Accessing a variable outside a c file - c

Question

I'm trying to do this code, and I split it up into .c files (lets say file1.c and file2.c) and file1.h file. I'm not allowed to change which parameters I can send to the function, so I need to find another way to "send"/access another variable. I tried to make the variable static in the header file file1.h, and include it in the file2.c. The function in file1.c look something like this:
int function(int *array, int a, int b){
...
...
if(global_variable == 1){
point = array[(a+b)/2];
}else if(global_variable == 0){
point = array[b];
}
and in the file2.c I have a function something like this:
double function2(t_sort_funcp fun, const case_t c, int array_length, result_t *buf, t_generate_array_funcp g_array){
int array[array_length];
switch (c)
{
case first:
global_variable = 1;
g_array(array, array_length);
return debugg(fun, array, array_length);
break;
case second:// Wors case is an inverted sorted array.
global_variable = 0;
g_array(array, array_length);
return debugg(fun, array, array_length);
break;
case third:
global_variable = 1;
g_array(array, array_length);
return debugg(fun, array, array_length);
break;
}
return 0;
}
In the file1.h I have:
#ifndef ALGORITHM_H
#define ALGORITHM_H
#include <stdbool.h> // bool
static int global_variable;
#endif
as you can see, I'm trying to change the global_variable variable in file2.c and use it in file1.c but that does not work, the if-statement in file1.c always executes the code in the else-statement, even if I changed the variable to 1.
NOTE: file2.c always executes before file1.c

Answer 1

Do it the opposite way
#ifndef ALGORITHM_H
#define ALGORITHM_H
#include <stdbool.h> // bool
extern int global_variable;
#endif
In one of the .c files
int global_variable;
Include the .h file in all files which require access to this variable.
static in global scope makes the variable only available in one compilation unit (file).

Answer 2

You can use extern with a cpp conditional.
Below are samples files.
FILE: file1.h
#ifndef ALGORITHM_H
#define ALGORITHM_H
#include <stdbool.h> // bool
#ifdef FILE1_C
int global_variable;
#else
extern int global_variable;
#endif
#endif
FILE: file1.c
// I am file1.c
#define FILE1_C
#include "file1.h"
FILE: file2.c
// I am file2.c
#include "file1.h"
UPDATE:
definitely wrong way. Why this weird ifdef complication –
0___________
No, it's not the wrong way. I do this all the time. The idea is that you have both definitions in the same place/file. If you put the global in a .c file, it's harder to see if you change the type.
Here's a more expanded case:
#ifndef ALGORITHM_H
#define ALGORITHM_H
#include <stdbool.h> // bool
#ifdef FILE1_C
#define EXTRN_FILE1 /**/
#else
#define EXTRN_FILE1 extern
#endif
EXTRN_FILE1 int global_variable;
EXTRN_FILE1 double global_variable_2;
#ifdef FILE1_C
int global_variable_3 = 37;
#else
extern int global_variable_3;
#endif
#endif
And, we may want to put the globals in a different .c file. If we rename FILE1_C to (e.g.) DEFINE_GLOBALS, we can put #define DEFINE_GLOBALS in any .c. If we change our minds later, it's trivial to move the #define from (e.g.) file1.c to file2.c.