Add the -d(“directory order”) option, which makes comparisons only on letters, numbers, and blanks .Make sure it works in conjuction with -f.

#include <stdio.h>
#include <ctype.h>

#define   NUMERIC  1  /*numeric sort */
#define     DECR   2   /*sort in decreasing order */
#define     FOLD   4   /* fold upper and lower cases */
#define     DIR    8  /*directory order              */
#define     LINES   100 /* max # of lines to be sorted */

int charcmp(char *, char *);
int numcp(char *, char *);
int readlines(char *lineptr[], int maxlines);
void qsort(char *v[], int left, int right, int (*comp)(void*, void*));
void writelines(char *lineptr[], int nlines, int order);

static char option = 0;

/*sort input lines       */
int main(int argc, char *argv[]){
    char *lineptr[LINES];  /*pointers to text lines */
    int nlines;           /* number of input lines read */
    int c, rc = 0;

    while(--argc > 0 && (*++argv)[0] == '-')
        while(c == *++argv[0])
          switch(c){
            case 'd':   /* directory order */
                 option |= DIR;
                 break;
            case 'f':  /*fold upper and lower cases */
                option |= FOLD;
                break;
            case 'n':   /*numeric sort */
                 option |= NUMERIC;
                 break;
            case 'r':  /* sort in decreasing order */
                  option |= DECR;
                  break;
            default:
                  printf("sort: illegal option %c\n", c);
                  argc = 1;
                  rc = -1;
                  break;
           }
       if(argc)
           printf("Usage: sort -dfnr \n");
       else{
           if((nlines = readlines(lineptr, LINES)) > 0){
              if(option & NUMERIC)
                 qsort((void **) lineptr, 0, nlines -1,
                       (int (*)(void *, void *)) numcmp);
             else
                qsort((void **) lineptr, 0, nlines -1,
                      (int (*) (void *, void *)) charcmp);
             writelines(lineptr, nlines, option & DECR);
           }else{
               printf("input too big to sort \n");
               rc = -1;
           }
       }
       return rc;
}
/*charcmp: return <0 if s<t, 0 if s==t, >0 if s>t */
int charcmp(char *s, char *t){
    char a, b;
    int fold = (option & FOLD) ? 1 : 0;
    int dir =(option & DIR) ? 1 : 0;
    do{
       if(dir){
          while(!isalnum(*s) && *s != ' ' && *s != '\0') 
              s++;
         while(!isalnum(*t) && *t != ' ' && *t != '\0')
              t++;
       }
        a = fold ? tolower(*s) : *s;
        s++;
        b =fold ? tolower(*t) : *t;
        t++;
       if(a ==b && a == '\0')
           return 0;
    }while(a == b);
    return a - b;
}