CODEWITHSUNDEEP
cc-preprocessor
user1527227
user1527227

Reputation: 2258

Define preprocessor value when value not defined

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?

Upvotes: 1

Views: 2173

Answers (1)

edtheprogrammerguy
edtheprogrammerguy

Reputation: 6049

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.

Upvotes: 3

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