Conditionals are useful in connection with macros or assertions, because those are the only ways that an expression's value can vary from one compilation to another. A `#if' directive whose expression uses no macros or assertions is equivalent to `#if 1' or `#if 0'; you might as well determine which one, by computing the value of the expression yourself, and then simplify the program.
For example, here is a conditional that tests the expression `BUFSIZE == 1020', where `BUFSIZE' must be a macro.
#if BUFSIZE == 1020 printf ("Large buffers!\n"); #endif /* BUFSIZE is large */
(Programmers often wish they could test the size of a variable or data
type in `#if', but this does not work. The preprocessor does not
sizeof, or typedef names, or even the type keywords
The special operator `defined' is used in `#if' expressions to test whether a certain name is defined as a macro. Either `defined name' or `defined (name)' is an expression whose value is 1 if name is defined as macro at the current point in the program, and 0 otherwise. For the `defined' operator it makes no difference what the definition of the macro is; all that matters is whether there is a definition. Thus, for example,
#if defined (vax) || defined (ns16000)
would succeed if either of the names `vax' and `ns16000' is defined as a macro. You can test the same condition using assertions (see section Assertions), like this:
#if #cpu (vax) || #cpu (ns16000)
If a macro is defined and later undefined with `#undef', subsequent use of the `defined' operator returns 0, because the name is no longer defined. If the macro is defined again with another `#define', `defined' will recommence returning 1.
Conditionals that test whether just one name is defined are very common, so there are two special short conditional directives for this case.
Macro definitions can vary between compilations for several reasons.