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Date:	Thu, 21 Feb 2008 09:54:46 -0800
From:	Joe Perches <>
To:	Johannes Berg <>
Cc:	David Miller <>,,
Subject: Re: New sparse warning in net/mac80211/debugfs_sta.c

On Thu, 2008-02-21 at 11:17 +0100, Johannes Berg wrote:
> Yeah, I saw that discussion. I think it's fine, it's just something we
> need to be aware of. In fact, I Joe had a patch (that seems to have
> gotten lost?) to make DECLARE_MAC_BUF() declare a structure with the u8
> pointer in it instead to get type checking for the args, which would
> make our code there not even compile, and imho rightfully so. I'll send
> in a patch to fix this (via John) and Joe can resend his patch to get
> typechecking there.

This removes the __pure from print_mac, so reject as appropriate...

Add some type safety to print_mac by using 
struct print_mac_buf * instead of char *.

Signed-off-by: Joe Perches <>

diff --git a/include/linux/if_ether.h b/include/linux/if_ether.h
index 42dc6a3..2f8df76 100644
--- a/include/linux/if_ether.h
+++ b/include/linux/if_ether.h
@@ -129,9 +129,16 @@ extern ssize_t sysfs_format_mac(char *buf, const unsigned char *addr, int len);
  *	Display a 6 byte device address (MAC) in a readable format.
-extern __pure char *print_mac(char *buf, const unsigned char *addr);
-#define MAC_BUF_SIZE	18
-#define DECLARE_MAC_BUF(var) char var[MAC_BUF_SIZE] __maybe_unused
+struct print_mac_buf {
+	char formatted_mac_addr[18];
+#define DECLARE_MAC_BUF(var) \
+	struct print_mac_buf __maybe_unused _##var; \
+	struct print_mac_buf __maybe_unused *var = &_##var
+extern char *print_mac(struct print_mac_buf *buf, const unsigned char *addr);
diff --git a/net/ethernet/eth.c b/net/ethernet/eth.c
index a7b4175..ce607ab 100644
--- a/net/ethernet/eth.c
+++ b/net/ethernet/eth.c
@@ -384,9 +384,10 @@ ssize_t sysfs_format_mac(char *buf, const unsigned char *addr, int len)
-char *print_mac(char *buf, const unsigned char *addr)
+char *print_mac(struct print_mac_buf *buf, const unsigned char *addr)
-	_format_mac_addr(buf, MAC_BUF_SIZE, addr, ETH_ALEN);
-	return buf;
+	_format_mac_addr(buf->formatted_mac_addr,
+			 sizeof(buf->formatted_mac_addr), addr, ETH_ALEN);
+	return buf->formatted_mac_addr;

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