-assets/fuzz_corpora txorphan
# If running for multiple targets
LLVM_PROFILE_FILE="$(pwd)/build/raw_profile_data/%m_%p.profraw" ./build/test/fuzz/test_runner.py ../qa-assets/fuzz_corpora
# Merge profiles
llvm-profdata merge build/raw_profile_data/*.profraw -o build/coverage.profdata

Generate report:

llvm-cov show \
    --object=build/bin/fuzz \
    -Xdemangler=llvm-cxxfilt \
    --instr-profile=build/coverage.profdata \
    --ignore-filename-regex="src/crc32c/|src/leveldb/|src/minisketch/|src/secp256k1/|src/test/" \
    --format=html \
    --show-instantiation-summary \
    --show-line-counts-or-regions \
    --show-expansions \
    --output-dir=build/coverage_report \
    --project-title="Bitcoin Core Fuzz Coverage Report"

The generated coverage report can be accessed at build/coverage_report/index.html.

Using IWYU

The `include-what-you-use` tool (IWYU) helps to enforce the source code organization policy in this repository.

To reproduce the IWYU CI job locally, run:

env -i HOME="$HOME" PATH="$PATH" USER="$USER" MAKEJOBS="-j1" FILE_ENV="./ci/test/00_setup_env_native_iwyu.sh" ./ci/test_run_all.sh || echo "IWYU failed"

In some cases, IWYU might suggest headers that seem unnecessary at first glance, but are actually required. For example, a macro may use a symbol that requires its own include. Another example is passing a string literal to a function that accepts a std::string parameter. An implicit conversion occurs at the callsite using the std::string constructor, which makes the corresponding header required. We accept these suggestions as is.

If the provided IWYU CI job still produces a false positive, reduce it to a minimal reproducer and report it upstream.

Use IWYU pragmas sparingly.

Use IWYU pragma: keep only as a narrow workaround when needed.

Use IWYU pragma: associated only when IWYU cannot infer the intended associated header.

Use IWYU pragma: export very sparingly, as this enforces transitive inclusion of headers and undermines the specific purpose of IWYU.

The acceptable cases for using IWYU pragma: export are:

  1. Facade headers. For example, see `compat/compat.h`.
  2. Drop-in replacement headers. For example, see `util/time.h`.
  3. Presenting a complete interface across multiple headers.

For IWYU pragmas, prefer adding a nearby source comment that explains why the annotation is needed.

Performance profiling with perf

Profiling is a good way to get a precise idea of where time is being spent in code. One tool for doing profiling on Linux platforms is called `perf`. It can observe a running process and sample (at some frequency) where its execution is.

Perf installation is contingent on which kernel version you're running; see this thread for specific instructions.

Certain kernel parameters may need to be set for perf to be able to inspect the running process's stack.

$ sudo sysctl -w kernel.perf_event_paranoid=-1
$ sudo sysctl -w kernel.kptr_restrict=0

Make sure you [understand the security trade-offs](https://lwn.net/Articles/420403/) of setting these kernel parameters.

To profile a running bitcoind process for 60 seconds, you could use an invocation of perf record like this:

$ perf record \
    -g --call-graph dwarf --per-thread -F 140 \
    -p `pgrep bitcoind` -- sleep 60

You could then analyze the results by running:

perf report --stdio | c++filt | less

or using a graphical tool like Hotspot.

Valgrind

Valgrind is a programming tool for memory debugging, memory leak detection, and profiling. The repo contains a Valgrind suppressions file (`valgrind.supp`) which includes known Valgrind warnings in our dependencies that cannot be fixed in-tree. Example use:

$ valgrind --suppressions=test/sanitizer_suppressions/valgrind.supp build/bin/test_bitcoin
$ valgrind --suppressions=test/sanitizer_suppressions/valgrind.supp --leak-check=full \
      --show-leak-kinds=all build/bin/test_bitcoin --log_level=test_suite
$ valgrind -v --leak-check=full build/bin/bitcoind -printtoconsole
$ ./build/test/functional/test_runner.py --valgrind

Sanitizers

Bitcoin Core can be compiled with various "sanitizers" enabled, which add instrumentation for issues regarding things like memory safety, thread race conditions, or undefined behavior. This is controlled with the -DSANITIZERS cmake build flag, which should be a comma separated list of sanitizers to enable. The sanitizer list should correspond to supported -fsanitize= options in your compiler. These sanitizers have runtime overhead, so they are most useful when testing changes or producing debugging builds.

