Thread: Check if non ASCII character is inputted

hello, I'm trying to put in my code to check if non ASCII character is a valid input if not get an error message.

Code:

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


int key= 65;


unsigned int easyendecrypt(char c);


int main(void) {
    int c;
    while ((c = getc(stdin)) !=EOF) {
        putc(easyendecrypt(c), stdout);
    }
    key = 'A';
    printf("\n");
    return 0;
}


unsigned int easyendecrypt(char c) {
    unsigned int k;

#ifdef DECRYPT
    fprintf(stderr, "decrypting with %c\n",key);
    k = c - key;
    key = k;
    return k;
#else
    fprintf(stderr,"encrypting with %c\n",key);
    k = c + key;
    key = c;
    return k;
#endif
}