Where do I find examples mixing C and Prolog?
You can download the source and check the sources of one or more of the packages. There is a complicated example in the form of an TeX tokeniser in the ltx2htm subdirectory of the sources. There are a couple of examples in the src/win32/ subtree of the sources for handling Windows API. Finally, there is the SWI-Prolog source-code itself that uses the foreign interface for realising many of the build-in predicates. The function is normally called pl_<predicate-name>. See pl-ext.c for the function-names of all predicates. Note however that the system itself sometimes uses more low-level constructs to manipulate Prolog data. Never use functions that have no prototype in SWI-Prolog.h or SWI-Stream.h.