[C] Preprocessors - #if, #ifdef, #if defined

In this posting, we will take a look at the differences among #if, #ifdef, and #if defined.

#if

#if directive only works with a true or false value.

#define TEST	1

#if TEST
  #error "This would be an error as well."
#endif

It can be used with an expression.

#define TEST	0

#if (TEST == 0)
  #error "This would be an error as well."
#endif

It is also able to extend with #elif and #else.

#define TEST	2

#if (TEST == 1)
  #error "This is an error 1."
#elif (TEST == 2)
  #error "This is an error 2."
#else
  #error "This is an error 3."
#endif

#ifdef

#ifdef directives checks if the name is defined.

It only checks for definition literally and values are not important. It works even it doesn't have value.

#define TEST	0
#define TEST	1
#define TEST

#ifdef TEST
  #error "All three cases are treated as if thsy were defined."
#endif

If you want to handle the opposite case, use the #ifndef directive.

#ifndef TEST
  #error "Error occurred because there is no definition of TEST."
#endif

#if defined

#if defined is almost the same as #ifdef except for it can handle multiple conditions.

#define TEST

#if defined (TEST) || !defined (USERNAME)
  #error "This is an error."
#endif

As you can see in the above example, the opposite case can be handled by #if !defined.

And you can add multiple conditions using logical OR (||) and logical AND (&&).

You can also extend it using #elif and #else.

#define USERNAME

#if !defined (USERNAME)
  #error "There is no username."
#elif !defined (USERAGE)
  #error "There is no userage."
#endif

Conclusion

These directives are checked during compile-time.

So it is generally used for building multiple targets via different defined options.

I hope you can clearly understand their differences and use them appropriately.