Question

I want to check if a certain structure was defined in a header file I included and define it if it is not. The header might, or might not be included in the source file and that is (obviously) known at compile time.

Essentially I want something similar to the ifndef guards used in header files and I know that I could use a check if the header file where the struct is defined is included by checking it’s guard, but I am wandering if I could check for the struct itself.

Answer 1

I have checked in ANSI C spec

http://www.open-std.org/jtc1/sc22/wg14/www/docs/n1570.pdf

here
Preprocessing directives only talk about identifier.

so it is NOT possible to check that this structure is defined or not.