illumos gate

This was pointed out by folks looking to integrate printf(1) into FreeBSD, who noted that the n$ support in my printf is not quite complete:

+ if (dollar) \\
+ *f = '$'; \\
} while (0)

static int asciicode(void);

This looks useful, but is incomplete. The n$ syntax can also be used
after an asterisk, and in fact this is required: positional (n$) and
non-positional arguments may not be mixed in a single format directive.

For example:
printf '[%3$*1$.*2$s]\\n' 20 10 abcdefghijklmn
and
printf '[%1$*2$.*3$s]\\n' abcdefghijklmn 20 10
do the same thing as
printf '[%*.*s]\\n' 20 10 abcdefghijklmn

Of the printf(1)s I have tried, this works in zsh's and ksh93's builtin.

This certainly needs quite a bit of additional code, but I think the n$
and * syntaxes are most useful together, since they are both meant for
situations where the format string is translated. If the format string
is not translated, * can be replaced by a parameter substitution (or
even hard-coding) and n$ can be replaced by reordering the arguments.