initializing global variables from a text file in C - c

Question

I have a beginner question in C. I'm having trouble in reading a file with values that initialize global variables to be shared in different C files. My true project has many variables to be used and changed across multiple files. My file that reads the parameters is:
#include <stdio.h>
#include <stdlib.h>
#include "parameters.h"
#include "prototypes.h"
#define MAX_LENGTH 100
int ReadParameters(void)
{
char line[MAX_LENGTH];
FILE *fp = fopen("parameters.in", "r");
if (! fp)
return EXIT_FAILURE;
int numread = 0;
while (fgets(line, MAX_LENGTH, fp)) {
numread += sscanf(line, "x = %d", &x);
numread += sscanf(line, "y = %d", &y);
numread += sscanf(line, "z = %d", &z);
}
fclose(fp);
return numread;
}
I defined the variables x, y, and z as external in a header file:
#ifndef PARAMETERS_H
#define PARAMETERS_H
extern int x;
extern int y;
extern int z;
#endif
The prototype of ReadParameters.c is in the header prototype.h and the values in x, y, and z are used in main.c:
#include <stdio.h>
#include <stdlib.h>
#include "parameters.h"
#include "prototypes.h"
int main()
{
ReadParameters();
printf("The value of x: %d\n", x);
printf("The value of y: %d\n", y);
printf("The value of z: %d\n", z);
x += 15;
y -= 5;
z -= 20;
printf("Now the value of x: %d\n", x);
printf("Now the value of y: %d\n", y);
printf("Now the value of z: %d\n", z);
return EXIT_SUCCESS;
}
When I compile I get the error of undefined reference to x, y, and z which I believe its because I only declared the variables not defined them. If I remove extern from the header file I get no errors and it runs but I read its not good practice to define variables in header files. If I make a another .C file and defined the variables there:
#include "parameters.h"
int x, y, z;
it works but is this ok to do? I'm new to programming and any suggestions would be appreciated. Thanks

Answer 1

extern means that the variables are available in some other compilation unit (ie. .c file). Typically you have one .h file for each .c file that "exposes" anything to other units.
So for something like this, I would probably have:
parameters.c
int x, y, z;
parameters.h
#ifndef PARAMETERS_H
#define PARAMETERS_H
extern int x;
extern int y;
extern int z;
#endif
main.c
#include "parameters.h"
int foo() {
x = 4; // setting x from parameters.c
}

Answer 2

extern tells the compiler that the definition of the variables lies else where( special case would be where variables are defined in the same compilation unit ). But linker should find these definitions. So, declare them extern in a header file(s) and include the header file in source files intended to use them. But don't forget to define them in exactly one source file. What you are doing seems ok to me.

Answer 3

it works but is this ok to do?
It is indeed regarded as bad practice, since it leads to "tight coupling" and spaghetti code.
The preferred way is this:
//parameters.h
void set_x (int x);
//parameters.c
#include "parameters.h"
static int private_x; // static means: only visible/accessible from parameters.c
void set_x (int x)
{
private_x = x;
}
// main.c
#include "parameters.h"
set_x (something);