One common case of nested conditionals is used to check for more than two possible alternatives. For example, you might have
#if X == 1 ... #else /* X != 1 */ #if X == 2 ... #else /* X != 2 */ ... #endif /* X != 2 */ #endif /* X != 1 */
Another conditional directive, `#elif', allows this to be abbreviated as follows:
#if X == 1 ... #elif X == 2 ... #else /* X != 2 and X != 1*/ ... #endif /* X != 2 and X != 1*/
`#elif' stands for "else if". Like `#else', it goes in the middle of a `#if'-`#endif' pair and subdivides it; it does not require a matching `#endif' of its own. Like `#if', the `#elif' directive includes an expression to be tested.
The text following the `#elif' is processed only if the original `#if'-condition failed and the `#elif' condition succeeds. More than one `#elif' can go in the same `#if'-`#endif' group. Then the text after each `#elif' is processed only if the `#elif' condition succeeds after the original `#if' and any previous `#elif' directives within it have failed. `#else' is equivalent to `#elif 1', and `#else' is allowed after any number of `#elif' directives, but `#elif' may not follow `#else'.