The main compile-time defines to keep in mind when reading the host*.c source file are these:
this host has getaddrinfo() and family, and thus we use that. The host may not be able to resolve IPv6, but we do not really have to take that into account. Hosts that are not IPv6-enabled have CURLRES_IPV4 defined.
is defined if libcurl is built to use c-ares for asynchronous name resolves. This can be Windows or *nix.
is defined if libcurl is built to use threading for asynchronous name resolves. The name resolve is done in a new thread, and the supported asynch API is be the same as for ares-builds. This is the default under (native) Windows.
If any of the two previous are defined, CURLRES_ASYNCH is defined too. If libcurl is not built to use an asynchronous resolver, CURLRES_SYNCH is defined.
The host*.c sources files are split up like this:
hostip.c - method-independent resolver functions and utility functions
hostasyn.c - functions for asynchronous name resolves
hostsyn.c - functions for synchronous name resolves
asyn-ares.c - functions for asynchronous name resolves using c-ares
asyn-thread.c - functions for asynchronous name resolves using threads
hostip4.c - IPv4 specific functions
hostip6.c - IPv6 specific functions
The hostip.h is the single united header file for all this. It defines the CURLRES_* defines based on the config*.h and curl_setup.h defines.