Avoiding Multiple Header Inclusion in ANSI C

Let us assume that you have 2 header files a.h and b.h and one source file main.c.

The files are in following relation:

Double-inclusion scenario

Let assume that this is the initial content of the files:

The content of b.h:

/*b.h*/
typedef enum
{
    val1 = 1,
    val2,
    val3
}EValues;

The content of a.h:

/*a.h*/
#include"b.h"

const int resource2 = val1+val2;

The content of main.c:

/*main.c*/
#include"a.h"
#include"b.h"
#include<stdio.h>

int main(void)
{
    printf("val1: %d\nval2: %d\nResource %d",val1,val2,resource2);
    return 0;
}

If you try to build the program you will encounter redeclaration errors:

../b.h:13:5: error: redeclaration of enumerator ‘val1’

../b.h:13:5: note: previous definition of ‘val1’ was here

../b.h:14:5: error: redeclaration of enumerator ‘val2’

../b.h:14:5: note: previous definition of ‘val2’ was here

../b.h:15:5: error: redeclaration of enumerator ‘val3’

../b.h:15:5: note: previous definition of ‘val3’ was here

../b.h:16:2: error: conflicting types for ‘EValues’

../b.h:16:2: note: previous declaration of ‘EValues’ was here

The solution for this problem is to use include guards.

#ifndef _HEADER_IDENTIFIER_
#define _HEADER_IDENTIFIER_

/*Header code*/

#endif

In order to make the program work corectly you will have to add an include guard to a.h and b.h:

The content of b.h:

/*b.h*/
#ifndef _B_H_
#define _B_H_

typedef enum
{
    val1 = 1,
    val2,
    val3
}EValues;

#endif

The content of a.h:

/*a.h*/
#ifndef _A_H_
#define _A_H_

#include"b.h"

const int resource2 = val1+val2;

#endif

The content of main.c:

/*main.c*/
#include"a.h"
#include"b.h"
#include<stdio.h>

int main(void)
{
    printf("val1: %d\nval2: %d\nResource %d",val1,val2,resource2);
    return 0;
}
/*Output
val1: 1
val2: 2
Resource 3
 */