Portability issues (1)

Differences in IPv6 stacks
Separate libraries (libinet6.a)
Because many of them are add-ons.
Library/include pathnames (e.g. /usr/local/v6/lib)
Level of support
Advanced API (old and new)
tcp/udp code differences
Spec is not clear enough.

Be very neutral to kernel
Assume nothing, use only safe behaviors.
Use GNU autoconf
Guess system type based on string/symbol in include file.
Sample can be found in many of KAME patches:
ftp://ftp.kame.net/pub/kame/misc/