Re: [PATCH 4/4] tracetool: show trace-events filename/lineno in fmt string errors

[Stefan Hajnoczi <stefanha@redhat.com>]

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On Thu, Aug 27, 2020 at 03:59:04PM +0100, Peter Maydell wrote:
> On Thu, 27 Aug 2020 at 15:29, Stefan Hajnoczi <stefanha@redhat.com> wrote:
> >
> > The compiler encounters trace event format strings in generated code.
> > Format strings are error-prone and therefore clear compiler errors are
> > important.
> >
> > Use the #line directive to show the trace-events filename and line
> > number in format string errors:
> > https://gcc.gnu.org/onlinedocs/gcc-10.2.0/cpp/Line-Control.html
> >
> > For example, if the cpu_in trace event's %u is changed to %p the
> > following error is reported:
> >
> >   trace-events:29:18: error: format ‘%p’ expects argument of type ‘void *’, but argument 7 has type ‘unsigned int’ [-Werror=format=]
> >
> > Line 29 in trace-events is where cpu_in is defined. This works for any
> > trace-events file in the QEMU source tree and the correct path is
> > displayed.
> >
> > Unfortunately there does not seem to be a way to set the column, so "18"
> > is not the right character on that line.
> 
> It's been pointed out to me that you could do this by
> making the generated code have suitable line breaks, padding,
> etc, so that the format string in the output ends up starting in
> the same column it was in the input trace file. Whether this is
> worthwhile I leave up to you :-)
> 
> (The argument number (7 in your example) is also of course off,
> and that I think we're also stuck with. Getting the file and line
> number right is a solid improvement on the current situation.)

Thanks for mentioning that trick. I will leave the patch series as-is
for now.

Stefan

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