sort input lines lexicographically or numerically.

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

#define MAXLINES  5000  /*max #lines to be sorted */
char *lineptr[MAXLINES];    /*pointers to text lines */

int readlines(char *lineptr[], int nlines);
void writelines(char *lineptr[], int nlines);

void qsort(void *lineptr[], int left, int right, 
           int (*comp)(void *, void *));
int numcmp(char *, char *);

/*sort input lines */
main(int argc, char *argv[]){
     int nlines;   /* number of input lines read */
     int numeric = 0;   /*1 if numeric sort */

      if (argc > 1 && strcmp(argv[1], "-n") == 0)
          numeric = 1;
      if ((nlines = readlines(lineptr, MAXLINES)) >= 0) {
         qsort((void **) lineptr, 0, nlines -1,
              (int (*) (void*, void*))(numeric ? numcmp : strcmp));
         writelines(lineptr, nlines);
         return 0;
      }else {
           printf("input too big to sort\n");
           return 1;
       }
}