Modify the programs entab and detab to accept a list of tab stops as arguments. Use the default tab settings if there are no arguments.

#include <stdio.h>

#define   MAXLINE  100 /*maximum line size */
#define   TABINC     8   /*default tab increment size */
#define   YES        1   
#define   NO         0

void settab(int argc, char *argv[], char *tab);
void entab(char *tab);
int tabpos(int pos, char *tab);

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

    settab(argc, argv, tab);    /* initializ tab stops */
    entab(tab);            /* replace blanks w/tab */
    return 0;
}

/* entab: replace strings of blanks with tabs and blanks */
 void entab(char *tab){
      int c, pos;
      int nb = 0;     /* # of blanks necessary  */
      int  nt = 0;   /* # of tabs necessary */
      for(pos = 1; (c =getchar()) != EOF; pos++){
         if(c == ' '){
           if(tabpos(pos, tab) == NO) 
                ++nb;        /* increment # of blanks */
         }else{
              nb = 0;   /* reset # of blanks */
              ++nt;     /* one more tab */
         }else{
             for(; nt > 0; nt--)
                  putchar('\t');   /* output tab(s)    */
             if(c == '\t')      /* forget the blank(s)  */
                 nb = 0;
             else              /* output blank(s) */
                for(; nb > 0 ;nb--)
                     putchar(' ');
            putchar(c);
           if(c == '\n')
              pos = 0;
           else if (c == '\t')
             while(tabpos(pos, tab) != YES)
                   ++pos;
         }
      }
 }   

#include <stdlib.h>
#define  MAXLINE   100     /*maximum line size */
#define  TABINC     8      /* default tab increment size */
#define   YES      1
#define   NO       0
/*settab: set tab stops  in array tab      */
void settab(int argc, char *argv[], char *tab){
     int i, 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{     /*user provided tab stops */
       for(i =1;i <= MAXLINE;i++)
            tab[i] = NO;    /* turn off all tab stops */
      while(--argc > 0) {
          pos = atoi(*++argv);
          if(pos > 0 && pos <= MAXLINE)
               tab[pos] = YES;
      }
    }
} 
/* tabpos.c */
#define  MAXLINE   100     /* maximum line size */
#define  YES        1     
/* tabpos: determine if pos is at a tab stop  */
int tabpos(int pos, char *tab){
     if(pos > MAXLINE)
        return YES;
     else
       return tab[pos];
}
/*detab: replace tab with blanks */
void detab(char *tab){
     int c, pos = 1;
     while((c =getchar()) != EOF)
        if(c =='\t'){
          do
             putchar(' ');
         while(tabpos(pos++, tab) != YES);
        }else if (c == '\n'){
            putchar(c);
            pos = 1;
        }else{         /* all other characters */
            putchar(c);
            ++pos;
        }
}