Develop Reference

Android JNI print int64_t to file

I am trying to use JNI code to print Android sensor timestamps to a file.
The timestamps are defined as int64_t.
The line to print the timestamp is:
fout<<timestamp<<std::endl;
Where fout is an open file stream in output mode.
When I try to compile, the following error results:
ambiguous overload for 'operator<<' (operand types are 'std::ofstream {aka std::basic_ofstream >}' and 'int64_t {aka long long int}') XXX.cpp /YYY/jni line ZZZ C/C++ Problem
I thought I would make an MWE for this using standard C++:
#include <iostream>
#include <cstdint>
int main(){
int64_t a;
std::cin>>a;
std::cout<<a<<std::endl;
}
But this compiles without an issue, which makes me think the problem is somehow in the way Eclipse compiles the code.
I am inside Eclipse 3.8.1. My default compiler is GCC 4.9.2.

I solved this problem by going into jni/Application.mk and using this line:
LOCAL_CPPFLAGS += -std=gnu++11
instead of this line:
LOCAL_CPPFLAGS+=-std=c++11.
Some have conjectured that the problem is due to different mappings of long long int in stlport.

Using opencv in native code for Android app development

I have namespace error building with ndk-build for native code in my Android app. The error sample is
C:/adt-bundle-windows-x86/ndk/sources/cxx-stl/gnu-libstdc++/4.6/include/bits
/allocator.h:54:1: error: unknown type name 'namespace'
C:/adt-bundle-windows-x86/ndk/sources/cxx-stl/gnu-libstdc++/4.6/include/bits
/allocator.h:55:1: error: expected ',' or ';' before '{' token
For OpenCV settings, my Application.mk file is
APP_STL := gnustl_static
APP_CPPFLAGS := -frtti -fexceptions
APP_ABI := armeabi
APP_PLATFORM := android-10
That means I am using gnu-libstdc++ in compiling the native code.
My jni.c has c extension. That is I receive from my third party and they prefer in .c extension as if they have to change to .cpp extension, they have to change a lot in their other libraries.
So far is OK, I did all project settings for OpenCV for native development and if I do ndk-build, I can make .so libraries.
The problem of those namespace error happened when I include OpenCV's header file #include <opencv2/contrib/detection_based_tracker.hpp> into jni.c and I got a lot of name space error. If I include that #include <opencv2/contrib/detection_based_tracker.hpp> into cpp file, no error.
My questions are
(1)Is the error is because of using .hpp file in .c file?
(2)If I have no choice and have to use that .c file, is there way around to remove that error?
Thanks

My assumption would be that the file is compiled as a "C" file instead of a "C++" file because of the extension ".c". That means you cannot use any "C++" Code in your jni.c, wike classes or namespaces. These are obviously used however in your file "detection_based_tracker.hpp" that you are using.
So the problem is not that you include a file named ".hpp", but that this file contains "C++" code wich the "C" compiler cannot handle.
One solution to this problem is to only use the "C" functions in opencv (for example "opencv2/imgproc/imgproc_c.h" instead of "opencv2/imgproc/imgproc.hpp"). However, your function "detection_based_tracker.hpp" might not have a "C" version, as far as I can see.
The other option is to add a second file "function.cpp" with the "C++" functions that use opencv. The functions from "function.cpp" can be declared in a file "functions.h" and included in your "jni.c", so you can still use opencv c++ functions. Be careful to only use C style functions then, though (no classes, namespaces, ...) in your "function.h" file, otherwise you will have the same problems as before.

Wrapp existing library (.so) in jni for android app

I wanna realize this idea. I spent several days searching for information, but could not find anything. All tutorials say how to write my own library with JNI, but how to wrap already existing? I need just simple tutorial step by step (and why? if it possible). So I wanna start create native android application.
What I have :
I create C++ library in QTCreator by tutorial from youtube: simple library on C++ (.so) with headers (.h) which do simple cout in console:
Not compiled code mylib.cpp:
#include "mylib.h"
MyLib::MyLib() { }
void MyLib::Test() {
qDebug() << "Hello from our DLL";
// .so
}
Header mylib.h:
#ifndef MYLIB_H
#define MYLIB_H
#include "mylib_global.h"
#include <QDebug>
class MYLIBSHARED_EXPORT MyLib
{
public:
MyLib();
void Test();
};
#endif // MYLIB_H
and mylib_global.h (I think it does't matter)
So after build I have myLib.so.
And now I need wrap it in my android app. So I don't understand what I need to do for it.
I'm develop in Android Studio. And what I know:
Create in java package LibWrappClass with native method - something like "simplePrint()":
public native void simplePrint();
I need to create in /src/main folder "jni". Create Android.mk, myLibWrapper.h and myLibWrapper.cpp. But I don't understand what I need to write in Android.mk for connect my myLib.so to "myLibWrapper.h", and where should I put my library with headers. Can anyone help?

