Does anyone have any idea why this program doesn't write whether the input number is a prime or not to a txt file?

Code:

#include <values.h>
#include <limits.h>
#include <stdio.h>
#include <process.h>
#include <conio.h>
int write_function();
int main(void)
{
printf(" Welcome to the Prime Number Program\n");
printf("This program will determine if a number is a prime number.\n");
write_function();
getch();
return 0;
}
int write_function()
{
FILE *prime_file;
int number, i, remainder;
int not_divisible = 1;
int biggest = 1;
prime_file = fopen("prime_io.txt", "w");
printf("Enter a number(e.g. 1972): ");
scanf("%d", &number);
for( i = 2; i < number - 1; i++ )
{
remainder = number % i;
if(remainder == 0 )
{
not_divisible = 0;
biggest = i;
}
}
if(not_divisible == 1)
fprintf(prime_file, "\nThe number %d is a prime number", number);
else
fprintf(prime_file, "\nThis number's highest divisor is %d and thus is not prime", biggest);
fclose(prime_file);
}