This document discusses logging in Android systems. It describes how to log from Java programs using android.util.Log and from native programs using liblog. It provides an overview of Android's logging system including the log device files and how logs can be read with logcat or adb. It also provides some tips for logging like dumping stack traces, dealing with character encoding, using logwrappers, and logging from init processes.

1. Logging System of
Android
2010.8.27
2010.9.2 updated
Tetsuyuki Kobayashi
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2. Who am I?
 20+ years involved in embedded systems
 10 years in real time OS, such as iTRON
 10 years in embedded Java Virtual Machine
 Now GCC, Linux, QEMU, Android, …
 Blogs
 http://d.hatena.ne.jp/embedded/ (Personal)
 http://blog.kmckk.com/ (Corporate)
 Twitter
 @tetsu_koba
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3. Today's topic
 Log from Java program
 Log from native program
 Overview of Android Logging
system
 Tips
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4. Log from Java program
 android.util.Log class
 System.out, System.err

5. android.util.Log
 Static methods
 Log.e, Log.w, Log.i, Log.d, Log.v
 e,w,i,d,v mean priority/log level
 writing log with tag, priority
 Tag and priority is used by filtering
Static final String LOG_TAG = ”Hello”;
...
Log.i(LOG_TAG, ”Hello to log”);

6. System.out/System.err
 Default instance is
com.android.internal.os.AndroidPri
ntStream
 These connects to android.util.Log
Tips: How to identify instance of System.out
System.out.println(”System.out=” +
System.out.toString());

7. Log from native program
 Use liblog library
 Include <android/log.h>
 __android_log_print
 Writing log with tag, priority
#include <android/log.h>
#define LOGI(...)
__android_log_print(ANDROID_LOG_INFO,LOG_TAG,__VA_ARG
S__)
…
LIGI(”i=%d, name=%sn”, i, name);

8. Stdout/stderr of native program
 These are NOT connected to logging
system.
 Init process set stdout/stderr to
/dev/null for its child processes.
 All Java processes, too.
 JNI native libs, too.
 Do not use printf (to stdout) in JNI
native libs.

9. How to read logs
 Logcat command
 Adb logcat
 Logcat pane in ADT, Eclipse
 Filtering by tag and priority

10. Log device files
 4 channels, each have ring buffer
 /dev/log/radio – radio-related messages (64KB)
 /dev/log/event – system/hardware events (256KB)
 /dev/log/system – system/framwork messages (64KB)
 /dev/log/main – everything else (64KB)
 File permission of each is 0662
 owner/group RW, other Write only
 owner=root, group=log
 Anyone can write logs, root or log group can read
them

11. Overview of Android Logging System
Target
Java program
Java program System.out
/System.err
com.android.internal.os
Native program android.util.Log com.android.internal.os Host
Native program android.util.Log AndroidPrintStream
AndroidPrintStream
ADT in Eclipse
stdout
logcat
logcat
stdout liblog
/stderr liblog adbserver
adbd
adbd adbserver
User
Kernel adb logcat
main 64KB radio
logger
logger 64KB
/dev/log/main /dev/log/main
/dev/log/radio event /dev/log/radio
system
/dev/log/event 256KB /dev/log/event
/dev/log/system 64KB /dev/log/system

12. Where to read in source code
 android.util.Log
 frameworks/base/core/java/android/util/Log.java
 com.android.internal.os.AndroidPrintStream
 frameworks/base/core/java/com/android/internal/os/Android
PrintStream.java
 Liblog
 system/core/liblog/
 Logcat command
 system/core/logcat/
 Adb command
 system/core/adb/
 Kernel log driver
 drivers/staging/android/logger.c

13. Tips
 Dumping stack trace
 Character Encoding
 Logwrapper
 Log at init process

14. Dumping stack trace
 3 arguments fuctions in android.util.Log
class
You can put 'new Throwable()'
 For example:
 Log.e(String tag, String msg, Throwable tr)
 Throwable.printStacktrace() also works
 Dump to System.err
 See also
 http://blog.kmckk.com/archives/2902690.html

15. Character Encoding
 Fixed to Utf-8
 Not Shift-JIS, Not EUC
 Use Utf-8 encoding terminal/window to
use logcat
 adb logcat
 invoke 'logcat' at serial console
 Logcat pane in ADT in Eclipse seems bug.
 Can not show Japanese character correctly.

16. Logwrapper
 Logwrapper redirects stdout/stderr to
Android Logging system
 Easy way to add 'usual' command as
Android service lunched from init.
 See also
 http://blog.kmckk.com/archives/2918551.html

17. Log at init process
 The first process 'init' does not use
Android Logging System.
 Init writes log to (the same node as)
'/dev/kmsg'
 The same way as 'printk' : kernel log

18. Q&A
Thank you for listening!
Any comments to blogs are welcome.
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