Internal of logging system of Android. This is updated edition.

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
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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”);
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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());
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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);
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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. 8

9. How to read logs
 Logcat command
 Adb logcat
 Logcat pane in ADT, Eclipse
 Filtering by tag and priority
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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 10

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 11

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
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 drivers/staging/android/logger.c

13. Tips
 Dumping stack trace
 Character Encoding
 Logwrapper
 Log at init process
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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
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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.
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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
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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
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18. Q&A
Thank you for listening!
Any comments to blogs are welcome.
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