Function not getting called from the main function

I am seriously worried that I don't understand the very fundamentals of C programming. I am reading numbers from a text file, and I am going to check if they are prime numbers or not. However, the problem is that the IsPrime function is never being called from the main function. I have tried making the number variable a global and a local variable, but it does not work. Is my basic understand that bad?

Reading from the text file is no problem. It looks like this:

73771782    81296771    79982326    75332246    10128193
81643413    76259734    94432076    50063976    91748657
42311916    -1920042    90747362    53851612    43498487
73193311    96685173    39019033    8630045     17

Because the main function never calls the IsPrime function, the consequence is that all numbers are printed out as is a prime number.

Here is the code:

#include <stdio.h>
#include <stdlib.h>
#include <string.h>

int isPrime(int number);

int main(int argc, char **argv) {
    int number = 0;
    int number2 = 0;
    FILE *fp = NULL;
    
    char file_name[] = "C:\\Users\\Erlen\\Desktop\\numbers.txt";
    fp = fopen(file_name, "r"); 
    
    if (fp == NULL) {
        printf("Error opening the file: \n");
        exit(0);
    }
    
    fscanf(fp, "%d", &number);
    
    while (number != number2) {
        if (isPrime(&number) == 1) {
            printf("Number: %d is a prime number \n", number);
        } else {
            printf("The number %d is a prime number \n", number);
        }
        number2 = number;
        fscanf(fp, "%d", &number);
    }
    printf("\n");
    
    return 0;
}

int isPrime(int number) {
    for (int i = 2; i < number; i++) {
        if (number % i == 0) {
            return 0;
        }
    }
    return 1;
}

Solution

There are multiple problems in your code:

you pass the address of number instead of its value to isPrime.
you should iterate while (fscanf(fp, "%d", &number) == 1) to handle all numbers in the file. Your current loop logic is cumbersome and would fail if the first number if 0 or if the file contains identical consecutive numbers.
it is idiomatic in C to only test if the return value of isPrime is non zero.
the message for composite numbers is incorrect.
the convention for the exit status is non zero for failure.
you should take the absolute value of negative numbers.
you should return 0 for 0 and 1 which are not considered prime numbers.
you can improve the performance of isPrime for large numbers by testing if i * i <= number instead of i < number, reducing the complexity to O(sqrt(N)).

Here is a modified version:

#include <stdio.h>

int isPrime(int number);

int main(int argc, char *argv[]) {
    int number;
    FILE *fp;
    char file_name[] = "C:\\Users\\Erlen\\Desktop\\numbers.txt";

    fp = fopen(file_name, "r"); 
    if (fp == NULL) {
        printf("Error opening the file: \n");
        return 1;
    }
    
    while (fscanf(fp, "%d", &number) == 1) {
        if (isPrime(number)) {
            printf("Number %d is a prime number\n", number);
        } else {
            printf("Number %d is a not a prime number\n", number);
        }
    }
    return 0;
}

int isPrime(int number) {
    if (number < 0)
        number = -number;
    if (number < 2)
        return 0;
    for (int i = 2; i < number; i++) {
        if (number % i == 0) {
            return 0;
        }
    }
    return 1;
}