#include <stdio.h>
#include "blank.h"
#include "boolean.h"

int main()
{
    bool result = blank("");

    printf("%dn", result);

    return 0;
}

// Header file for blank function

bool blank(char string[]);

#include "boolean.h"
#include "blank.h"
#include <regex.h>

bool blank(char string[])
{

    regex_t regex_blank;
    int blank = regcomp(&regex_blank, "[:blank:]", 0);

    blank = regexec(&regex_blank, string, 0, NULL, 0);

    if  ( string == NULL || blank == 1 )
        return true;
    else
        return false;
}

// Boolean

// Define true
#ifndef true
#define true 1
#endif

// Define false
#ifndef false
#define false 0
#endif

typedef int bool;

gcc Test.c blank.c -o b

In file included from Test.c:2:0:
blank.h:3:1: error: unknown type name ‘bool’
blank.c: In function ‘blank’:
blank.c:11:46: error: ‘NULL’ undeclared (first use in this function)
blank.c:11:46: note: each undeclared identifier is reported only once for each function it appears in

gcc Test.c blank.c -o b

gcc *.c

#ifndef BLANK_H_INCLUDED
#define BLANK_H_INCLUDED

bool blank(char string[]);

#endif

// Header file for blank function

#include "boolean.h"
bool blank(char string[]);

#ifndef BLANK_H
#define BLANK_H 1
// Header file for blank function

bool blank(char string[]);
#endif