Develop Reference

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

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.

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");
}

Class 'stat64' doesn't have constructor stat64(const char *,stat64*)

I am trying to build my android application with support for OpenSceneGraph. I am trying to render a simple box from my project's raw folder. However, when I build the project I am getting this compile error in the FileUtils class of the OpenSceneGraph library:
Class 'stat64' doesn't have constructor stat64(const char *,stat64*)
Any ideas what is causing the problem?

I'm also faced with such problem when I have tried to compile OpenSceneGraph to Android. The problem is that NDK's <sys/stat.h> header for Android API less that 21 doesn't contain stat64 function definition so compiler thinks that stat64 is a struct. Problem can be solved by adding
#if defined(ANDROID) || defined(__ANDROID__)
#if __ANDROID_API__ < 21
#define stat64 stat
#endif
#endif
after src/osgDB/FileUtils.cpp:92.

Android NDK - JNI - HAL

On Android NDK project we need to access bluetooth using a HAL interface. Our intention is to do something like this:
#include <hardware/hardware.h>
#include <hardware/bluetooth.h>
....
void load_Module(....)
{
hw_module_t* module;
hw_device_t* device;
bdt_log("Loading HAL lib + extensions");
err = hw_get_module(BT_HARDWARE_MODULE_ID, (hw_module_t const**)&module);
if (err == 0){
err = module->methods->open(module, BT_HARDWARE_MODULE_ID, &device);
if (err == 0) {
bt_device = (bluetooth_device_t *)device;
sBtInterface = bt_device->get_bluetooth_interface();
}
}
}
Everybody seems to do the same (in google example code). There appears to be a problem when we try to compile:
#include <hardware/hardware.h> no such file
We properly added a permission to manifest and properly added libraries at Android.mk
Do we need to add something more? Is there a working project that I can refer to?

You're trying to use libhardware on Android NDK project. libhardware is a part of Android framework, not a part of NDK.
https://github.com/android/platform_hardware_libhardware
So your project doesn't have true binary compatibility of Android if you can build your project with libhardware. It would work on some devices, some versions, but no guarantee.
From my experience, I believe you can build your code on AOSP. You need to modify Android.mk for AOSP not NDK.

"Invalid arguments" when porting C++ code to Android

I am trying to port the following piece of C++ code to Android as part of some NDK work:
#include <fstream>
// ...
inline void trim(string & str, const string & delim = " \t\r\n")
{
string::size_type pos = str.find_last_not_of(delim);
if(pos == string::npos) str.erase(str.begin(), str.end());
else
{
str.erase(pos+1);
pos = str.find_first_not_of(delim);
if(pos != string::npos) str.erase(0, pos);
}
}
What I get is the following message at str.erase(pos+1); and str.erase(0, pos):
Invalid arguments '
Candidates are:
std::basic_string<char,std::char_traits<char>,std::allocator<char>> & erase(?, ?)
__gnu_cxx::__normal_iterator<char *,std::basic_string<char,std::char_traits<char>,std::allocator<char>>> erase(__gnu_cxx::__normal_iterator<char *,std::basic_string<char,std::char_traits<char>,std::allocator<char>>>)
__gnu_cxx::__normal_iterator<char *,std::basic_string<char,std::char_traits<char>,std::allocator<char>>> erase(__gnu_cxx::__normal_iterator<char *,std::basic_string<char,std::char_traits<char>,std::allocator<char>>>, __gnu_cxx::__normal_iterator<char *,std::basic_string<char,std::char_traits<char>,std::allocator<char>>>)
'
I tried what was suggested here but it did not help. Instead, now I am getting a new message for #include <fstream>:
Unresolved inclusion: <fstream>
What do I need to do to resolve this? Thank you.
UPDATE: I am trying to add this piece of code to an existing C++ code in an Android project that built and ran fine. When I applied the change linked to above, I not only got the unresolved inclusion for fstream, I also got a bunch of other errors. For instance, for the following block form the existing code:
#include <jni.h>
#include <opencv2/core/core.hpp>
#include <opencv2/imgproc/imgproc.hpp>
#include <opencv2/features2d/features2d.hpp>
#include <vector>
I am also getting Unresolved inclusion errors. So it made things worse.

Try to add the following line to Application.mk of your project.
APP_STL := gnustl_static

Build issue when using <cmath> with android ndk

I have a simple file stlTest2.cpp like this:
#include <jni.h>
#include <cmath>
bool isnan (void);
There is something more complicated in some code I am porting. My question is this. Why would this work when building using GCC outside of the NDK, but not with using the NDK? There error it gives is this:
jni/stlTest2.cpp:6: error: expected unqualified-id before 'sizeof'
jni/stlTest2.cpp:6: error: expected ')' before 'sizeof'
The immediate reason for this is that math.h (included via <cmath>) defines isnan as a macro. Why is the build outside of the ndk not including the #define from math.h, but this is? If I comment out the includes in the code, all is fine, but that is not acceptable as this problem repeats itself.... a lot.

The isnan macro was added in C99. In C++11 it was added as a function in the std namespace, overloaded for float and double and long double (so a typical <cmath> header, such as your non-NDK GCC is probably using, might have something like this:
#undef isnan
inline bool isnan(float ...) { ... }
inline bool isnan(double ...) { ... }
inline bool isnan(long double ...) { ... }
), but apparently the NDK hasn't gotten the memo, and is still providing the C99 macro as a convenience. (isnan was never a macro in C++, but before TR1 added std::tr1::isnan, there wasn't really a C++ alternative, so multiple C++ compilers provided the C99 macro.)
Do you need to be able to use the compiler-provided isnan or std::isnan? If not, then you can just change this:
#include <cmath>
to this:
#include <cmath>
#undef isnan
(and similarly for any other problematic macros, provided you don't need them).

In $ndk\sources\cxx-stl\gnu-libstdc++\libs\armeabi\include\bits\c++config.h (change armeabi to whatever is appropriate) change this:
/* #undef _GLIBCXX_USE_C99_MATH */
to
#define _GLIBCXX_USE_C99_MATH 1
Then clean and build your project again.