Scope and conditional #define in C - c

Question

I want to use local .h file for define same names used in my lib. My lib has a default value definition for this names, but I'd like change this default value using local .h file. However, I'm having unwanted behaviour. How can I solve this?
test.c
#include <stdio.h>
#include <string.h>
#include <stdint.h>
#include "conf.h"
#include "mylib.h"
int main ()
{
printf("Value in main: %d\n", NAMEDEFINITION);
fn();
return 0;
}
conf.h
#define NAMEDEFINITION 42
mylib.h
#include <stdio.h>
#include <string.h>
#include <stdint.h>
#ifndef NAMEDEFINITION
#define NAMEDEFINITION 84
#endif
void fn();
mylib.c
#include "mylib.h"
void fn()
{
printf("Value in fn: %d\n", NAMEDEFINITION);
return;
}
My compiling line and output:
user#local:~/user/test/c$ gcc test.c mylib.c -o test
user#local:~/user/test/c$ ./test
Value in main: 42
Value in fn: 84
[EDITED]
I'd like NAMEDEFINITION be "42" when I define it in conf.h, then print "42" in two main() calls. When it is not defined in conf.h, it would be its default value, "84" (print "84" in two main() calls.

Answer 1

Value in main: 42
Reason for this is that in test.c you have included conf.h Now even though you include mylib.h, NAMEDEFINITION is visible in test.c and NAMEDEFINITION will have a value of 42. The ifdef in mylib.h will not be valid.
Value in fn: 84
In mylib.c you have not included conf.h. So the line #ifndef NAMEDEFINITION will be true and the value of NAMEDEFINITION will be 84.
If you want the value in fn to also print 42, you need to include conf.h in the mylib.c or mylib.h
Then if you comment out the line #define NAMEDEFINITION 42 in conf.h the value 84 will be printed both times.

Answer 2

I see 3 ways you could go with this:
1. Make config file mandatory part of your library
mylib.h
#include <stdio.h>
#include <string.h>
#include <stdint.h>
#include "conf.h"
#ifndef NAMEDEFINITION
#error NAMEDEFINITION hasn't beed defined! Please edit conf.h!
#endif
void fn();
2. Define NAMEDEFINITION with compilation option
Example:
-D NAMEDEFINITION=42
With this approach you'd not use conf.h file at all. Downside to this method is that you have to make sure that you remember to include this option for your both .c file compilations.
3. Include conf.h with compilation option
Example:
-include conf.h
This is similar to method 2, but instead of defining symbol directly, you would force the inclusion of the conf.h instead.
What ever you do, make sure your header files have include guards. (Thanks #ChristianGibbons)

Answer 3

The explanation is very simple.
First example (main)
in the conf.h you define it. When you include mylib.h it is already defined and it is not redefined. So the value is 42
In the second example (fn) you just include mylib.h and #ifndef condition is true.
Macros are expanded before the compilation and are valid only in one compilation unit.,