8.8 What's the deal with _INCLUDE_xxxx_SOURCE?

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_HPUX_SOURCE" is specified.  While this has turned into one the
most frequently asked of FAQ's 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 the HPUX_SOURCE
namespace.

(Thanks to Marc Sabatella, HP)