Nonstandard Predefined Macros

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Nonstandard predefined macros

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 documentation, 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; for more information, see Invoking the C preprocessor. Some nonstandard predefined macros describe the operating system in use, with more or less specificity, as in the following examples.

- 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, as in the following six examples.

- 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 as the names for the macros. Some predefine both names. What happens in GNU C depends on the system you are using.
- 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, as in the following three examples.

- 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.

The previous three 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.

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