Some examples:

# Enable both the address sanitizer and the undefined behavior sanitizer
cmake -B build -DSANITIZERS=address,undefined

# Enable the thread sanitizer
cmake -B build -DSANITIZERS=thread

If you are compiling with GCC you will typically need to install corresponding "san" libraries to actually compile with these flags, e.g. libasan for the address sanitizer, libtsan for the thread sanitizer, and libubsan for the undefined sanitizer. If you are missing required libraries, the build will fail with a linker error when testing the sanitizer flags.

The test suite should pass cleanly with the thread and undefined sanitizers. You may need to use a suppressions file, see test/sanitizer_suppressions. They may be used as follows:

export LSAN_OPTIONS="suppressions=$(pwd)/test/sanitizer_suppressions/lsan"
export TSAN_OPTIONS="suppressions=$(pwd)/test/sanitizer_suppressions/tsan:halt_on_error=1:second_deadlock_stack=1"
export UBSAN_OPTIONS="suppressions=$(pwd)/test/sanitizer_suppressions/ubsan:print_stacktrace=1:halt_on_error=1:report_error_type=1"

See the CI config for more examples, and upstream documentation for more information about any additional options.

Not all sanitizer options can be enabled at the same time, e.g. trying to build with -DSANITIZERS=address,thread will fail in the build as these sanitizers are mutually incompatible. Refer to your compiler manual to learn more about these options and which sanitizers are supported by your compiler.

Additional resources:

* AddressSanitizer * LeakSanitizer * MemorySanitizer * ThreadSanitizer * UndefinedBehaviorSanitizer * GCC Instrumentation Options * Google Sanitizers Wiki

Development guidelines

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

Locking/mutex usage notes

The code is multi-threaded and uses mutexes and the LOCK and 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 (or use -DCMAKE_BUILD_TYPE=Debug) 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 FillableSigningProvider class and its cs_KeyStore lock for example).

Threads

: Started from main() in bitcoind.cpp. Responsible for starting up and shutting down the application.

: Performs various loading tasks that are part of init but shouldn't block the node from being started: external block import, reindex, reindex-chainstate, main chain activation, spawn indexes background sync threads and mempool load.

: Parallel script validation threads for transactions in blocks.

: Thread to listen for RPC and REST connections.

: Threads to service RPC and REST requests.

: One thread per indexer.

: Does asynchronous background tasks like dumping wallet contents, dumping addrman and running asynchronous validationinterface callbacks.

: Thread for tor connections.

- ThreadMessageHandler (`b-msghand`) : Application level message handling (sending and receiving). Almost all net_processing and validation logic runs on this thread.

- ThreadDNSAddressSeed (`b-dnsseed`) : Loads addresses of peers from the DNS.

- ThreadMapPort (`b-mapport`) : Universal plug-and-play startup/shutdown.

- ThreadSocketHandler (`b-net`) : Sends/Receives data from peers on port 8333.

- ThreadOpenAddedConnections (`b-addcon`) : Opens network connections to added nodes.

- ThreadOpenConnections (`b-opencon`) : Initiates new connections to peers.

- ThreadI2PAcceptIncoming (`b-i2paccept`) : Listens for and accepts incoming I2P connections through the I2P SAM proxy.

General Bitcoin Core

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

Logging

The macros LogInfo, LogDebug, LogTrace, LogWarning and LogError are available for logging messages. They should be used as follows:

most of the time, and it should be used for log messages that are useful for debugging and can reasonably be enabled on a production system (that has sufficient free storage space). They will be logged if the program is started with -debug=category or -debug=1, or the category is enabled through the logging RPC.

messages or for infrequent and important events such as a new block tip being found or a new outbound connection being made.

severe problems that require the node (or a subsystem) to shut down entirely (e.g., insufficient storage space).

severe problems that the node admin should address, but are not severe enough to warrant shutting down the node (e.g., system time appears to be wrong, unknown soft fork appears to have activated).

