linux-kernel - "cat /proc/20502/wchan" vs strace vs man page

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Message-ID: <9b0752e70709240054t7b21928eh3b4215f39cd328a5@mail.gmail.com>
Date:	Mon, 24 Sep 2007 09:54:17 +0200
From:	"Johan Walles" <johan.walles@...il.com>
To:	linux-kernel@...r.kernel.org
Subject: "cat /proc/20502/wchan" vs strace vs man page

Try this:
1. At one prompt, do "cat".
Now, switch to another prompt and...
2. Do "pstree -p|grep cat" to find out the PID of your cat command.  I
get 20502.
3. Do "cat /proc/20502/wchan".  I get "0" here.
4. Do "strace -p 20502".  I get "read(0,  <unfinished ...>" here.

So wchan says cat is waiting for "0".  Strace says cat is waiting for
read (which sounds much more probable to me).

Is "0" the correct contents of /proc/20502/wchan?  I expected it to
say something about "read".

According to "man proc", wchan "is the "channel" in which the process
is waiting.  It is the address of a system call".  "0" doesn't seem to
fit that description.  Is "0" or the man page wrong?

I'm on Ubuntu 7.04 Feisty, i686, have manpages "2.39-1", kernel "Linux
version 2.6.20-16-generic (root@...ranova) (gcc version 4.1.2 (Ubuntu
4.1.2-0ubuntu4)) #2 SMP Fri Aug 31 00:55:27 UTC 2007".

I'm not subscribed to the list, please CC me on replies.

  Cheers //Johan
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