Re: [Qemu-devel] [RFC PATCH] vl.c: add -semihosting-config "arg" sub-argument

[Eric Blake <eblake@redhat.com>]

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On 04/21/2015 10:54 AM, Peter Maydell wrote:
> On 21 April 2015 at 17:48, Eric Blake <eblake@redhat.com> wrote:
>> On 04/21/2015 09:55 AM, Peter Maydell wrote:
>>> If you want pure POSIX shell then it is slightly
>>> uglier as we have to invoke sed:
>>>
>>> argstr=""
>>> for arg in "$@"; do
>>>      o=$(printf '%s.\n' "$arg" | sed s/,/,,/g)
>>>      argstr=${argstr:+$argstr,}arg=${o%.}
>>> done
>>
>> It's possible to use IFS= and case to avoid the need to fork, while
>> still writing portable shell, but the solution then takes LOTS more
>> lines of code, so I'm not even going to bother to try writing it here.
>>
>> However, one thing this does NOT handle is an argument that is a literal
>> ','.  That is, if I pass 'a', ',' and 'b' as my arguments, this script
>> results in $argstr being a,,,,b, which the qemu parser would read as a
>> single argument of 'a,,b' rather than the three intended arguments.
> 
> Hmm? You get "arg=a,arg=,,,arg=b". Does the parser not read that
> the way I would expect?

D'oh. I missed the 'arg=' insertion.

> 
>> In order to handle that, you'd need some sort of solution that requires
>> an unambiguous separation between arguments.  For example,
>>
>> --semihosting-options opt=a,,,,b
>> vs.
>> --semihosting-options opt=a,opt=,,,opt=b
> 
> The latter is what the script works with (except 'arg' rather
> than 'opt').

Indeed, we have an unambiguous representation already, and I just
overlooked it.

> 
> Incidentally if you have a better idea for achieving the
> desired goal than this messing around with comma-escaping
> I would really like to know. I don't like either of the
> current two proposals very much :-(

Libvirt is already used to doing comma escaping; so while I may not like
the resulting syntax, I _do_ like the consistency factor (that is, it's
much easier to reuse the same mechanism everywhere than it is to invent
yet another mechanism).

-- 
Eric Blake   eblake redhat com    +1-919-301-3266
Libvirt virtualization library http://libvirt.org


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