Re: getopt() not exposing __optpos - shell needs it

[Rich Felker <dalias@libc.org>]

On Tue, Aug 29, 2017 at 01:32:01PM +0200, Denys Vlasenko wrote:
> On Mon, Aug 28, 2017 at 5:28 PM, Rich Felker <dalias@libc.org> wrote:
> > On Mon, Aug 28, 2017 at 12:18:57PM +0200, Denys Vlasenko wrote:
> >> I am using getopt() in busybox hush shell.
> >> "unset" builtin, for example: it takes -v and -f options.
> >> This works fine.
> >>
> >> However, POSIX requires that shells has a "getopts" builtin:
> >> http://pubs.opengroup.org/onlinepubs/9699919799/utilities/getopts.html
> >>
> >> It is basically an API binding to access getopt() in the shell code:
> >> it uses OPTIND and (in bash) OPTERR on entry, returns a single-char
> >> variable on return and updates OPTIND and OPTARG. Sounds familiar, right?
> >>
> >> When I try to do that (use getopt() to implement "getopts"), it hits a snag.
> >> Unlike normal getopt() usage in C programs, where it is called in a loop
> >> with the same argv[] array until parsing is finished,
> >> when it is used from "getopts", each successive call will (usually) have
> >> the same argv[] CONTENTS, but not the ADDRESSES.
> >> (The reason is in how shell works: it re-creates command arguments just before
> >> running a command, since there can be variable substitution, globbing, etc).
> >
> > First, some background out of the spec to establish what is supposed
> > to work and what's not:
> >
> >     If the application sets OPTIND to the value 1, a new set of
> >     parameters can be used: either the current positional parameters
> >     or new arg values. Any other attempt to invoke getopts multiple
> >     times in a single shell execution environment with parameters
> >     (positional parameters or arg operands) that are not the same in
> >     all invocations, or with an OPTIND value modified to be a value
> >     other than 1, produces unspecified results.
> >
> > What this means is that, when you use getopts(1), you need to either
> > use the exact same arguments (as you said, *string contents*, not
> > likely to be the same argv[] pointers) or reset it with OPTIND=1.
> >
> > It seems to me that the easiest, fully-portable fix is just the
> > obvious quadratic-time solution: on each run of getopts(1), reset
> > getopt(3) to the start and call it ++N times.
> 
> This has several problems:
> It prints multiple messages "invalid option -q"
> when there are options which are not in optstring.

opterr=0;

Either leave it 0 and always do your own error printing, or set it
nonzero just before the last call (for the current option) so that
only that one prints an error.

> It mangles optarg if an option without argument follows
> an option with an argument.

Maybe I'm missing what you're trying to say, but all the state is
clobbered; I don't see how optarg is a problem specifically. You can
clear or set it to a sentinel value before the relevant call if you're
trying to determine if the call set it. Across other calls (not the
one for the current option) I don't see why it matters at all what
happens to it.

Rich