Add the option -f to fold upper and lower case together, so that case distinctions are not made during sorting;for e.g, a and A compare equal.

#include<stdio.h>
#include <string.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        LINES      100 /* max # of lines to be sorted */

int charcmp(char *, char *);
int numcmp(char *, char *);
int readlines(char *lineptr[], int maximum);
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 '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 -fnr \n");
           else{
               if((nlines = readlines(lineptr, LINES)) > 0){
                  if(option & NUMERIC)
                    qsort((void **) lineptr, 0, nlines - 1,
                           (int (*)(void *, void*)) numcmp);
                 else if (option & FOLD)
                   qsort((void **) lineptr, 0, nlines- 1,
                       (int (*)(void *, void *)) charcmp);
                else
                   qsort((void **) lineptr, 0, nlines -1,
                         (int (*)(void *, void *)) strcmp);
               }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){
    for(;tolower(*s) == tolower(*t);s++, t++)
        if (*s == '\0')
             return 0;
    return tolower(*s) - tolower(*t);
}