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Re: [Qemu-block] [PATCH v2] qemu-io: Reinitialize optind to 1 (not 0) be

From: Eric Blake
Subject: Re: [Qemu-block] [PATCH v2] qemu-io: Reinitialize optind to 1 (not 0) before parsing inner command.
Date: Mon, 7 Jan 2019 11:46:28 -0600
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:60.0) Gecko/20100101 Thunderbird/60.3.1

On 1/7/19 11:17 AM, Max Reitz wrote:
> On 03.01.19 10:47, Richard W.M. Jones wrote:
>> On FreeBSD 11.2:
>>   $ nbdkit memory size=1M --run './qemu-io -f raw -c "aio_write 0 512" $nbd'
>>   Parsing error: non-numeric argument, or extraneous/unrecognized suffix -- 
>> aio_write
>> After main option parsing, we reinitialize optind so we can parse each
>> command.  However reinitializing optind to 0 does not work on FreeBSD.
>> What happens when you do this is optind remains 0 after the option
>> parsing loop, and the result is we try to parse argv[optind] ==
>> argv[0] == "aio_write" as if it was the first parameter.
>> The FreeBSD manual page says:
>>   In order to use getopt() to evaluate multiple sets of arguments, or to
>>   evaluate a single set of arguments multiple times, the variable optreset
>>   must be set to 1 before the second and each additional set of calls to
>>   getopt(), and the variable optind must be reinitialized.
> [...]
>> Note I didn't set optreset.  It's not present in glibc and the "hard
>> reset" is not necessary in this context.
> But it sure sounds like FreeBSD requires you to set it, doesn't it?

The reason BSD and glibc have a hard reset path is because of hidden
state - both BSD and glibc track state that remembers if the options
began with '+' or '-' (both of those are extensions beyond POSIX), and
whether POSIXLY_CORRECT was set.  Beyond that hidden state is a corner
case of one more piece of state that you can trigger using only POSIX:
if the user passes './prog -ab' while you had code:

swich (getopt(argc, argv, "ab")) {
  case 'a': optind = 1; ...

then things fall apart for both BSD and glibc, because getopt() has to
track invisible state in order to remember that the next call will
process the -b portion of the merged short-option in argv[optind==1]
rather than repeating the -a half and before moving on to optind==2.
But this latter corner case can only happen when getopt() did not return -1.

At the end of the day, both GNU optind=0 and BSD optreset=1 are
sufficient to force a hard reset of all hidden state.  But if you don't
use POSIX extensions, and always run getopt() until a -1 return, then
setting optind=1 is a portable soft reset, regardless of how the hidden
state is implemented, and regardless of how (or even if) libc offers a
hard reset, even though POSIX itself is currently lacking that mention.
(I should probably file a POSIX defect to get that wording listed in POSIX)

Eric Blake, Principal Software Engineer
Red Hat, Inc.           +1-919-301-3226
Virtualization:  qemu.org | libvirt.org

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