C header files without #ifndef still work...why? [duplicate] - c

Question

This question already has answers here:
Why are typedef identifiers allowed to be declared multiple times?
(3 answers)
Closed 5 years ago.
Why don't I get the error messages?
I have three header files named DataType.h, printInt.h, printStr.h, and one myApp.c.
DataType.h
typedef int Integer;
typedef char String;
printInt.h
#include "DataType.h"
void printInt(Integer);
printInt.c
#include "printInt.h"
#include <stdio.h>
void printInt(Integer number){
printf("%d\n", number);
}
printStr.h
#include "DataType.h"
void printStr(String*);
printStr.c
#include "printStr.h"
#include <stdio.h>
void printStr(String *str){
printf("%s\n", str);
}
myApp.c
#include "printStr.h"
#include "printInt.h"
Integer main(void){
printInt(20);
printStr("hello");
return 0;
}
Clearly, I have included the DataType.h twice, and I did not use #ifndef to avoid redefinition of Integer and String. Please, someone, tell me how I can get the error messages to demonstrate the directive is working properly.
#ifndef __DATATYPE_H
#define __DATATYPE_H
typedef int Integer;
typedef char String;
#endif
Regardless having #ifndef or not, the gcc complier (version 5.4.0) does not generate any error messages. What's wrong?

Answer 1

Definitions of typedef and prototypes of functions can occur as many times as you want. For example:
typedef int lala;
typedef int lala;
void somePrototype();
void somePrototype();
int main() {
return 0;
}
will compile just fine: https://ideone.com/4EjfaR
Try adding the definition of a function to a header file. You will see then that you get a redefinition error and will require a header guard.