Passing the value of pointer - c

Question

I wanted to create a simple program to better understand how the pointers work and I came across a problem. I want to work on 3 files main.c modul.c and modul.h.
modul.h
typedef struct
{
int data;
}w_options;
int show(); //prototype of function
modul.c
#include <stdio.h>
#include <string.h>
#include <stdlib.h>
#include "modul.h"
int show()
{
w_options *color;
color = (w_options *)malloc(sizeof(w_options));
printf("%d\n", color->data);
if (color->data == 1)
{
printf("Good\n");
}
else
{
printf("Bad\n");
}
}
main.c
#include <stdio.h>
#include <string.h>
#include <stdlib.h>
#include "modul.h"
int main()
{
w_options *color;
color = (w_options *)malloc(sizeof(w_options));
color->data=1;
printf("%d\n", color->data);
show();
}
In main.c I am setting the value of color->data to 1 and it's working, it's printing 1. But I would like to pass this set value to modul.c. That's way I created simple if instruction to check if the value is passed. Unfortunately the value isn't passed and I don't have idea how to fix it. I need this kind of solution for my bigger program.
The Output:
1
0
Bad

Answer 1

You just have to pass it as an argument to your function. And, as your function returns nothing, declare it as void.
modul.h
typedef struct
{
int data;
}w_options;
void show(w_options *color); //prototype of function
modul.c
#include <stdio.h>
#include <string.h>
#include <stdlib.h>
#include "modul.h"
void show(w_options *color)
{
if (color->data == 1)
{
printf("Good\n");
}
else
{
printf("Bad\n");
}
}
main.c
#include <stdio.h>
#include <string.h>
#include <stdlib.h>
#include "modul.h"
int main()
{
w_options *color;
color = (w_options *)malloc(sizeof(w_options));
color->data=1;
printf("%d\n", color->data);
show(color);
return EXIT_SUCCESS;
}

Answer 2

Currently you aren't passing any values.
In modul.c you're creating one pointer named color that has no initialized values, you are only allocating memory.
color = (w_options *)malloc(sizeof(w_options));
printf("%d\n", color->data);
It's an accident that it happens to print 0 as the current value. This isn't guaranteed at all.
In main.c you're creating another different pointer, also named color but in a different scope, whose own color->data is set to 1.
color = (w_options *)malloc(sizeof(w_options));
color->data=1;
printf("%d\n", color->data);
This correctly prints 1 as the current value because you've initialized it properly.
If you want show to use a pointer, pass the pointer to it as an argument and use it on the other end.
main.c
...
show(color);
...
modul.c
...
int show(w_options *color)
{
// this is a parameter now, receiving the value from its caller
//w_options *color;
//color = (w_options *)malloc(sizeof(w_options));
printf("%d\n", color->data);
if (color->data == 1)
{
printf("Good\n");
}
else
{
printf("Bad\n");
}
}
...