Extend entab and detab to accept the shorthand entab -m +n to mean tab stops every n columns, starting at column m.Choose convenient(for the user) default behavior.

#include <stdio.h>

#define   MAXLINE   100    /*maximum line size */
#define   TABINC     8    /*default tab increment size */
#define     YES      1   
#define     NO       0
void esettab(int argc,  char *argv[], char *tab);
void entab(char *tab);

/* replace strings of blanks with tabs */
int main(int argc, char *argv[]){
    char tab[MAXLINE + 1];

    esettab(argc, argv, tab);    /* initiliaze tab stops */
    entab(tab);                 /* replace blanks w/tab */
    return 0;
}
/*esettab.c */
#include<stdlib.h>
#define  MAXLINE   100   /* maximum line size */
#define  TABINC     8    /*default tab increment size */
#define   YES       1   
#define   NO        0
/* esettab: set tab stops in array tab */
void esettab(int argc, char *argv[], char *tab){
    int i, inc, pos;

   if(argc <= 1)    /* default tab stops */
     for(i =1; i <= MAXLINE;i++)
         if(i % TABINC == 0)
            tab[i] = YES;
         else
           tab[i] = NO;
   else if (argc == 3 && /*user provided range   */
              *argv[1] == '-' && *argv[2] == '+'){
       pos = atoi(&(*++argv)[1]);
       inc = atoi(&(*+=argv)[1]);
       for(i = 1; i <= MAXLINE;i++)
           if(i != pos)
                tab[i] = NO;
           else{
               tab[i] = YES;
               pos += inc;
           }
   }else{    /* user provided tab stops */
       for(i = 1; i <= MAXLINE;i++)
           tab[i] = NO;   /* turn off tab stops */
       while(--argc > 0) { /* walk through argument list */
             pos = atoi(*++argv);
             if(pos > 0 && pos <= MAXLINE)
                 tab[pos] = YES;
    }
}