Difference between preprocessor directives #if and #ifdef

Question

What is the difference (if any) between the two following preprocessor control statements.

#if

and

#ifdef

Answer 1

You can demonstrate the difference by doing:

#define FOO 0
#if FOO
  // won't compile this
#endif
#ifdef FOO
  // will compile this
#endif

#if checks for the value of the symbol, while #ifdef checks the existence of the symbol (regardless of its value).