LogDebug for log messages that would be unusable on a production system, e.g. due to being too noisy in normal use, or too resource intensive to process. These will be logged if the startup options -debug=category -loglevel=category:trace or `-debug=1 -loglevel=trace` are selected.

Note that the format strings and parameters of LogDebug and LogTrace are only evaluated if the logging category is enabled, so you must be careful to avoid side-effects in those expressions.

While LogInfo, LogWarning and LogError messages should be rare, in case there are circumstances where they are not, those messages are automatically rate-limited to prevent potential disk-filling attacks. For the cases where this protection is undesirable, rate-limiting can be avoided with the util::log::NO_RATE_LIMIT tag, eg LogInfo(util::log::NO_RATE_LIMIT, "UpdateTip: new best=%s ...",...).

General C++

For general C++ guidelines, you may refer to the [C++ Core Guidelines](https://isocpp.github.io/CppCoreGuidelines/).

Common misconceptions are clarified in those sections:

Guidelines](https://isocpp.github.io/CppCoreGuidelines/CppCoreGuidelines#rf-conventional).

- 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.

unique_ptr for allocations in a function.

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

C++ data structures

- 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.

another data structure (even if of the same type).

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

including &vch[0] for an empty vector. Use vch.data() and `vch.data() + vch.size()` instead.

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. Initializing the members in the declaration makes it easy to spot uninitialized ones.

class A
{
    uint32_t m_count{0};
}

- Rationale: This is a precaution to avoid unintended conversions.

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.

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

- Rationale: Compared to Foo(const vector<int>&) this avoids the need for a (potentially expensive) conversion to vector if the caller happens to have the input stored in another type of container. However, be aware of the pitfalls documented in span.h.

void Foo(std::span<const int> data);

std::vector<int> vec{1,2,3};
Foo(vec);

- Rationale: Scoped enumerations avoid two potential pitfalls/problems with traditional C++ enumerations: implicit conversions to int, and name clashes due to enumerators being exported to the surrounding scope.

enum class Tabs {
    info,
    console,
};

int GetInt(Tabs tab)
{
    int ret = [&]() {
        switch (tab) {
        case Tabs::info: return 0;
        case Tabs::console: return 1;
        } // no default case, so the compiler can warn about missing cases
        assert(false);
    }();
    LogInfo("Tab %s", ret);
    return ret;
}

Rationale: The comment documents skipping default: label, and it complies with clang-format rules. The assertion prevents firing of -Wreturn-type warning on some compilers.

Strings and formatting

- 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.

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

strings.

- Do not use it when passing a sized array (so along with .size()). Use .data() instead to get a pointer to the raw data.

- Rationale: Although this is guaranteed to be safe starting with C++11, .data() communicates the intent better.

- Do not use it when passing strings to tfm::format, strprintf, LogInfo, LogDebug, etc.

- Rationale: This is redundant. Tinyformat handles strings.

- Do not use it to convert to QString. Use QString::fromStdString().

- Rationale: Qt has built-in functionality for converting their string type from/to C++. No need to roll your own.

- In cases where you do call .c_str(), you might want to additionally check that the string does not contain embedded '\0' characters, because it will (necessarily) truncate the string. This might be used to hide parts of the string from logging or to circumvent checks. If a use of strings is sensitive to this, take care to check the string for embedded NULL characters first and reject it if there are any.

Shadowing

Although the shadowing warning (-Wshadow) is not enabled by default (it prevents issues arising from using a different variable with the same name), please name variables so that their names do not shadow variables defined in the source code.

When using nested cycles, do not name the inner cycle variable the same as in the outer cycle, etc.

Lifetimebound

The [Clang lifetimebound attribute](https://clang.llvm.org/docs/AttributeReference.html#lifetimebound) can be used to tell the compiler that a lifetime is bound to an object and potentially see a compile-time warning if the object has a shorter lifetime from the invalid use of a temporary. You can use the attribute by adding a LIFETIMEBOUND annotation defined in src/attributes.h; please grep the codebase for examples.

