netdev - Re: [PATCH v4] net: mac802154: comparision issue of type cast, finding by EXTRA

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Date:	Thu, 25 Apr 2013 17:40:29 +0800
From:	Chen Gang <gang.chen@...anux.com>
To:	David Miller <davem@...emloft.net>
CC:	Christophe Aeschlimann <c.aeschlimann@...-group.ch>,
	sergei.shtylyov@...entembedded.com,
	alex.bluesman.smirnov@...il.com, dbaryshkov@...il.com,
	linux-zigbee-devel@...ts.sourceforge.net, netdev@...r.kernel.org
Subject: Re: [PATCH v4] net: mac802154: comparision issue of type cast, finding
 by EXTRA_CFLAGS=-W


Need change ~(u8)0 to 0xff, or in mac802154_wpan_xmit(), "u8 chan"
will never == MAC802154_CHAN_NONE.

The related warning:
  net/mac802154/wpan.c:325:2: warning: comparison is always false
  due to limited range of data type [-Wtype-limits]

The related unit test:
  cc -Wall -O2 -o test test.c
  ./test
  will output 'different'

  (after use '0xff' instead of '~(u8)0', it will output 'equal')

-----------------------test code begin--------------------------------

#include <stdio.h>

typedef unsigned char u8;

int main()
{
	u8 uc = 0xff;

	if (uc == ~(u8)0)
		printf("\nequal\n");
	else
		printf("\ndifferent\n");

	return 0;
}

-----------------------test code end----------------------------------


Signed-off-by: Chen Gang <gang.chen@...anux.com>
---
 net/mac802154/mac802154.h | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/net/mac802154/mac802154.h b/net/mac802154/mac802154.h
index a4dcaf1..703c121 100644
--- a/net/mac802154/mac802154.h
+++ b/net/mac802154/mac802154.h
@@ -90,7 +90,7 @@ struct mac802154_sub_if_data {
 
 #define MAC802154_MAX_XMIT_ATTEMPTS	3
 
-#define MAC802154_CHAN_NONE		(~(u8)0) /* No channel is assigned */
+#define MAC802154_CHAN_NONE		0xff /* No channel is assigned */
 
 extern struct ieee802154_reduced_mlme_ops mac802154_mlme_reduced;
 extern struct ieee802154_mlme_ops mac802154_mlme_wpan;
-- 
1.7.11.7

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