What do parentheses mean in an #if defined preprocessor operator?

c, c-preprocessor, preprocessor-directive

Solution

These three preprocessor directives:

#if defined (AR7x00)

#if defined AR7x00

#ifdef AR7x00

all mean exactly the same thing: that the following code is to be processed only if the macro `AR7x00` is currently defined.

The `#ifdef ...` directive is simply a convenient alternative to `#if defined ...`. There's also a `#ifndef ...` directive; `#ifndef FOO` is equivalent to `#if ! defined FOO`.

As for the parentheses, the syntax for the `defined` operator allows for either an identifier, or an identifier in parentheses, with no difference in meaning. I'm not entirely sure why the parentheses are optional; I suspect it's just historical. (The language reference in the 1978 first edition of K&R doesn't mention the `defined` operator. The second edition shows both forms, with and without parentheses.)

Strictly speaking, these are not grouping parentheses of the kind you see in ordinary expressions; they're specifically part of the syntax of the `defined` operator, which can only be used in preprocessor `#if` directives. In particular, this:

#if defined ((AR7x00))

is a syntax error.

Problem

(I am working on a SDK wherein I have the code of the particular SDK in reference and I am not able to trace out the flow of the program.) What does ``` #if defined (AR7x00) ``` mean? Specifically, what is the purpose of parentheses in a such a preprocessor operator?

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