Re: [iproute PATCH v3 6/6] misc/ifstat: simplify unsigned value comparison

[Phil Sutter]

On Fri, Jun 24, 2016 at 09:20:32AM +, David Laight wrote:
> From: Phil Sutter
> > Sent: 23 June 2016 18:34
> > 
> > By directly comparing the value of both unsigned variables, casting to
> > signed becomes unnecessary.
> > 
> > This also fixes for compiling with older versions of gcc (at least
> > <=3.4.6) which emit the following warning:
> > 
> > | ifstat.c: In function `update_db':
> > | ifstat.c:542: warning: comparison is always false due to limited range of 
> > data type
> > 
> > Signed-off-by: Phil Sutter 
> > ---
> >  misc/ifstat.c | 2 +-
> >  1 file changed, 1 insertion(+), 1 deletion(-)
> > 
> > diff --git a/misc/ifstat.c b/misc/ifstat.c
> > index abbb4e732fcef..9a44da487599e 100644
> > --- a/misc/ifstat.c
> > +++ b/misc/ifstat.c
> > @@ -539,7 +539,7 @@ static void update_db(int interval)
> > int i;
> > 
> > for (i = 0; i < MAXS; i++) {
> > -   if ((long)(h1->ival[i] - n->ival[i]) < 
> > 0) {
> > +   if (h1->ival[i] < n->ival[i]) {
> > memset(n->ival, 0, 
> > sizeof(n->ival));
> > break;
> 
> That isn't the same check.
> The original code is using modulo arithmetic.

Oh, right! The code behaves differently if h1->ival[i] is close to
ULONG_MAX and n->ival[i] is very small. Though I don't see where this
becomes relevant. Am I missing another scenario?

Thanks, Phil

RE: [iproute PATCH v3 6/6] misc/ifstat: simplify unsigned value comparison

[David Laight]

From: Phil Sutter
> Sent: 23 June 2016 18:34
> 
> By directly comparing the value of both unsigned variables, casting to
> signed becomes unnecessary.
> 
> This also fixes for compiling with older versions of gcc (at least
> <=3.4.6) which emit the following warning:
> 
> | ifstat.c: In function `update_db':
> | ifstat.c:542: warning: comparison is always false due to limited range of 
> data type
> 
> Signed-off-by: Phil Sutter 
> ---
>  misc/ifstat.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/misc/ifstat.c b/misc/ifstat.c
> index abbb4e732fcef..9a44da487599e 100644
> --- a/misc/ifstat.c
> +++ b/misc/ifstat.c
> @@ -539,7 +539,7 @@ static void update_db(int interval)
>   int i;
> 
>   for (i = 0; i < MAXS; i++) {
> - if ((long)(h1->ival[i] - n->ival[i]) < 
> 0) {
> + if (h1->ival[i] < n->ival[i]) {
>   memset(n->ival, 0, 
> sizeof(n->ival));
>   break;

That isn't the same check.
The original code is using modulo arithmetic.

David

[iproute PATCH v3 6/6] misc/ifstat: simplify unsigned value comparison

[Phil Sutter]

By directly comparing the value of both unsigned variables, casting to
signed becomes unnecessary.

This also fixes for compiling with older versions of gcc (at least
<=3.4.6) which emit the following warning:

| ifstat.c: In function `update_db':
| ifstat.c:542: warning: comparison is always false due to limited range of 
data type

Signed-off-by: Phil Sutter 
---
 misc/ifstat.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/misc/ifstat.c b/misc/ifstat.c
index abbb4e732fcef..9a44da487599e 100644
--- a/misc/ifstat.c
+++ b/misc/ifstat.c
@@ -539,7 +539,7 @@ static void update_db(int interval)
int i;
 
for (i = 0; i < MAXS; i++) {
-   if ((long)(h1->ival[i] - n->ival[i]) < 
0) {
+   if (h1->ival[i] < n->ival[i]) {
memset(n->ival, 0, 
sizeof(n->ival));
break;
}
-- 
2.8.2