SQLite aggregate function example

aggsum.c

// gcc -shared aggsum.c -o aggsum.dll -s -static
#include "sqlite3ext.h"
SQLITE_EXTENSION_INIT1

static void sum_step (sqlite3_context *ctx, int argc, sqlite3_value **argv) {
	double* total = (double*)sqlite3_aggregate_context(ctx, sizeof(double));
	if (total == NULL) 
		return sqlite3_result_error_nomem(ctx);

	*total += sqlite3_value_double(argv[0]);	
}

static void sum_final (sqlite3_context *ctx) {
	double* total = (double*)sqlite3_aggregate_context(ctx, sizeof(double));
	sqlite3_result_double(ctx, *total);
}

#ifdef _WIN32
__declspec(dllexport)
#endif
int sqlite3_aggsum_init(sqlite3 *db, char **pzErrMsg, const sqlite3_api_routines *pApi) {
	SQLITE_EXTENSION_INIT2(pApi);
	return sqlite3_create_function (db, "aggsum", 1, SQLITE_UTF8, NULL, NULL, sum_step, sum_final);
}