The strspn function searches string for characters not found in the set string.

The strspn function returns the index of the first character located in string that does not match a character in set. If the first character in string does not match a character in set, a value of 0 is returned. If all characters in string are found in set, the length of string is returned.

strchr, strcspn, strpbrk, strpos, strrchr, strrpbrk, strrpos

#include <string.h>
#include <stdio.h> /* for printf */

void tst_strspn (char *digit_str) {
  char octd [] = "01234567";
  int i;

  i = strspn (digit_str, octd);

  if (digit_str [i] != '\0')
    printf ("%c is not an octal digit\n", digit_str [i]);
}