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

[Denys Vlasenko <vda.linux@googlemail.com>]

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