Threads and synchronization

get compile-time warnings about potential race conditions or deadlocks in code.

- In functions that are declared separately from where they are defined, the thread safety annotations should be added exclusively to the function declaration. Annotations on the definition could lead to false positives (lack of compile failure) at call sites between the two.

- Prefer locks that are in a class rather than global, and that are internal to a class (private or protected) rather than public.

- Combine annotations in function declarations with run-time asserts in function definitions (AssertLockNotHeld() can be omitted if LOCK() is called unconditionally after it because LOCK() does the same check as AssertLockNotHeld() internally, for non-recursive mutexes):

// txmempool.h
class CTxMemPool
{
public:
    ...
    mutable RecursiveMutex cs;
    ...
    void UpdateTransactionsFromBlock(...) EXCLUSIVE_LOCKS_REQUIRED(::cs_main, cs);
    ...
}

// txmempool.cpp
void CTxMemPool::UpdateTransactionsFromBlock(...)
{
    AssertLockHeld(::cs_main);
    AssertLockHeld(cs);
    ...
}

// validation.h
class Chainstate
{
protected:
    ...
    Mutex m_chainstate_mutex;
    ...
public:
    ...
    bool ActivateBestChain(
        BlockValidationState& state,
        std::shared_ptr<const CBlock> pblock = nullptr)
        EXCLUSIVE_LOCKS_REQUIRED(!m_chainstate_mutex)
        LOCKS_EXCLUDED(::cs_main);
    ...
    bool PreciousBlock(BlockValidationState& state, CBlockIndex* pindex)
        EXCLUSIVE_LOCKS_REQUIRED(!m_chainstate_mutex)
        LOCKS_EXCLUDED(::cs_main);
    ...
}

// validation.cpp
bool Chainstate::PreciousBlock(BlockValidationState& state, CBlockIndex* pindex)
{
    AssertLockNotHeld(m_chainstate_mutex);
    AssertLockNotHeld(::cs_main);
    {
        LOCK(cs_main);
        ...
    }

    return ActivateBestChain(state, std::shared_ptr<const CBlock>());
}

deadlocks are introduced. This is defined by default when building with -DCMAKE_BUILD_TYPE=Debug.

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);
{
    ...
}

Scripts

Write scripts in Python or Rust rather than bash, when possible.

Source code organization

when performance due to inlining is critical.

- Rationale: Shorter and simpler header files are easier to read and reduce compile time.

definitions from, even if those headers are already included indirectly through other headers.

- Rationale: Excluding headers because they are already indirectly included results in compilation failures when those indirect dependencies change. Furthermore, it obscures what the real code dependencies are. The Using IWYU section describes a tool to help enforce this.

fully specified types such as std::string.

- Rationale: Avoids symbol conflicts.

should be placed on the same line as the brace closing the namespace, e.g.

namespace mynamespace {
...
} // namespace mynamespace

namespace {
...
} // namespace

- Rationale: Avoids confusion about the namespace context.

GUI

- 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.

add new interfaces::Node and interfaces::Wallet method calls, even if they may be fast now, in case they are changed to lock or communicate across processes in the future.

Prefer to offload work from the GUI thread to worker threads (see RPCExecutor in console code as an example) or take other steps (see https://doc.qt.io/archives/qq/qq27-responsive-guis.html) to keep the GUI responsive.

- Rationale: Blocking the GUI thread can increase latency, and lead to hangs and deadlocks.

Subtrees

Several parts of the repository are subtrees of software maintained elsewhere.

Normally, these are maintained by active developers of Bitcoin Core, in which case changes should go directly upstream without being PRed directly against the project. They will be merged back in the next subtree merge.

Others are external projects without a tight relationship with our project. Changes to these should also be sent upstream, but bugfixes may also be prudent to PR against a Bitcoin Core subtree, so that they can be integrated quickly. Cosmetic changes should be taken upstream.

