C [C99] has peculiar rules for inline functions that are different from the
C++ rules.  Unlike C++ where inline is "fire and forget", in C a programmer
must pay attention to the function's storage class / visibility.  The main
problem is with the case where a compiler decides to not inline a call to the
function declared as inline.
Some relevant links:
- http://infocenter.arm.com/help/index.jsp?topic=/com.arm.doc.faqs/ka15831.html
- http://www.drdobbs.com/the-new-c-inline-functions/184401540
The summary is that either the inline functions should be declared 'static
inline' or one of the compilation units (.c files) must provide a callable
externally visible function definition.  In the former case, the compiler would
automatically create a local non-inlined function instance in every compilation
unit where it's needed.  In the latter case the single external definition is
used to satisfy any non-inlined calls in all compilation units.  As things
stand right now, we can get an undefined reference error under certain
combinations of compilers and compiler options.  For example, this is what I
get on FreeBSD when compiling with clang 4.0.0 and -O1:
  In function `abd_free': /usr/src/sys/cddl/contrib/opensolaris/uts/common/fs/zfs/abd.c:385:
  undefined reference to `abd_is_linear'