Alignment on XScale ARM

Hi,

we use an XScale 422 (ARM) with a big endian Linux system based on the buildroot environment. It seems there is an alignment problem we have. I've tracked down the problem to the following.
1. The data received via a netlink communication from the kernel is stored into a buffer buf. The data is correct.
2. The address of the buffer is converted to a pointer to nlmsghdr:

        struct nlmsghdr *h;
        h = (struct nlmsghdr*) buf;
        printf("IPSECTOOLS: Length  %04d\n", h->nlmsg_len);
        printf("IPSECTOOLS: Type %02d\n", h->nlmsg_type);
  The results of printf are nonsens. The first fields seem to have an offset of 2 bytes, at the end there also seem to be swapped bytes.
3. To test what might happen, I've created an own struct
      struct aligntest{
               __u32 first;
               __u32 int second;
               __u32 third;
               __u32 fourth;
      };
    When you now set a point to a buffer, again the values are wrong:
        s1 = (struct aligntest*) buf;
        printf("1: %08X\n", s1->first);
        printf("2: %08X\n", s1->second);
        printf("3: %08X\n", s1->third);
        printf("4: %08X\n", s1->fourth);

It seems, that there is a problem with the unsigned int values. When you add the attribute "packed" to the definition of struct aligntest ( __attribute__(__packed__) ), all values are displayed correctly. So, any hints what the problem can be?

Hi,

we use an XScale 422 (ARM) with a big endian Linux system based on the buildroot environment. It seems there is an alignment problem we have. I've tracked down the problem to the following. 1. The data received via a netlink communication from the kernel is stored into a buffer buf. The data is correct. 2. The address of the buffer is converted to a pointer to nlmsghdr:

         struct nlmsghdr *h;
         h = (struct nlmsghdr*) buf; 
         printf("IPSECTOOLS: Length  %04d\n", h->nlmsg_len);
         printf("IPSECTOOLS: Type %02d\n", h->nlmsg_type);
The results of printf are nonsens. The first fields seem to have an 
offset of 2 bytes, at the end there also seem to be swapped bytes. 3. 
To test what might happen, I've created an own struct
       struct aligntest{
                __u32 first;
                __u32 int second;
                __u32 third;
                __u32 fourth;
       };
When you now set a point to a buffer, again the values are wrong:
         s1 = (struct aligntest*) buf;
         printf("1: %08X\n", s1->first);
         printf("2: %08X\n", s1->second);
         printf("3: %08X\n", s1->third);
         printf("4: %08X\n", s1->fourth);

It seems, that there is a problem with the unsigned int values. When you add the attribute "packed" to the definition of struct aligntest ( __attribute__(__packed__) ), all values are displayed correctly. So, any hints what the problem can be?

The ARM processor traditionally cannot access misaligned short and int values in memory.  By using the packed attribute you tell the compiler that the structure should not have inserted padding to align its members, and that accesses are to be performed wth byte accesses and the value reconstructed that way.