Let’s start by writing a function to check for a prime number. We’ll iterate from 2 to the square root of the number, as a number greater than its square root won't be a divisor if no smaller number has been.

#include <stdio.h>
#include <stdbool.h>
#include <math.h>

bool isPrime(int num) {
    if (num <= 1) return false;
    for (int i = 2; i <= sqrt(num); i++) {
        if (num % i == 0) return false;
    }
    return true;
}

int main() {
    int number;
    printf("Enter an integer: ");
    scanf("%d", &number);

    if (isPrime(number))
        printf("%d is a prime number.", number);
    else
        printf("%d is not a prime number.", number);

    return 0;
}
            

This code introduces the use of the sqrt() function from the math library to optimize the prime checking process, reducing the number of iterations needed.

Compile the code using GCC or your preferred C compiler:

gcc -o primeCheck primeCheck.c -lm

Note the -lm flag is used to link the math library.

Execute your program to see if the entered number is prime:

./primeCheck

This will prompt you to enter a number and display whether it is prime.