By default, the posix profile includes <stddef.h> which is a linux-ism. it doesnt works on OSX (between others). See the location of stddef in a standard linux distribution:

/usr/include/linux/stddef.h

in OSX, this include does not exists which results in no definition of NULL and other basic types for C. Here's a sample program which fails to compile in OSX.

====
using Posix;
void main () {
  void *p = null;
}
====

if we do print ("%p\n", p); then <stdio.h> is included..so it builds.

Should we include stdio.h by default in posix profile instead of stddef? i dont know if stddef exists in windows or BSD. but would be good to test.