Another good trick for seeing what headers (and the pre-processor) does in general is using the -E flag (for gcc / clang) it will show the file with the headers

Another good trick for seeing what headers (and the pre-processor) does in general is using the -E flag (for gcc / clang) it will show the file with the headers included so you can see what happens. This is really useful when using macros and include guards to see what you are getting. clang -E myfile.c > pre.txt will give you an editable file to see what is going on.