How do I Print to Console using dart:ffi in Flutter?

Question

Trying to print to console when running my Flutter app (on Android) from the following C++ code I am running through dart:ffi :

#include <iostream>

std::cout << "Hello, World!";

Does not give me any output in the terminal. How would I print to the Flutter terminal from C++?

---

I know my functions otherwise work correctly because I am getting correct actions on pointers / return values.

Answer 1

Edit: When only running on android, there is a simpler way. I have updated the answer below.

You'll have to pass a wrapper to the print function into the native code

void wrappedPrint(Pointer<Utf8> arg){
  print(Utf8.fromUtf8(arg));
}

typedef _wrappedPrint_C = Void Function(Pointer<Utf8> a);
final wrappedPrintPointer = Pointer.fromFunction<_wrappedPrint_C>(_wrappedPrint_C);

final void Function(Pointer) initialize =
  _nativeLibrary
    .lookup<NativeFunction<Void Function(Pointer)>>("initialize")
    .asFunction<void Function(Pointer)>();

initialize(wrappedPrintPointer);

and then use that in your C library:

void (*print)(char *);

void initialize(void (*printCallback)(char *)) {
    print = printCallback;
    print("C library initialized");
}

void someOtherFunction() {
    print("Hello World");
}

---

When only running on android, things get simpler. Instead of all of the above, do:

Simply use the android logging mechanism, it will show up on the console, at least when using flutter run . I'm assuming flutter attaches to logcat using the app's PID.

To do so, update the CMakeLists.txt with:

find_library( # Defines the name of the path variable that stores the
              # location of the NDK library.
              log-lib

              # Specifies the name of the NDK library that
              # CMake needs to locate.
              log )

# Links your native library against one or more other native libraries.
target_link_libraries( # Specifies the target library.
                       <your-libs-name-here>

                       # Links the log library to the target library.
                       ${log-lib} )

and in your c lib do:

#include <android/log.h>

void someOtherFunction() {
      __android_log_print(ANDROID_LOG_DEBUG, "flutter", "Hello world! You can use %s", "formatting");
}