I did something similar (without macros, with a bit more type safety since the keywords have an associated type, and with a shortcut for boolean arguments):

https://github.com/CaptainCrowbar/kwargs

You can pre-initialize any member you want to something other than 0, such as -1. The C standard permits multiple initializations of the same member, and defines the value to be that of the last initialization. So if the application sets the member again, it all works properly.

Both GCC and clang complain about such initializer "overriding", but I habitually disable those warnings whenever I start a new project. (I also contributed the patch to clang to be able to disable that warning =)

Initializer overrides and missing braces are the two dumbest diagnostics that, sadly, both GCC and clang spit out. Using '{ 0 }' is the _only_ way to initialize a compound automatic variable where you do not know (and shouldn't depend on) the internal layout, and it's perfectly legal C.