There is a tool in test/lint/git-subtree-check.sh (instructions) to check a subtree directory for consistency with its upstream repository.

The tool instructions also include a list of the subtrees managed by Bitcoin Core.

To fully verify or update a subtree, add it as a remote:

git remote add libmultiprocess https://github.com/bitcoin-core/libmultiprocess.git

To update the subtree:

git fetch libmultiprocess
git subtree pull --prefix=src/ipc/libmultiprocess libmultiprocess master --squash

The ultimate upstream of the few externally managed subtrees are:

- Upstream at https://github.com/google/leveldb ; maintained by Google. Open important PRs to the Bitcoin Core subtree to avoid delay. - Note: Follow the instructions in Upgrading LevelDB when merging upstream changes to the LevelDB subtree.

- Used by leveldb for hardware acceleration of CRC32C checksums for data integrity. - Upstream at https://github.com/google/crc32c ; maintained by Google.

Upgrading LevelDB

Extra care must be taken when upgrading LevelDB. This section explains issues you must be aware of.

File Descriptor Counts

In most configurations, we use the default LevelDB value for max_open_files, which is 1000 at the time of this writing. If LevelDB actually uses this many file descriptors, it will cause problems with Bitcoin's select() loop, because it may cause new sockets to be created where the fd value is >= 1024. For this reason, on 64-bit Unix systems, we rely on an internal LevelDB optimization that uses mmap() + close() to open table files without actually retaining references to the table file descriptors. If you are upgrading LevelDB, you must sanity check the changes to make sure that this assumption remains valid.

In addition to reviewing the upstream changes in env_posix.cc, you can use lsof to check this. For example, on Linux this command will show open .ldb file counts:

$ lsof -p $(pidof bitcoind) |\
    awk 'BEGIN { fd=0; mem=0; } /ldb$/ { if ($4 == "mem") mem++; else fd++ } END { printf "mem = %s, fd = %s\n", mem, fd}'
mem = 119, fd = 0

The mem value shows how many files are mmap'ed, and the fd value shows how many file descriptors these files are using. You should check that fd is a small number (usually 0 on 64-bit hosts).

See the notes in the SetMaxOpenFiles() function in dbwrapper.cc for more details.

Consensus Compatibility

It is possible for LevelDB changes to inadvertently change consensus compatibility between nodes. This happened in Bitcoin 0.8 (when LevelDB was first introduced). When upgrading LevelDB, you should review the upstream changes to check for issues affecting consensus compatibility.

For example, if LevelDB had a bug that accidentally prevented a key from being returned in an edge case, and that bug was fixed upstream, the bug "fix" would be an incompatible consensus change. In this situation, the correct behavior would be to revert the upstream fix before applying the updates to Bitcoin's copy of LevelDB. In general, you should be wary of any upstream changes affecting what data is returned from LevelDB queries.

Scripted diffs

For reformatting and refactoring commits where the changes can be easily automated using a bash script, we use scripted-diff commits. The bash script is included in the commit message and our CI job checks that the result of the script is identical to the commit. This aids reviewers since they can verify that the script does exactly what it is supposed to do. It is also helpful for rebasing (since the same script can just be re-run on the new master commit).

To create a scripted-diff:

- -BEGIN VERIFY SCRIPT- - -END VERIFY SCRIPT-

The scripted-diff is verified by the tool test/lint/commit-script-check.sh. The tool's default behavior, when supplied with a commit is to verify all scripted-diffs from the beginning of time up to said commit. Internally, the tool passes the first supplied argument to git rev-list --reverse to determine which commits to verify script-diffs for, ignoring commits that don't conform to the commit message format described above.

For development, it might be more convenient to verify all scripted-diffs in a range A..B, for example:

test/lint/commit-script-check.sh origin/master..HEAD

Suggestions and examples

If you need to replace in multiple files, prefer git ls-files to find or globbing, and git grep to grep, to avoid changing files that are not under version control.

For efficient replacement scripts, reduce the selection to the files that potentially need to be modified, so for example, instead of a blanket git ls-files src | xargs sed -i s/apple/orange/, use git grep -l apple src | xargs sed -i s/apple/orange/.

