Re: [PATCH v2] eal: remove variable length array

["Wiles, Keith" <keith.wiles@intel.com>]

> On Dec 14, 2018, at 2:40 PM, Jeff Shaw <jeffrey.b.shaw@intel.com> wrote:
> 
> Compilers that do not support the C99 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>
> ---
> 
> V2:
> - Reference C99 in commit message instead of C11.
> - Remove unnecessary cast of alloca() returning void *.
> 
> ---
> 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..6d7a220b0 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 = alloca(buf_len);

Looks like some formatting problems with tab and/or spaces. DPDK is tabs of size 8 and no spaces before tabs.

The code looks fine to me, except for the calling vsnprintf twice. Could we have used char buffer[SOME_NUMBER_THAT_IS_REASONABLE];

This would remove the two calls to vsnprintf() and alloca usage.

If everyone is OK with it then I am ok after the formatting problems get fixed.
> 
> -		va_end(ap);
> -
> -		va_start(ap, fmt);
> -		vsnprintf(buffer, sizeof(buffer), fmt, ap);
> -		va_end(ap);
> +	va_start(ap, fmt);
> +	vsnprintf(buffer, buf_len, fmt, ap);
> +	va_end(ap);
> 
> -		rte_log(RTE_LOG_ERR, RTE_LOGTYPE_PMD, "%s: %s",
> -			func_name, buffer);
> -	}
> +	rte_log(RTE_LOG_ERR, RTE_LOGTYPE_PMD, "%s: %s", func_name, buffer);
> }
> 
> /*
> -- 
> 2.14.3
> 

Regards,
Keith