Re: [dpdk-dev] [PATCH] eal: remove variable length array

[Jeff Shaw <jeffrey.b.shaw@intel.com>]

On Fri, Dec 14, 2018 at 10:59:28AM -0800, Jeff Shaw wrote:
> On Fri, Dec 14, 2018 at 10:36:03AM -0800, Stephen Hemminger wrote:
> > On Fri, 14 Dec 2018 08:38:27 -0800
> > Jeff Shaw <jeffrey.b.shaw@intel.com> wrote:
> > 
> > > Compilers that do not support the C11 standard, or do not implement
> > > gcc extensions, may not support variable length arrays.
> > > 
> > > The code prior to this commit produced the following warning when
> > > compiled with "-Wvla -std=c90".
> > > 
> > >   warning: ISO C90 forbids variable length array ‘array’ [-Wvla]
> > > 
> > > This commit removes the variable length array from the PMD debug
> > > trace function by allocating memory dynamically on the stack using
> > > alloca().
> > > 
> > > Signed-off-by: Jeff Shaw <jeffrey.b.shaw@intel.com>
> > > ---
> > >  lib/librte_eal/common/include/rte_dev.h | 19 +++++++++----------
> > >  1 file changed, 9 insertions(+), 10 deletions(-)
> > > 
> > > diff --git a/lib/librte_eal/common/include/rte_dev.h b/lib/librte_eal/common/include/rte_dev.h
> > > index a9724dc91..af772872b 100644
> > > --- a/lib/librte_eal/common/include/rte_dev.h
> > > +++ b/lib/librte_eal/common/include/rte_dev.h
> > > @@ -47,22 +47,21 @@ __attribute__((format(printf, 2, 0)))
> > >  static inline void
> > >  rte_pmd_debug_trace(const char *func_name, const char *fmt, ...)
> > >  {
> > > +	char *buffer;
> > > +	int buf_len;
> > >  	va_list ap;
> > >  
> > >  	va_start(ap, fmt);
> > > +	buf_len = vsnprintf(NULL, 0, fmt, ap) + 1;
> > > +	va_end(ap);
> > >  
> > > -	{
> > > -		char buffer[vsnprintf(NULL, 0, fmt, ap) + 1];
> > > +	buffer = (char *)alloca(buf_len);
> > 
> > alloca is void * so cast is not necessary.
> 
> I get a compiler warning on Windows, hence the cast. I suppose we could
> disable the warning.
> 
> > 
> > You might be able to skip the buffering step entirely by using rte_log
> > a little more creatively, see how other logging works.
> 
> Probably right... this eventuall needs to be done in ~100 other places,
> most of which aren't used with rte_log(), so it seemed fitting.
> 
> > 
> > But to go further since rte_pmd_debug_trace is not used anywhere in
> > current code base in DPDK, it should just be removed.
> 
> Much better solution :)

Actually, it cannot be removed. It is used by librte_eventdev. When
RTE_LIBRTE_EVENTDEV_DEBUG is defined, the RTE_PMD_DEBUG_TRACE macro
uses rte_pmd_debug_trace.

I will remove the cast and update the commit message to reference C99
instead of C11 as requested by Mattias R.