if i am defining such function in java file
/**
* Adds two integers, returning their sum
*/
public native int add( int v1, int v2 );
so i need to code in c file
JNIEXPORT jint JNICALL Java_com_marakana_NativeLib_add
(JNIEnv * env, jobject obj, jint value1, jint value2) {
printf("\n this is log messge \n");
return (value1 + value2);
}
then from where this printf will print it message ? In logcate i dont get it?
How can i debug any NDK application by putting log messages?
use __android_log_print()
instead. You have to include header <android/log.h>
Sample Example. __android_log_print(ANDROID_LOG_DEBUG, "LOG_TAG", "\n this is log messge \n");
You can also use format specifier like printf -
__android_log_print(ANDROID_LOG_DEBUG, "LOG_TAG", "Need to print : %d %s",int_var, str_var);
Make sure you also link against the logging library, in your Android.mk file:
LOCAL_LDLIBS := -llog
Ohh.. forgot .. The output will be shown in Logcat
with tag LOG_TAG
Easy Approach
Add the following lines to your common header file.
#include <android/log.h>
#define LOG_TAG "your-log-tag"
#define LOGD(...) __android_log_print(ANDROID_LOG_DEBUG, LOG_TAG, __VA_ARGS__)
#define LOGE(...) __android_log_print(ANDROID_LOG_ERROR, LOG_TAG, __VA_ARGS__)
// If you want you can add other log definition for info, warning etc
Now just call LOGD("Hello world") or LOGE("Number = %d", any_int)
like printf in c
.
Don't forget to include the common header file.
Remove the logging
If you define LOGD(...)
empty, all logs will be gone. Just comment after LOGD(...)
.
#define LOGD(...) // __android_log..... rest of the code
There is two options:
1) replace printf with __android_log_print. You can do this easily with define at beginning of code:
#define printf(...) __android_log_print(ANDROID_LOG_DEBUG, "TAG", __VA_ARGS__);
Of course this will require changing all source code that has printf.
2) redirect stdout and stderr to Android logcat (not sure if this will work on non-rooted device): http://developer.android.com/guide/developing/debugging/debugging-log.html#viewingStd
not need to root device, acroding to http://developer.android.com/guide/developing/debugging/debugging-log.html#viewingStd, below can work prefectly.
$ adb shell
$ su
$ stop
$ setprop log.redirect-stdio true
$ start
done!
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