Re: [patch] add ether_(aton, ntoa)

[Strake <strake888@gmail.com>]

On 05/05/2013, Rich Felker <dalias@aerifal.cx> wrote:
> On Sun, May 05, 2013 at 06:24:19PM +0200, Szabolcs Nagy wrote:
>> but even if the representation and alignment is the same
>> an object with effective type (const char*) cannot be
>> accessed through a (char*) lvalue expression within strtoul
>>
>> so the aliasing rules are violated as well
>> (a compiler may reorder the loads from x and the stores
>> to *(char**)&x arbitarily)
>
> Indeed this is the real issue, but it's fairly hard to hit. If the
> compiler can only see the code in the caller and not inside strtoul,
> it can't do any reordering, because it must assume the callee could
> convert the pointer back to the right type and perform legal writes
> through it. The aliasing violation only has pratical consequences if
> you assume optimization can take place across translation units, e.g.
> LTO. In any case, it's policy not to do this kind of aliasing
> violation, especially when the identical result can easily be achieved
> using a temp variable of the right type.

All very esoteric. This is properly the compiler's worry, not mine.
Alas, lossage finds safety in standardization. But what would I know?
I'm not a committee.

From 3f301831e3e42ce23d5d3c0f3721232d66b93114 Mon Sep 17 00:00:00 2001
From: Strake <strake888@gmail.com>
Date: Sun, 14 Apr 2013 12:09:30 -0500
Subject: [PATCH] add ether_(aton, ntoa)

---
 include/netinet/ether.h  | 14 ++++++++++++++
 src/network/ether_aton.c | 30 ++++++++++++++++++++++++++++++
 src/network/ether_ntoa.c | 17 +++++++++++++++++
 3 files changed, 61 insertions(+)
 create mode 100644 include/netinet/ether.h
 create mode 100644 src/network/ether_aton.c
 create mode 100644 src/network/ether_ntoa.c

diff --git a/include/netinet/ether.h b/include/netinet/ether.h
new file mode 100644
index 0000000..c5179d5
--- /dev/null
+++ b/include/netinet/ether.h
@@ -0,0 +1,14 @@
+#ifndef _NETINET_ETHER_H
+#define _NETINET_ETHER_H
+
+#include <netinet/if_ether.h>
+
+char *ether_ntoa (const struct ether_addr *);
+
+struct ether_addr *ether_aton (const char *);
+
+char *ether_ntoa_r (const struct ether_addr *, char *);
+
+struct ether_addr *ether_aton_r (const char *, struct ether_addr *);
+
+#endif
diff --git a/src/network/ether_aton.c b/src/network/ether_aton.c
new file mode 100644
index 0000000..55f56ec
--- /dev/null
+++ b/src/network/ether_aton.c
@@ -0,0 +1,30 @@
+#include <stdlib.h>
+#include <string.h>
+#include <netinet/ether.h>
+
+static struct ether_addr a;
+
+struct ether_addr *ether_aton_r (const char *x, struct ether_addr *p_a) {
+	struct ether_addr a;
+	for (int ii = 0; ii < 6; ii++) {
+		unsigned long int n;
+		if (ii != 0) {
+			if (x[0] != ':') return 0; /* bad format */
+			else x++;
+		}
+		{
+			char *y;
+			n = strtoul (x, &y, 16);
+			x = y;
+		}
+		if (n > 0xFF) return 0; /* bad byte */
+		a.ether_addr_octet[ii] = n;
+	}
+	if (x[0] != 0) return 0; /* bad format */
+	memmove (p_a, &a, sizeof (struct ether_addr));
+	return p_a;
+}
+
+struct ether_addr *ether_aton (const char *x) {
+	return ether_aton_r (x, &a);
+}
diff --git a/src/network/ether_ntoa.c b/src/network/ether_ntoa.c
new file mode 100644
index 0000000..bcc773a
--- /dev/null
+++ b/src/network/ether_ntoa.c
@@ -0,0 +1,17 @@
+#include <stdio.h>
+#include <netinet/ether.h>
+
+static char x[18];
+
+char *ether_ntoa_r (const struct ether_addr *p_a, char *x) {
+	char *y;
+	y = x;
+	for (int ii = 0; ii < 6; ii++) {
+		x += sprintf (x, ii == 0 ? "%.2X" : ":%.2X", p_a -> ether_addr_octet[ii]);
+	}
+	return y;
+}
+
+char *ether_ntoa (const struct ether_addr *p_a) {
+	return ether_ntoa_r (p_a, x);
+}
-- 
1.7.11.1