Why is stddef.h not in /usr/include?

Question

I have compiled the gnu standard library and installed it in $GLIBC_INST.

Now, I try to compile a very simple programm (using only one #include

Answer 1

Why is that file not under /usr/include?

Because there's absolutely no requirement for standard headers to be located at /usr/include/.

The implementation could place them anywhere. The only guarantee is that when you do #include , the compiler/preprocessor correctly locates and includes it. Since you disable that with -nostdinc option of gcc, you are on your own (to correctly give the location of that header).