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Date: Fri, 17 Apr 2009 06:51:45 -0400 (EDT)
From: Steven Rostedt <rostedt@goodmis.org>
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To: Peter Zijlstra <a.p.zijlstra@chello.nl>
cc: =?ISO-8859-15?Q?Fr=E9d=E9ric_Weisbecker?= <fweisbec@gmail.com>,
       Ingo Molnar <mingo@elte.hu>, Zhaolei <zhaolei@cn.fujitsu.com>,
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       KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>,
       LKML <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH 1/2 v2] tracing/events: provide string with undefined
 size  support
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On Fri, 17 Apr 2009, Peter Zijlstra wrote:

> On Fri, 2009-04-17 at 06:10 -0400, Steven Rostedt wrote:
> 
> > When I replied to Frederic, I thought I could come up with a way to do 
> > something like you are proposing. Instead, I only ended up with the 
> > variant that Frederic implemented.
> > 
> > I've done what you are suggesting several times in tracing. Logdev does 
> > this in its tracing.
> > 
> > The problem that we have, is that we don't have actual code. We have a 
> > TRACE_EVENT macro that is doing the work for us. This, unfortunately, 
> > limits what we can do.
> 
> Can't you do things like:
> 
> #define __string(x) unsigned long length_##x, offset_##x;
> 
>   TP_STRUCT__entry
> 
>   size = sizeof(struct foo);
> 
> #define __string(x) length_##x = strlen(x) + 1; size += length_##x;
> 
>   TP_STRUCT__entry
> 
>   entry = ring_buffer_lock_reserve(size);
>   offset = sizeof(stuct foo);
> 
> #define __string(x) \
> 	__entry->x = offset_##x = offset; offset += length_##x;
> 
>   TP_STRUCT__entry
> 
>   TP_fast_assign
> 
> #define _string(x) strcpy(&entry->data[offset_##x], x);
> 
>   TP_STRUCT__entry
> 
>   ring_buffer_unlock();
> 
> also, you don't need that __ending_string() thing, you can always end a
> struct with char data[0], its 0 size ;-)
> 

This looks very similar to what I suggested in the cross email.

Frederic,

You want to take a crack at implementing this. I think my description had 
a bit more details, but is basically the same as what Peter is describing 
here.

I'd just make __string become an offset (as described in my email)

Then the user could just index directly. They only need to worry about the 
print, the assignment would be automated.

-- Steve