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

int main()
{
    char *digits="0123456789";

    char x[] = "2abc";
    int result = atoi(x);
    printf ("case a) string %s is %d as decimal\n",x,result);

    if (strlen(x) == 0 || (strspn(x,digits) != strlen(x)))
        printf ("case b) %s is NOT a simple positive integer!\n"
        "        there are either no digits or non-digits\n", x);
        
    char y[] = "";
    int result_y = atoi(y);
    printf ("case c) a null string results in %d\n",result_y);      
	return 0;
}
/* prints:
   case a) string 2abc is 2 as decimal
   case b) 2abc is NOT a simple positive integer!
           there are either no digits or non-digits
   case c) a null string results in 0
*/