If you compile a program, and give -D_XOPEN_SOURCE=700 to the compiler, and the program has #include <sys/file.h> the compiler gives an error like this:
/usr/include/sys/file.h:226:2: error: unknown type name 'u_int'
u_int xf_flag; /* flags (see fcntl.h) */
and then bails out.
Why is the u_int typedef used instead of 'unsigned int' itself? It's unnecessary other than to save a couple keystrokes while typing.
The current implementation of the feature test macros like _XOPEN_SOURCE that request strict standards compliance is that the application is assumed not to need any extensions. For the most part, defining such a macro hides things not belonging to the selected standard from header files listed in the standard. If a header file is used that is not in the standard, this is a bad idea: it either fails (as you see here) or exposes extensions to the standard.