Add preprocessor directives in four header files.

src/createCustomer.h

@@ -1,3 +1,5 @@
+#ifndef CREATE_CUSTOMER_H
+#define CREATE_CUSTOMER_H
 #include <stdio.h>
 #include <stdlib.h>
 #include <stdbool.h>
@@ -7,3 +9,4 @@
 int generateID();
 void collectCustomerProperties();
 void writeCustomerPropertiesToFile(customer_t *);
+#endif

src/customerProperties.h

@@ -1,3 +1,5 @@
+#ifndef CUSTOMER_PROPERTIES_H
+#define CUSTOMER_PROPERTIES_H
 typedef struct Customer
 {
 	unsigned int ID;
@@ -6,3 +8,4 @@ typedef struct Customer
 	char *forename, *surname;
 	float balance;	
 }customer_t;
+#endif

src/loginCustomer.h

@@ -1,3 +1,5 @@
+#ifndef LOGIN_CUSTOMER_H
+#define LOGIN_CUSTOMER_H
 #include <stdio.h>
 #include <stdlib.h>
 #include <stdbool.h>
@@ -8,3 +10,4 @@
 bool checkLogin(bool);
 void collectCustomerDataForLogin(int);
 bool loginCustomer(customer_t *);
+#endif

src/stringManipulation.h

@@ -1,3 +1,5 @@
+#ifndef STRING_MANIPULATION_H
+#define STRING_MANIPULATION_H
 #include <stdlib.h>
 #include <stdio.h>
 #include <string.h>
@@ -8,3 +10,4 @@ char *generateCheckString(unsigned int, char *);
 unsigned int toUnsignedInteger(char *);
 unsigned int power(unsigned int, unsigned int);
 bool everyCharacterIsDigit(char *);
+#endif