After adding the native method in your java code, You simply build the project. Now you need to move to the location where the class files are written by your IDE. Since you use Android Studio, it must be somewhere your project folder with path
out/production/YourModuleName
Open the location in commandline and run the javah command to generate the header file for your native function
javah -d <your jni folder path> <com.YourPackage.YourClass>
The class YourClass should be where you have declared the native method. This command will create a header file with name something like com_YourPackage_YourClass.h with a function declaration looks like
JNIEXPORT void JNICALL Java_com_YourPackage_YourClass_simplePrint
(JNIEnv *, jobject);
Implement this function in a C/C++ file with whatever operations you have to perform on jni side.
Then, Define the Android.mk file, In this case it will be something like
LOCAL_PATH := $(call my-dir)
include $(CLEAR_VARS)
LOCAL_MODULE := lib
LOCAL_SRC_FILES := lib.c
include $(BUILD_SHARED_LIBRARY)
If you have multiple source files add them in LOCAL_SRC_FILES separated by space.
Next, go to your jni folder and build the project using command ndk-build. This will place the .so file inside the android folder libs/armeabi
Finally rebuild the Android project from Android studio and run.
For detailed instructions and complete source checkout this gist

how to enable ALOGD in android?

In CPP code, for ex in camera HAL, ALOGD messages are not being printed.
Like in set_preview_window()
ALOGD("set_preview_window : X, rc %d", rc);
How to enable them?

try to add following statements in the files which you wanted to catch logs.
#define LOG_NDEBUG 0
#define LOG_NIDEBUG 0
#define LOG_NDDEBUG 0
After compiled, you can try to get logs through adb logcat.

These macros (ALOGX...) are defined in some system core C/C++ headers.
You can find an example in system/core/include/log/log.h (AOSP 6.0.0r1)
Sometimes you need to add liblog in LOCAL_SHARED_LIBRARIES of the corresponding Android.mk :
LOCAL_SHARED_LIBRARIES := ... liblog
Also, you might need to add two lines at the top of C/C++ concerned source files :
#define LOG_NDEBUG 0
#define LOG_TAG "LibName"
Don't forget LOG_NDEBUG 0 if you want ALOGV() logs.
After rebuilding the lib/module, you should be able to see logs in your logcat.

See the definition of ALOGD to get a hint.
In my case i had to do
setprop persist.testapp.debug.log 5
5 was debug level.

To get logs from CPP to Android you ca use the following
__android_log_print(ANDROID_LOG_ERROR, "TRACKERS", "%s", Str);
and to use this you need to import the following library
#include <android/log.h>
if you Want o get simple logs you ca use the following
LOG("Add description here");

Android - NDK strange error

I start getting funny errors with NDK with fairly complex program:
#include <stdio.h>
#include <iostream>
int main( int argc, char **argv ) {
return 0;
}
>call c:\android-ndk-r9\ndk-build.cmd
"Compile++ thumb : test <= test.cpp
In file included from C:/workspace/c++11_test//jni/test.cpp:11:
In file included from c:/android-ndk-r9/sources/cxx-stl/stlport/stlport\iostream:43:
In file included from c:/android-ndk-r9/sources/cxx-stl/stlport/stlport\stl/_istream.h:31:
In file included from c:/android-ndk-r9/sources/cxx-stl/stlport/stlport\stl/_ostream.h:380:
In file included from c:/android-ndk-r9/sources/cxx-stl/stlport/stlport\stl/_ostream.c:26:
In file included from c:/android-ndk-r9/sources/cxx-stl/stlport/stlport\stl/_num_put.h:180:
In file included from c:/android-ndk-r9/sources/cxx-stl/stlport/stlport\stl/_num_put.c:26:
c:/android-ndk-r9/sources/cxx-stl/stlport/stlport\stl/_limits.h:217:48: error: non-type template argument evaluates to -2147483648, which cannot be narrowed to type 'wchar_t'
[-Wc++11-narrowing]
: public _STLP_PRIV _Integer_limits<wchar_t, WCHAR_MIN, WCHAR_MAX, -1, true>
^
c:/android-ndk-r9/platforms/android-14/arch-arm/usr/include\../include/wchar.h:76:22: note: expanded from macro 'WCHAR_MIN'
#define WCHAR_MIN INT_MIN
^
c:/android-ndk-r9/platforms/android-14/arch-arm/usr/include\sys/limits.h:69:18: note: expanded from macro 'INT_MIN'
#define INT_MIN (-0x7fffffff-1) /* min value for an int */
^
1 error generated.
make: * [C:/workspace/c++11_test//obj/local/armeabi/objs/test/test.o] Error 1
It's ndk-r9. 4.8 complie it just fine, only clang stumbles on it.
Do I need to define something to make clang work?
I've tried to Google errors, but got nothing relevant...
Obviously, I can turn it off with -Wno-c++11-narrowing, but I don't want disable narrowing checks.
And it's only shows with stlport_static, there is NO error with gnustl_static

You appear to have enabled C++11, since you are getting this warning.
STLport support for C++11 is not very good. I suspect this is the reason for your woes.
A fix requires changes to STLport, or removing the "-std=c++11" switch from the clang command-line.
IMHO, I would recommend switching to the gnustl (aka. libstdc++), unless there are other reasons that prevent you from doing so (licensing: libstdc++ is GPL3 with some significant exceptions that may or may not be valid for clang - you should determine that for yourself), legacy support, size etc.). I have had very good experiences with that.