#define IDENTIFIER without a token

Question

What does the following statement mean:

#define FAHAD

I am familiar with the statements like:

#define FAHAD 1

Answer 1

Defining a constant without a value acts as a flag to the preprocessor, and can be used like so:

#define MY_FLAG
#ifdef MY_FLAG
/* If we defined MY_FLAG, we want this to be compiled */
#else
/* We did not define MY_FLAG, we want this to be compiled instead */
#endif