Define preprocessor value when value not defined

Question

Sometimes I see a define preprocessor but not value assigned to it. For example:

#define VAR

What is assigned to VAR when no value is specified?

I am also reading a text and I see:

#ifndef ERROR_FUNCTIONS_H
#define ERROR_FUNCTIONS_H

/* Error diagnostic routines */

void errMsg(const char *format, ...);

#ifdef __GNUC__

    /* This macro stops 'gcc -Wall' complaining that "control reaches
       end of non-void function" if we use the following functions to
       terminate main() or some other non-void function. */

#define NORETURN __attribute__ ((__noreturn__))
#else
#define NORETURN
#endif                  

Is ERROR_FUNCTIONS_H a header file? Or is that just a constant defined by define preprocessor?

Answer 1

It is just 'defined' so you can do checks like #ifdef ERROR_FUNCTIONS_H

So if you have

 #define ERROR_FUNCTIONS_H

You can later do:

#ifdef ERROR_FUNCTIONS_H
 //do something
#endif 

and the code inside the #ifdef will be compiled.

ERROR_FUNCTIONS_H is not a header file. It is just a #define for the preprocessor to use.

What you have in your example:

#ifndef ERROR_FUNCTIONS_H
#define ERROR_FUNCTIONS_H

is often put at the top of a .h file to make sure it is only included once so you don't get multiple defines.