Also, it is good to keep the selection of files as specific as possible — for example, replace only in directories where you expect replacements — because it reduces the risk that a rebase of your commit by re-running the script will introduce accidental changes.

Some good examples of scripted-diff:

uses an elegant script to replace occurrences of multiple terms in all source files.

replaces specific terms in a list of specific source files.

does a global replacement but excludes certain directories.

To find all previous uses of scripted diffs in the repository, do:

git log --grep="-BEGIN VERIFY SCRIPT-"

Release notes

Release notes should be written for any PR that:

Release notes should be added to a PR-specific release note file at /doc/release-notes-<PR number>.md to avoid conflicts between multiple PRs. All release-notes* files are merged into a single release-notes-<version>.md file prior to the release.

RPC interface guidelines

A few guidelines for introducing and reviewing new RPC interfaces:

- Rationale: Consistency with the existing interface.

style or spelling convention in the API for the type of object in question (blockhash, for example), and if so, try to use that. If not, use snake case fee_delta (and not, e.g. feedelta or camel case feeDelta).

- Rationale: Consistency with the existing interface.

arguments unless absolutely necessary.

- Rationale: Introduces hand-rolled string manipulation code at both the caller and callee sites, which is error-prone, and it is easy to get things such as escaping wrong. JSON already supports nested data structures, no need to re-invent the wheel.

- Exception: AmountFromValue can parse amounts as string. This was introduced because many JSON parsers and formatters hard-code handling decimal numbers as floating-point values, resulting in potential loss of precision. This is unacceptable for monetary values. Always use AmountFromValue and ValueFromAmount when inputting or outputting monetary values. The only exceptions to this are prioritisetransaction and getblocktemplate because their interface is specified as-is in BIP22.

default value, both cases should fail in the same way. The easiest way to follow this guideline is to detect unspecified arguments with params[x].isNull() instead of params.size() <= x. The former returns true if the argument is either null or missing, while the latter returns true if is missing, and false if it is null.

- Rationale: Avoids surprises when switching to name-based arguments. Missing name-based arguments are passed as 'null'.

do different things.

- Rationale: This is impossible to use with bitcoin-cli, and can be surprising to users.

- Exception: Some RPC calls can take both an int and bool, most notably when a bool was switched to a multi-value, or due to other historical reasons. Always have false map to 0 and true to 1 in this case.

Disallow usage of boolean verbosity (see ParseVerbosity() in util.h).

- Rationale: Integer verbosity allows for multiple values. Undocumented boolean verbosity is deprecated and new RPC methods should prevent its use.

- Rationale: bitcoin-cli and the GUI debug console use this table to determine how to convert a plaintext command line to JSON. If the types don't match, the method can be unusable from there.

introduce new methods that differ in behavior based on the presence of a wallet.

- Rationale: As well as complicating the implementation and interfering with the introduction of multi-wallet, wallet and non-wallet code should be separated to avoid introducing circular dependencies between code units.

- Rationale: If a RPC response is not a JSON object, then it is harder to avoid API breakage if new data in the response is needed.

getblockchaininfo's state immediately prior to the call's execution. Wallet RPCs whose behavior does not depend on the current chainstate may omit this call.

- Rationale: In previous versions of Bitcoin Core, the wallet was always in-sync with the chainstate (by virtue of them all being updated in the same cs_main lock). In order to maintain the behavior that wallet RPCs return results as of at least the highest best-known block an RPC client may be aware of prior to entering a wallet RPC call, we must block until the wallet is caught up to the chainstate as of the RPC call's entry. This also makes the API much easier for RPC clients to reason about.

RPCExamples help documentation.

- Rationale: Prevent accidental transactions by users and encourage the use of bech32 addresses by default.

timestamps in the documentation.

- Rationale: User-facing consistency.

to JSON strings, not fs::PathToString and fs::PathFromString

- Rationale: JSON strings are Unicode strings, not byte strings, and RFC8259 requires JSON to be encoded as UTF-8.

