The ANSI standard clearly states what identifiers it reserves, and says
the rest are available to you, the programmer.  Many "important things"
like "ulong" are *not* specified by ANSI, so ANSI header files are not
allowed by the standard to define them.  Each standard supported by HP-UX
(POSIX1, POSIX2, XPG2, XPG3, XPG4, AES, etc) has its own set of reserved
identifiers and header files, and the convention is to require
"-D_POSIX_SOURCE" (et al) to enabled their respective namespaces.  Since
HP could not predict what future standards would come along and claim
more header files and identifiers, it proved much simpler to make the
namespace as restrictive as possible unless "-D_HP-UX_SOURCE" is
specified.  While this has turned into one the most frequently asked of
FAQs about HP-UX, at least once you learn this, you don't have to deal
with inconsistencies again.  Whereas, had we allowed all non-standard
headers to define all non-standard symbols, you'd find identifiers
randomly "disappearing" from headers over time as they were claimed by
various standards.

Also check the man page for "cc -Ae"; it enables the HP-UX_SOURCE
namespace.