DuckDB Extension Macros

A modern, Rust 2024 Edition compatible procedural macro for creating DuckDB loadable extensions.

Overview

This crate provides the #[duckdb_extension] attribute macro that simplifies the creation of DuckDB loadable extensions in Rust. It serves as a modern alternative to DuckDB's official duckdb-loadable-macros crate, which only supports Rust 2021 Edition.

Features

• Rust 2024 Edition Support: Fully compatible with modern Rust editions
• Drop-in Replacement: API-compatible with duckdb-loadable-macros
• Flexible Configuration: Configure extension name and API version via attributes or environment variables
• Comprehensive Error Handling: Proper error propagation from Rust to DuckDB's C API
• Safe FFI Integration: Manages unsafe operations with proper error handling
• Modern Dependencies: Uses up-to-date versions of darling, syn, quote, and proc-macro2

Installation

Add this to your Cargo.toml:

[dependencies]
duckdb = { version = "1.4.2", features = ["vtab-loadable"] }  # or your preferred version
duckdb-ext-macros = "0.1.0"
libduckdb-sys = { version = "1.4.2", features = ["loadable-extension"] }

Quick Start

use duckdb_ext_macros::duckdb_extension;

#[duckdb_extension(name = "my_extension", api_version = "v1.2.0")]
fn my_extension_init(connection: duckdb::Connection) -> Result<(), String> {
    // Your extension initialization logic here
    // Register functions, types, or perform setup

    println!("My extension loaded successfully!");
    Ok(())
}

Usage

Basic Usage

The simplest usage with default values:

#[duckdb_extension]
fn my_extension(connection: duckdb::Connection) -> Result<(), String> {
    // Extension logic
    Ok(())
}

This will:

• Use your cargo package name as the extension name (converted to snake_case)
• Use DuckDB API version "v1.2.0" as the minimum required version
• Generate a C entrypoint function named {package_name}_init_c_api

Custom Extension Name

#[duckdb_extension(name = "custom_extension")]
fn init(connection: duckdb::Connection) -> Result<(), String> {
    Ok(())
}

Custom API Version

#[duckdb_extension(api_version = "v1.1.0")]
fn init(connection: duckdb::Connection) -> Result<(), String> {
    Ok(())
}

Environment Variables

You can also configure the extension via environment variables, which is useful for build scripts or CI/CD:

# Set extension name
export DUCKDB_EXTENSION_NAME="my_extension"

# Set minimum API version
export DUCKDB_MINIMUM_API_VERSION="v1.2.0"

Environment variables are used as fallbacks when macro attributes are not provided.

How It Works

The #[duckdb_extension] macro transforms your Rust function into a DuckDB-compatible C entrypoint. Here's what happens under the hood:

1. Argument Parsing: The macro parses attributes (name, api_version) or reads environment variables
2. Name Resolution: Determines the extension name with fallback logic
3. API Version Resolution: Determines the minimum required DuckDB API version
4. Code Generation: Generates a C-compatible function with the signature {extension_name}_init_c_api
5. FFI Integration: Creates safe wrappers around DuckDB's C API calls
6. Error Handling: Converts Rust errors to DuckDB error messages

Generated Code Example

For this input:

#[duckdb_extension(name = "my_ext")]
fn init(conn: duckdb::Connection) -> Result<(), String> { Ok(()) }

The macro generates:

#[unsafe(no_mangle)]
pub extern "C" fn my_ext_init_c_api(
    info: libduckdb_sys::duckdb_extension_info,
    access: *const libduckdb_sys::duckdb_extension_access
) -> bool {
    // FFI and error handling code
}

Comparison with Official duckdb-loadable-macros

Feature	duckdb-ext-macros	duckdb-loadable-macros
Rust Edition	2024 Edition	2021 Edition only
Dependencies	Modern versions	Older versions

Migration from duckdb-loadable-macros

If you're currently using duckdb-loadable-macros, migration is straightforward:

1. Update Cargo.toml:

   # Remove or replace:
   # duckdb-loadable-macros = "..."
   
   # Add:
   duckdb-ext-macros = "0.1.0"

2. Update imports:

   // Change from:
   // use duckdb_loadable_macros::duckdb_entrypoint;
   
   // To:
   use duckdb_ext_macros::duckdb_extension;

3. Update attribute:

   // Change from:
   // #[duckdb_entrypoint]
   
   // To:
   #[duckdb_extension]

The API is designed to be compatible, so your existing extension logic should work without changes.

Configuration Options

Macro Attributes

• name: The extension name (used in the C entrypoint function)
• api_version: Minimum DuckDB C API version required (default: "v1.2.0")

Environment Variables

• DUCKDB_EXTENSION_NAME: Extension name (fallback if name attribute not provided)
• DUCKDB_MINIMUM_API_VERSION: Minimum API version (fallback if api_version attribute not provided)

Name Resolution Order

The extension name is determined in this order:

1. name attribute in the macro
2. DUCKDB_EXTENSION_NAME environment variable
3. CARGO_PKG_NAME environment variable (cargo package name)

API Version Resolution Order

The minimum API version is determined in this order:

1. api_version attribute in the macro
2. DUCKDB_MINIMUM_API_VERSION environment variable
3. Default: "v1.2.0"

Error Handling

The macro provides comprehensive error handling:

1. Compile-time Errors: Invalid macro syntax or configuration
2. Runtime Errors: Extension initialization failures
3. FFI Errors: C API interaction failures

Errors are propagated to DuckDB with descriptive messages. If error string allocation fails, a fallback message is used.

License

This project is licensed under the same terms as DuckDB (MIT License).

Acknowledgments

• DuckDB team for the excellent database and original duckdb-loadable-macros
• The Rust community for excellent procedural macro libraries
• All contributors to this project