A few guidelines for modifying existing RPC interfaces:

- Rationale: Changes in RPC JSON structure can break downstream application compatibility. Implementation of deprecatedrpc provides a grace period for downstream applications to migrate. Release notes provide notification to downstream users.

Feature deprecation and removal process

Bitcoin Core uses a structured process for deprecating and removing features to give downstream users and applications time to migrate.

General principles

major release.

RPC methods and fields

Deprecated features should remain accessible via this flag during the grace period so downstream users are not immediately broken.

that re-enables it. For example: ` "\nDeprecated in v25.0, use the newfoo RPC instead. Start bitcoind with" " -deprecatedrpc=foo to continue using this RPC.\n" `

Startup options

option is used, so users are notified at startup.

removed in a future release.

REST interface

with the version they were deprecated.

ZMQ

they were deprecated.

Internal interface guidelines

Internal interfaces between parts of the codebase that are meant to be independent (node, wallet, GUI), are defined in `src/interfaces/`. The main interface classes defined there are `interfaces::Chain`, used by wallet to access the node's latest chain state, `interfaces::Node`, used by the GUI to control the node, `interfaces::Wallet`, used by the GUI to control an individual wallet and `interfaces::Mining`, used by RPC to generate block templates. There are also more specialized interface types like `interfaces::Handler` `interfaces::ChainClient` passed to and from various interface methods.

Interface classes are written in a particular style so node, wallet, and GUI code doesn't need to run in the same process, and so the class declarations work more easily with tools and libraries supporting interprocess communication:

virtual](https://en.cppreference.com/w/cpp/language/abstract_class). This allows multiple implementations to inherit from the same interface class, particularly so one implementation can execute functionality in the local process, and other implementations can forward calls to remote processes.

implementing new functionality. Any substantial new node or wallet functionality should be implemented in `src/node/` or `src/wallet/` and just exposed in `src/interfaces/` instead of being implemented there, so it can be more modular and accessible to unit tests.

be other interface classes. Interface methods shouldn't pass references to objects that can't be serialized or accessed from another process.

Examples:

`c++ // Good: takes string argument and returns interface class pointer virtual unique_ptr<interfaces::Wallet> loadWallet(std::string filename) = 0;

// Bad: returns CWallet reference that can't be used from another process virtual CWallet& loadWallet(std::string filename) = 0; `

`c++ // Good: accepts and returns primitive types virtual bool findBlock(const uint256& hash, int& outheight, int64t& out_time) = 0;

// Bad: returns pointer to internal node in a linked list inaccessible to // other processes virtual const CBlockIndex* findBlock(const uint256& hash) = 0; `

`c++ // Good: takes plain callback type and returns interface pointer using TipChangedFn = std::function<void(int blockheight, int64t block_time)>; virtual std::unique_ptr<interfaces::Handler> handleTipChanged(TipChangedFn fn) = 0;

// Bad: returns btcsignals connection specific to local process using TipChangedFn = std::function<void(int blockheight, int64t block_time)>; virtual btcsignals::scoped_connection connectTipChanged(TipChangedFn fn) = 0; `

Rationale: consistency and friendliness to code generation tools.

Example:

`c++ // Good: method names are unique virtual bool disconnectByAddress(const CNetAddr& net_addr) = 0; virtual bool disconnectById(NodeId id) = 0;

// Bad: methods are overloaded by type virtual bool disconnect(const CNetAddr& net_addr) = 0; virtual bool disconnect(NodeId id) = 0; `

Internal interface naming style

UpperCamelCase.

Rationale: consistency and friendliness to code generation tools.

Examples:

`c++ // Good: lowerCamelCase method name virtual void blockConnected(const CBlock& block, int height) = 0;

// Bad: uppercase class method virtual void BlockConnected(const CBlock& block, int height) = 0; `

`c++ // Good: UpperCamelCase standalone function name std::unique_ptr<Node> MakeNode(LocalInit& init);

// Bad: lowercase standalone function std::unique_ptr<Node> makeNode(LocalInit& init); `

Note: This last convention isn't generally followed outside of `src/interfaces/`, though it did come up for discussion before in #14635.