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(cpp-295.info)Nonstandard Predefined

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Nonstandard Predefined Macros
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   The C preprocessor normally has several predefined macros that vary
between machines because their purpose is to indicate what type of
system and machine is in use.  This manual, being for all systems and
machines, cannot tell you exactly what their names are; instead, we
offer a list of some typical ones.  You can use `cpp -dM' to see the
values of predefined macros; see Note: Invocation.

   Some nonstandard predefined macros describe the operating system in
use, with more or less specificity.  For example,

`unix'
     `unix' is normally predefined on all Unix systems.

`BSD'
     `BSD' is predefined on recent versions of Berkeley Unix (perhaps
     only in version 4.3).

   Other nonstandard predefined macros describe the kind of CPU, with
more or less specificity.  For example,

`vax'
     `vax' is predefined on Vax computers.

`mc68000'
     `mc68000' is predefined on most computers whose CPU is a Motorola
     68000, 68010 or 68020.

`m68k'
     `m68k' is also predefined on most computers whose CPU is a 68000,
     68010 or 68020; however, some makers use `mc68000' and some use
     `m68k'.  Some predefine both names.  What happens in GNU C depends
     on the system you are using it on.

`M68020'
     `M68020' has been observed to be predefined on some systems that
     use 68020 CPUs--in addition to `mc68000' and `m68k', which are
     less specific.

`_AM29K'
`_AM29000'
     Both `_AM29K' and `_AM29000' are predefined for the AMD 29000 CPU
     family.

`ns32000'
     `ns32000' is predefined on computers which use the National
     Semiconductor 32000 series CPU.

   Yet other nonstandard predefined macros describe the manufacturer of
the system.  For example,

`sun'
     `sun' is predefined on all models of Sun computers.

`pyr'
     `pyr' is predefined on all models of Pyramid computers.

`sequent'
     `sequent' is predefined on all models of Sequent computers.

   These predefined symbols are not only nonstandard, they are contrary
to the ANSI standard because their names do not start with underscores.
Therefore, the option `-ansi' inhibits the definition of these symbols.

   This tends to make `-ansi' useless, since many programs depend on the
customary nonstandard predefined symbols.  Even system header files
check them and will generate incorrect declarations if they do not find
the names that are expected.  You might think that the header files
supplied for the Uglix computer would not need to test what machine
they are running on, because they can simply assume it is the Uglix;
but often they do, and they do so using the customary names.  As a
result, very few C programs will compile with `-ansi'.  We intend to
avoid such problems on the GNU system.

   What, then, should you do in an ANSI C program to test the type of
machine it will run on?

   GNU C offers a parallel series of symbols for this purpose, whose
names are made from the customary ones by adding `__' at the beginning
and end.  Thus, the symbol `__vax__' would be available on a Vax, and
so on.

   The set of nonstandard predefined names in the GNU C preprocessor is
controlled (when `cpp' is itself compiled) by the macro
`CPP_PREDEFINES', which should be a string containing `-D' options,
separated by spaces.  For example, on the Sun 3, we use the following
definition:

     #define CPP_PREDEFINES "-Dmc68000 -Dsun -Dunix -Dm68k"

This macro is usually specified in `tm.h'.