should be it

external/duckdb/extension/demo_capi/CMakeLists.txt

@@ -0,0 +1,27 @@
+cmake_minimum_required(VERSION 2.8.12...3.29)
+
+project(DemoCapiExtension)
+
+include_directories(include)
+
+# Configure extension: the minimum CAPI version where the extension still
+# compiles should be chosen here
+set(EXTENSION_NAME demo_capi)
+
+set(EXTENSION_FILES add_numbers.cpp capi_demo.cpp)
+
+option(
+  USE_UNSTABLE_C_API
+  "Use the unstable C Extension API (extension will be tied to exact DuckDB version)"
+  TRUE)
+
+if(USE_UNSTABLE_C_API)
+  build_loadable_extension_capi_unstable(${EXTENSION_NAME} ${EXTENSION_FILES})
+else()
+  # Minimum supported DuckDB version
+  set(CAPI_MAJOR 1)
+  set(CAPI_MINOR 2)
+  set(CAPI_PATCH 0)
+  build_loadable_extension_capi(${EXTENSION_NAME} ${CAPI_MAJOR} ${CAPI_MINOR}
+                                ${CAPI_PATCH} ${EXTENSION_FILES})
+endif()

external/duckdb/extension/demo_capi/add_numbers.cpp

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+#include "add_numbers.h"
+
+#include "duckdb_extension.h"
+
+DUCKDB_EXTENSION_EXTERN
+
+// Scalar function that adds two numbers together
+static void AddNumbersTogether(duckdb_function_info info, duckdb_data_chunk input, duckdb_vector output) {
+	// get the total number of rows in this chunk
+	idx_t input_size = duckdb_data_chunk_get_size(input);
+	// extract the two input vectors
+	duckdb_vector a = duckdb_data_chunk_get_vector(input, 0);
+	duckdb_vector b = duckdb_data_chunk_get_vector(input, 1);
+	// get the data pointers for the input vectors (both int64 as specified by the parameter types)
+	auto a_data = (int64_t *)duckdb_vector_get_data(a);
+	auto b_data = (int64_t *)duckdb_vector_get_data(b);
+	auto result_data = (int64_t *)duckdb_vector_get_data(output);
+	// get the validity vectors
+	auto a_validity = duckdb_vector_get_validity(a);
+	auto b_validity = duckdb_vector_get_validity(b);
+	if (a_validity || b_validity) {
+		// if either a_validity or b_validity is defined there might be NULL values
+		duckdb_vector_ensure_validity_writable(output);
+		auto result_validity = duckdb_vector_get_validity(output);
+		for (idx_t row = 0; row < input_size; row++) {
+			if (duckdb_validity_row_is_valid(a_validity, row) && duckdb_validity_row_is_valid(b_validity, row)) {
+				// not null - do the addition
+				result_data[row] = a_data[row] + b_data[row];
+			} else {
+				// either a or b is NULL - set the result row to NULL
+				duckdb_validity_set_row_invalid(result_validity, row);
+			}
+		}
+	} else {
+		// no NULL values - iterate and do the operation directly
+		for (idx_t row = 0; row < input_size; row++) {
+			result_data[row] = a_data[row] + b_data[row];
+		}
+	}
+}
+
+// Register the AddNumbersFunction
+void RegisterAddNumbersFunction(duckdb_connection connection) {
+	// create a scalar function
+	auto function = duckdb_create_scalar_function();
+	duckdb_scalar_function_set_name(function, "add_numbers_together");
+
+	// add a two bigint parameters
+	duckdb_logical_type type = duckdb_create_logical_type(DUCKDB_TYPE_BIGINT);
+	duckdb_scalar_function_add_parameter(function, type);
+	duckdb_scalar_function_add_parameter(function, type);
+
+	// set the return type to bigint
+	duckdb_scalar_function_set_return_type(function, type);
+
+	duckdb_destroy_logical_type(&type);
+
+	// set up the function
+	duckdb_scalar_function_set_function(function, AddNumbersTogether);
+
+	// register and cleanup
+	duckdb_register_scalar_function(connection, function);
+	duckdb_destroy_scalar_function(&function);
+}

external/duckdb/extension/demo_capi/capi_demo.cpp

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+#include "add_numbers.h"
+#include "duckdb_extension.h"
+
+DUCKDB_EXTENSION_ENTRYPOINT(duckdb_connection connection, duckdb_extension_info info, duckdb_extension_access *access) {
+
+	// Register a demo function
+	RegisterAddNumbersFunction(connection);
+
+#ifdef DUCKDB_EXTENSION_API_VERSION_UNSTABLE
+	// Test using the unstable API
+	duckdb_arrow result;
+	auto api_result = duckdb_query_arrow(connection, "SELECT 1 as a", &result);
+
+	if (api_result != duckdb_state::DuckDBSuccess) {
+		access->set_error(info, "Arrow Query failed during initialization");
+		return false;
+	}
+
+	duckdb_destroy_arrow(&result);
+#endif
+
+	// Return true to indicate succesful initialization
+	return true;
+}

external/duckdb/extension/demo_capi/include/add_numbers.h

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+#pragma once
+
+#include "duckdb_extension.h"
+
+void RegisterAddNumbersFunction(duckdb_connection connection);