linux-kernel - Re: [PATCH 1/2 v2] tracing/events: provide string with undefined size support

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Date:	Fri, 17 Apr 2009 12:39:34 +0200
From:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
To:	Steven Rostedt <rostedt@...dmis.org>
Cc:	Frédéric Weisbecker <fweisbec@...il.com>,
	Ingo Molnar <mingo@...e.hu>, Zhaolei <zhaolei@...fujitsu.com>,
	Tom Zanussi <tzanussi@...il.com>,
	Li Zefan <lizf@...fujitsu.com>,
	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 1/2 v2] tracing/events: provide string with undefined
 size  support

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 ;-)

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