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(cpp-295.info)Assertions

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Assertions
----------

   "Assertions" are a more systematic alternative to macros in writing
conditionals to test what sort of computer or system the compiled
program will run on.  Assertions are usually predefined, but you can
define them with preprocessing directives or command-line options.

   The macros traditionally used to describe the type of target are not
classified in any way according to which question they answer; they may
indicate a hardware architecture, a particular hardware model, an
operating system, a particular version of an operating system, or
specific configuration options.  These are jumbled together in a single
namespace.  In contrast, each assertion consists of a named question and
an answer.  The question is usually called the "predicate".  An
assertion looks like this:

     #PREDICATE (ANSWER)

You must use a properly formed identifier for PREDICATE.  The value of
ANSWER can be any sequence of words; all characters are significant
except for leading and trailing whitespace, and differences in internal
whitespace sequences are ignored.  Thus, `x + y' is different from
`x+y' but equivalent to `x + y'.  `)' is not allowed in an answer.

   Here is a conditional to test whether the answer ANSWER is asserted
for the predicate PREDICATE:

     #if #PREDICATE (ANSWER)

There may be more than one answer asserted for a given predicate.  If
you omit the answer, you can test whether *any* answer is asserted for
PREDICATE:

     #if #PREDICATE

   Most of the time, the assertions you test will be predefined
assertions.  GNU C provides three predefined predicates: `system',
`cpu', and `machine'.  `system' is for assertions about the type of
software, `cpu' describes the type of computer architecture, and
`machine' gives more information about the computer.  For example, on a
GNU system, the following assertions would be true:

     #system (gnu)
     #system (mach)
     #system (mach 3)
     #system (mach 3.SUBVERSION)
     #system (hurd)
     #system (hurd VERSION)

and perhaps others.  The alternatives with more or less version
information let you ask more or less detailed questions about the type
of system software.

   On a Unix system, you would find `#system (unix)' and perhaps one of:
`#system (aix)', `#system (bsd)', `#system (hpux)', `#system (lynx)',
`#system (mach)', `#system (posix)', `#system (svr3)', `#system
(svr4)', or `#system (xpg4)' with possible version numbers following.

   Other values for `system' are `#system (mvs)' and `#system (vms)'.

   *Portability note:* Many Unix C compilers provide only one answer
for the `system' assertion: `#system (unix)', if they support
assertions at all.  This is less than useful.

   An assertion with a multi-word answer is completely different from
several assertions with individual single-word answers.  For example,
the presence of `system (mach 3.0)' does not mean that `system (3.0)'
is true.  It also does not directly imply `system (mach)', but in GNU
C, that last will normally be asserted as well.

   The current list of possible assertion values for `cpu' is: `#cpu
(a29k)', `#cpu (alpha)', `#cpu (arm)', `#cpu (clipper)', `#cpu
(convex)', `#cpu (elxsi)', `#cpu (tron)', `#cpu (h8300)', `#cpu
(i370)', `#cpu (i386)', `#cpu (i860)', `#cpu (i960)', `#cpu (m68k)',
`#cpu (m88k)', `#cpu (mips)', `#cpu (ns32k)', `#cpu (hppa)', `#cpu
(pyr)', `#cpu (ibm032)', `#cpu (rs6000)', `#cpu (sh)', `#cpu (sparc)',
`#cpu (spur)', `#cpu (tahoe)', `#cpu (vax)', `#cpu (we32000)'.

   You can create assertions within a C program using `#assert', like
this:

     #assert PREDICATE (ANSWER)

(Note the absence of a `#' before PREDICATE.)

   Each time you do this, you assert a new true answer for PREDICATE.
Asserting one answer does not invalidate previously asserted answers;
they all remain true.  The only way to remove an assertion is with
`#unassert'.  `#unassert' has the same syntax as `#assert'.  You can
also remove all assertions about PREDICATE like this:

     #unassert PREDICATE

   You can also add or cancel assertions using command options when you
run `gcc' or `cpp'.  Note: Invocation.