how to clean old log from logcat, the logcat -c doesn't work

from cmd: C:\android-sdk-w开发者_如何学JAVAindows\platform-tools>adb shell

and then logcat -b radio

it prints old historic log. I tried to call logcat -c like this: 1. adb shell 2. logcat -c and it returns no error 3. exit and reenter adb shell 4. logcat -b radio(I got the same with or without step 3)

and it didn't work it always got the old log as well. Any idea what is wrong? Thanks!

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"logcat -b radio -c",to chean the old radio log,and then "logcat -b radio".