Assertions are a deprecated 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.
Assertions were intended to provide a more systematic way to describe the compiler's target system. However, in practice they are just as unpredictable as the system-specific predefined macros. In addition, they are not part of any standard, and only a few compilers support them. Therefore, the use of assertions is less portable than the use of system-specific predefined macros. We recommend you do not use them at all.
An assertion looks like this:
#predicate (answer)
predicate must be a single identifier. answer can be any
sequence of tokens; all characters are significant except for leading
and trailing whitespace, and differences in internal whitespace
sequences are ignored. (This is similar to the rules governing macro
redefinition.) Thus, (x + y) is different from (x+y) but
equivalent to ( x + y ). Parentheses do not nest inside an
answer.
To test an assertion, you write it in an #if. For example, this
conditional succeeds if either vax or ns16000 has been
asserted as an answer for machine.
#if #machine (vax) || #machine (ns16000)
You can test whether any answer is asserted for a predicate by omitting the answer in the conditional:
#if #machine
Assertions are made with the #assert directive. Its sole
argument is the assertion to make, without the leading # that
identifies assertions in conditionals.
#assert predicate (answer)
You may make several assertions with the same predicate and different answers. Subsequent assertions do not override previous ones for the same predicate. All the answers for any given predicate are simultaneously true.
Assertions can be canceled with the #unassert directive. It
has the same syntax as #assert. In that form it cancels only the
answer which was specified on the #unassert line; other answers
for that predicate remain true. You can cancel an entire predicate by
leaving out the answer:
#unassert predicate
In either form, if no such assertion has been made, #unassert has
no effect.
You can also make or cancel assertions using command line options. See Invocation.