I can't wrap my head around this sentence from the GNU C PREPROCESSOR documentation

Question

Link to the page

It is in the 9th row under the "tokenization" section in the "Preprocessor overview" It's as follows:

The only identifier which can be considered a preprocessing keyword is defined.

I tried

#define DEFINED

but it works just fine.

I also asked chatGPT and searched the web for it but nothing helped.

What does this tell us in practice? What limitations does this cause us?

Answer 1

First note that C is a case-sensitive language, so DEFINED and defined mean different things. What the text refers to is the special identifier defined only valid inside pre-processor macros. Usage is like this:

#if defined(__STDC__) && (__STDC__VERSION == 201710L)
  puts("This C compiler is up to date.");
#endif

This is equivalent to:

#ifdef __STDC__
  #if (__STDC__VERSION == 201710L)
    puts("This C compiler is up to date.");
  #endif
#endif

As we can see, the benefit of define is that it allows simpler pre-processor checks without resorting to multiple nested #if/#ifdef. Suppose we want to check of a compiler is following standard C and also supports VLA:

#if defined(__STDC__) && !defined(__STDC_NO_VLA__)
  puts("This compiler is useful");
#else
  puts("This compiler is useless");
#endif

That's a whole lot more convenient code with no code repetition, compared to this mess:

#ifdef __STDC__
  #ifndef __STDC_NO_VLA__
    puts("This compiler is useful");
  #else
    puts("This compiler is useless");
  #endif
#else
  puts("This compiler is useless");
#endif