We're returning the calculated checksum result modulo 65536, given that
Internet checksums are 16-bit; throw in a cast to let MSVC know that
this is intended.
This can prevent bizarre failures if, for example, you've done a
configuration in the top-level source directory, leaving behind one
config.h file, and then do an out-of-tree build in another directory,
with different configuration options. This way, we always pick up the
same config.h, in the build directory.
We use AC_TYPE_UINTPTR_T in the configure script, so it'll be defined if
we use autotools, and MSVC has supported it since at least Visual Studio
.NET 2003, so we'll use it instead of "unsigned long"; "unsigned long"
causes warnings on LLP64 platforms, as it's not wide enough for a
pointer on those platforms.
The purpose of this macro was to enable the file-by-file switch to NDO,
after which only tcpdump.c had a use of it and the definitions guarded
by it. Update tcpdump.c not to require them any more and dismiss the
unused definitions.
And, as we require at least autoconf 2.61, and as autoconf 2.61 and
later have AC_TYPE_UINTn_T and AC_TYPE_INTn_T macros, we use them to
define the uintN_t and intN_t macros if the system doesn't define them
for us.
This lets us get rid of bitypes.h as well.
The Wireshark routine is based on the BSD in-kernel portable checksum
routine (thus BSD-licensed); it takes a vector of pointers and lengths
and checksums the concatenation of the buffers in question (just as the
BSD in-kernel routine checksums a chain of mbufs).
This simplifies the "with a pseudo-header" checksums; hopefully it'll
fix up the problems being seen on some big-endian platforms, which might
be due to hand-calculating some or all of the checksum and doing so
incorrectly. It also gets rid of some code that might be dereferencing
unaligned pointers.
