A field width, or precision, or both may be indicated by  an
     asterisk  (*)  .  In this case, an argument of type int sup-
     plies the field width  or  precision.  Arguments  specifying
     field width, or precision, or both must appear in that order
     before the argument, if any, to  be  converted.  A  negative
     field  width  is  taken  as  a - flag followed by a positive
     field width. A negative precision is taken as if the  preci-
     sion were omitted. In format strings containing the %n$ form
     of a conversion specification, a field  width  or  precision
     may  be  indicated by the sequence *m$, where m is a decimal
     integer in the range [1, NL_ARGMAX] giving the  position  in
     the  argument list (after the format argument) of an integer
     argument containing the field width or precision, for  exam-
     ple:

     printf("%1$d:%2$.*3$d:%4$.*3$d\n", hour, min, precision, sec);

     The format can contain either numbered  argument  specifica-
     tions (that is, %n$ and *m$), or unnumbered argument specif-
     ications (that is, % and *), but normally not both. The only
     exception to this is that %% can be mixed with the %n$ form.
     The results  of  mixing  numbered  and  unnumbered  argument
     specifications  in  a format string are undefined. When num-
     bered argument specifications are used, specifying  the  Nth
     argument  requires  that all the leading arguments, from the
     first to the (N-1)th, are specified in the format string.

printf("%-25s", foo)
vs
printf("%-*s", max, foo)   # assuming max=25