Inheritance and Header Files
----------------------------

   "Inheritance" is what happens when one object or file derives some
of its contents by virtual copying from another object or file.  In the
case of C header files, inheritance means that one header file includes
another header file and then replaces or adds something.

   If the inheriting header file and the base header file have different
names, then inheritance is straightforward: simply write `#include
"BASE"' in the inheriting file.

   Sometimes it is necessary to give the inheriting file the same name
as the base file.  This is less straightforward.

   For example, suppose an application program uses the system header
file `sys/signal.h', but the version of `/usr/include/sys/signal.h' on
a particular system doesn't do what the application program expects.
It might be convenient to define a "local" version, perhaps under the
name `/usr/local/include/sys/signal.h', to override or add to the one
supplied by the system.

   You can do this by using the option `-I.' for compilation, and
writing a file `sys/signal.h' that does what the application program
expects.  But making this file include the standard `sys/signal.h' is
not so easy--writing `#include <sys/signal.h>' in that file doesn't
work, because it includes your own version of the file, not the
standard system version.  Used in that file itself, this leads to an
infinite recursion and a fatal error in compilation.

   `#include </usr/include/sys/signal.h>' would find the proper file,
but that is not clean, since it makes an assumption about where the
system header file is found.  This is bad for maintenance, since it
means that any change in where the system's header files are kept
requires a change somewhere else.

   The clean way to solve this problem is to use `#include_next', which
means, "Include the *next* file with this name."  This directive works
like `#include' except in searching for the specified file: it starts
searching the list of header file directories *after* the directory in
which the current file was found.

   Suppose you specify `-I /usr/local/include', and the list of
directories to search also includes `/usr/include'; and suppose that
both directories contain a file named `sys/signal.h'.  Ordinary
`#include <sys/signal.h>' finds the file under `/usr/local/include'.
If that file contains `#include_next <sys/signal.h>', it starts
searching after that directory, and finds the file in `/usr/include'.