DragonFly submit List (threaded) for 2007-05
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Re: [PATCH] ICMP extensions for MPLS support for traceroute(8)
:> +#if BYTE_ORDER == BIG_ENDIAN
:> + unsigned char version:4;
:> + unsigned char reserved1:4;
:> +#else
:> + unsigned char reserved1:4;
:> + unsigned char version:4;
:> +#endif
:> + unsigned char reserved2;
:> + unsigned short checksum;
:> +};
:
:Please don't add new code that depends on the order of bitfields.
:(Or bitfields in general for wire protocols).
:
:Joerg
What Joerg is saying is that the patch looks great! But we're
trying to get away from using bitfields so if it would not be too
much trouble, could you just make that a u_char and extract the fields
manually with a macro?
Note that when used as a u_char, I'm pretty sure that no endian
conversions are needed when doing a manual extraction. Endian
conversions on bitfield ordering within single bytes are an artifact
of the way the compiler assigns the bitfield indices. The actual
storage (within a char) is the same.
Take for example the IP header (now there's something I would like to
clean up and just make _IP_VHL the default). Note that the IP_VHL_*()
macros do not require any endian conditionalization.
struct ip {
#ifdef _IP_VHL
u_char ip_vhl; /* version << 4 | header length >> 2 */
#elif _BYTE_ORDER == _LITTLE_ENDIAN
u_int ip_hl:4, /* header length */
ip_v:4; /* version */
#elif _BYTE_ORDER == _BIG_ENDIAN
u_int ip_v:4, /* version */
ip_hl:4; /* header length */
#else
. ..
#endif
#define IP_VHL_HL(vhl) ((vhl) & 0x0f)
#define IP_VHL_V(vhl) ((vhl) >> 4)
#define IP_VHL_BORING 0x45
-Matt
Matthew Dillon
<dillon@backplane.com>
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