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dart:ffi SQLite mini tutorial

In this mini tutorial we learn how to bind SQLite, a native library, in Dart using Dart's new foreign function interface dart:ffi. We build a package which provides a Dartlike SQLite API using objects and Iterators. Inside the package we write Dart code which directly invokes C functions and manipulates C memory.

Binding C Functions to Dart

The first step is to load a Native Library:

import "dart:ffi";

DynamicLibrary sqlite = dlopenPlatformSpecific("sqlite3");

In a DynamicLibrary we can lookup functions. Let's lookup the function sqlite3_prepare_v2 in the SQLite library. That function has the following signature in the library header file.

SQLITE_API int sqlite3_prepare_v2(
  sqlite3 *db,            /* Database handle */
  const char *zSql,       /* SQL statement, UTF-8 encoded */
  int nByte,              /* Maximum length of zSql in bytes. */
  sqlite3_stmt **ppStmt,  /* OUT: Statement handle */
  const char **pzTail     /* OUT: Pointer to unused portion of zSql */
);

In order to lookup a function, we need a C signature and a Dart signature.

typedef sqlite3_prepare_v2_native_t = Int32 Function(
    DatabasePointer database,
    CString query,
    Int32 nbytes,
    Pointer<StatementPointer> statementOut,
    Pointer<CString> tail);

typedef Sqlite3_prepare_v2_t = int Function(
    DatabasePointer database,
    CString query,
    int nbytes,
    Pointer<StatementPointer> statementOut,
    Pointer<CString> tail);

With these two signatures we can lookup the C function and expose it as a Dart function with asFunction.

Sqlite3_prepare_v2_t sqlite3_prepare_v2 = sqlite
    .lookup<NativeFunction<sqlite3_prepare_v2_native_t>>("sqlite3_prepare_v2")
    .asFunction();

Browse the code: platform specific dynamic library loading, C signatures, Dart signatures and bindings, and dart:ffi dynamic library interface.

Managing C Memory

In order to call sqlite3_prepare_v2 to prepare a SQLite statement before executing, we need to be able to pass C pointers to C functions.

Database and Statement pointers are opaque pointers in the SQLite C API. We specify these as classes extending Pointer<Void>.

class DatabasePointer extends Pointer<Void> {}
class StatementPointer extends Pointer<Void> {}

Strings in C are pointers to character arrays.

class CString extends Pointer<Uint8> {}

Pointers to C integers, floats, an doubles can be read from and written through to dart:ffi. However, before we can write to C memory from dart, we need to allocate some memory.

Pointer<Uint8> p = allocate(); // Infers type argument allocate<Uint8>(), and allocates 1 byte.
p.store(123);                  // Stores a Dart int into this C int8.
int v = p.load();              // Infers type argument p.load<int>(), and loads a value from C memory.

Note that you can only load a Dart int from a C Uint8. Trying to load a Dart double will result in a runtime exception.

We've almost modeled C Strings. The last thing we need is to use this Pointer as an array. We can do this by using elementAt.

CString string = allocate(count: 4).cast(); // Allocates 4 bytes and casts it to a string.
string.store(73);                           // Stores 'F' at index 0.
string.elementAt(1).store(73);              // Stores 'F' at index 1.
string.elementAt(2).store(70);              // Stores 'I' at index 2.
string.elementAt(3).store(0);               // Null terminates the string.

We wrap the above logic of allocating strings in the constructor CString.allocate.

Now we have all ingredients to call sqlite3_prepare_v2.

Pointer<StatementPointer> statementOut = allocate();
CString queryC = CString.allocate(query);
int resultCode = sqlite3_prepare_v2(
    _database, queryC, -1, statementOut, fromAddress(0));

With dart:ffi we are responsible for freeing C memory that we allocate. So after calling sqlite3_prepare_v2 we read out the statement pointer, and free the statement pointer pointer and CString which held the query string.

StatementPointer statement = statementOut.load();
statementOut.free();
queryC.free();

Browse the code: CString class, code calling sqlite3_prepare_v2, and dart:ffi pointer interface.

Dart API

We would like to present the users of our package with an object oriented API - not exposing any dart:ffi objects to them.

The SQLite C API returns a cursor to the first row of a result after executing a query. We can read out the columns of this row and move the cursor to the next row. The most natural way to expose this in Dart is through an Iterable. We provide our package users with the following API.

class Result implements Iterable<Row> {}

class Row {
  dynamic readColumnByIndex(int columnIndex) {}
  dynamic readColumn(String columnName) {}
}

However, this interface does not completely match the semantics of the C API. When we start reading the next Row, we do no longer have access to the previous Row. We can model this by letting a Row keep track if its current or not.

class Row {
  bool _isCurrentRow = true;

  dynamic readColumnByIndex(int columnIndex) {
    if (!_isCurrentRow) {
      throw Exception(
          "This row is not the current row, reading data from the non-current"
          " row is not supported by sqlite.");
    }
    // ...
    }
}

A second mismatch between Dart and C is that in C we have to manually release resources. After executing a query and reading its results we need to call sqlite3_finalize(statement).

We can take two approaches here, either we structure the API in such a way that users of our package (implicitly) release resources, or we use finalizers to release resources. In this tutorial we take the first approach.

If our users iterate over all Rows, we can implicitly finalize the statement after they are done with the last row. However, if they decide they do not want to iterate over the whole result, they need to explicitly state this. In this tutorial, we use the ClosableIterator abstraction for Iterators with backing resources that need to be closed.

Result result = d.query("""
  select id, name
  from Cookies
  ;""");
for (Row r in result) {
  String name = r.readColumn("name");
  print(name);
}
// Implicitly closes the iterator.

result = d.query("""
  select id, name
  from Cookies
  ;""");
for (Row r in result) {
  int id = r.readColumn("id");
  if (id == 1) {
    result.close(); // Explicitly closes the iterator, releasing underlying resources.
    break;
  }
}

Browse the code: Database, Result, Row, and CloseableIterator.

Architecture Overview

The following diagram summarized what we have implemented as package developers in this tutorial.

architecture

As the package developers wrapping an existing native library, we have only written Dart code - not any C/C++ code. We specified bindings to the native library. We have provided our package users with an object oriented API without exposing any dart:ffi objects. And finally, we have implemented the package API by calling the C API.

Current dart:ffi Development Status

In this minitutorial we used these dart:ffi features:

  • Loading dynamic libararies and looking up C functions in these dynamic libraries.
  • Calling C functions, with dart:ffi automatically marshalling arguments and return value.
  • Manipulating C memory through Pointers with allocate, free, load, store, and elementAt.

Features which we did not use in this tutorial:

  • @struct on subtypes of Pointer to define a struct with fields. (However, this feature is likely to change in the future.)

Features which dart:ffi does not support yet:

  • Callbacks from C back into Dart.
  • Finalizers
  • C++ Exceptions (Not on roadmap yet.)

Platform limitations:

  • dart:ffi is only enabled on 64 bit Windows, Linux, and MacOS. (Arm64 and 32 bit Intel are under review.)
  • dart:ffi only works in JIT mode, not in AOT.

It is possible to work around some of the current limitations by adding a C/C++ layer. For example we could catch C++ exceptions in a C++ layer, and rethrow them in Dart. The architecture diagram would change to the following in that case.

architecture2

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