Example to print all prime numbers between two numbers (entered by the user) by making a user-defined function.

To understand this example, you should have the knowledge of following C programming topics:

To find all prime numbers between two integers, `checkPrimeNumber()`

function is created. This function checks whether a number is prime or not.

```
#include <stdio.h>
int checkPrimeNumber(int n);
int main()
{
int n1, n2, i, flag;
printf("Enter two positive integers: ");
scanf("%d %d", &n1, &n2);
printf("Prime numbers between %d and %d are: ", n1, n2);
for(i=n1+1; i<n2; ++i)
{
// i is a prime number, flag will be equal to 1
flag = checkPrimeNumber(i);
if(flag == 1)
printf("%d ",i);
}
return 0;
}
// user-defined function to check prime number
int checkPrimeNumber(int n)
{
int j, flag = 1;
for(j=2; j <= n/2; ++j)
{
if (n%j == 0)
{
flag =0;
break;
}
}
return flag;
}
```

**Output**

Enter two positive integers: 12 30 Prime numbers between 12 and 30 are: 13 17 19 23 29

If the user enters larger number first, this program will not work as intended. To solve this issue, you need to swap numbers first.