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Re: [lwip-users] [PATCH] fix warning for gcc and possible unaligned acce

From: Pedro Alves
Subject: Re: [lwip-users] [PATCH] fix warning for gcc and possible unaligned access
Date: Sat, 22 Apr 2006 13:34:08 +0100
User-agent: Thunderbird 1.5 (Windows/20051201)

Timmy Brolin wrote:
Curt McDowell wrote:

Pedro Alves wrote
Seems like the right thing to do.
If the packing is removed from struct ip_addr, and struct ip_addr2 is removed, then there is no need to memcpy anymore.

A simple:

sipaddr = hdr->sipaddr;

will do.

memcpy may still be needed because hdr->sipaddr is misaligned at +2 due to the format of the ARP packet. My theory is that struct ip_addr2 was originally invented because the 4-byte struct/uint32 copy was crashing on someone's 2-byte
aligned CPU!

Curt McDowell
Broadcom Corp.

Either ip_addr2 or memcpy is needed.
The code "sipaddr = hdr->sipaddr;" would crash on any CPU which do not support unaligned accesses.

huh? Not if struct ip_addr is NOT packed, hdr is an instance of a packed struct, and sipaddr is a struct ip_addr, not struct ip_addr2.

Are you saying that:

#include <stdio.h>

struct test_me
  unsigned char a;
  unsigned long b;
  unsigned char c;
} __attribute__ ((packed));

int main(int argc, char** argv)
 volatile char dummy_offset_alignment_for_next_var;
 volatile struct test_me tm;
 volatile unsigned long i;
 volatile unsigned long i2;

 tm.a = 0xdd;
 tm.c = 0xee;
 i = 0x55ffaa33;
 tm.b = i;
 i2 = tm.b;

 printf("sizeof(struct test_me) = 0x%x\n", sizeof(struct test_me));
 printf ("tm.a = 0x%x\n", (unsigned long)tm.a);
 printf ("tm.b = 0x%x\n", (unsigned long)tm.b);
 printf ("tm.c = 0x%x\n", (unsigned long)tm.c);
 printf ("i2 = 0x%x\n", i2);

 return 0;

Won't work as expected? That would beat the whole purpose of __attribute__((packed)), no?

P.S.: No use testing in x86, as that supports unaligned accesses anyway.

Pedro Alves

The current ip_addr2 solution is technically more efficient than memcpy since it copies the ip_addr with just two 16bit reads+writes. The only solution which would be slightly more efficient is to copy hdr->sipaddr to a 32bit register using two 16bit accesses, and then write the 32bit register to sipaddr. But since this is not a critical code path, it is more important to keep the code as clean as possible rather than trying to save one single memory access.
The ip_addr2 solution makes the code nice and clean.

Timmy Brolin

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