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Dart FFI Calls

FFI lets Dart call native code directly. Dart opens a native library, looks up a function in it, and calls that function like a Dart function.

Looking up the abs function

The program opens the library and looks up abs with its two typedefs. Calling the function on -42 prints 42.

import 'dart:ffi';

typedef AbsNative = Int32 Function(Int32);
typedef AbsDart = int Function(int);

void main() {
  var lib = DynamicLibrary.open('libc.so.6');
  var abs = lib.lookupFunction<AbsNative, AbsDart>('abs');
  print(abs(-42));
}

Abs over several values

The same looked-up function runs three times. A positive number and zero pass through unchanged, and -9 becomes 9.

import 'dart:ffi';

typedef AbsNative = Int32 Function(Int32);
typedef AbsDart = int Function(int);

void main() {
  var lib = DynamicLibrary.open('libc.so.6');
  var abs = lib.lookupFunction<AbsNative, AbsDart>('abs');
  print(abs(7));
  print(abs(0));
  print(abs(-9));
}

Absolute differences of pairs

Each pair is a record of two numbers. The program subtracts the pair and passes the difference to abs, so every result is positive or zero.

import 'dart:ffi';

typedef AbsNative = Int32 Function(Int32);
typedef AbsDart = int Function(int);

void main() {
  var lib = DynamicLibrary.open('libc.so.6');
  var abs = lib.lookupFunction<AbsNative, AbsDart>('abs');
  var pairs = [(10, 4), (3, 8), (6, 6)];
  for (var (a, b) in pairs) {
    print(abs(a - b));
  }
}

Example

import 'dart:ffi';

typedef AbsNative = Int32 Function(Int32);
typedef AbsDart = int Function(int);

void main() {
  var lib = DynamicLibrary.open('libc.so.6');
  var abs = lib.lookupFunction<AbsNative, AbsDart>('abs');
  var readings = [-12, 5, -3, 0, 8];
  for (var reading in readings) {
    print(abs(reading));
  }
}

Key points

  • FFI calls native code directly from Dart.
  • DynamicLibrary.open opens the library, and lookupFunction finds one function in it by name.
  • Two typedefs describe the function, one in native types and one in Dart types.
  • The library name is platform-specific.
  • The name libc.so.6 is the Linux name, and the shipped code in this lesson is the Linux shape.
  • Only integer functions are shown in this lesson.
  • String and pointer functions need allocation helpers from a package, which this course does not use.

In this example, the looked-up abs function normalises each reading in the list. The negative readings -12 and -3 print as 12 and 3, and the other readings print unchanged.

Try it Yourself

Edit the code, then press Run.

Running is simulated · DartPad

Exercise

Why does this lesson show only integer functions through FFI?