Date: Wed, 13 Aug 2014 16:02:15 -0700 From: Marcel Moolenaar <marcel@xcllnt.net> To: Phil Shafer <phil@juniper.net> Cc: arch@freebsd.org, Poul-Henning Kamp <phk@phk.freebsd.dk>, Marcel Moolenaar <marcel@freebsd.org>, John-Mark Gurney <jmg@funkthat.com>, "Simon J. Gerraty" <sjg@juniper.net> Subject: Re: XML Output: libxo - provide single API to output TXT, XML, JSON and HTML Message-ID: <613EB1A5-2932-446A-A9A2-8CBDD060A00B@xcllnt.net> In-Reply-To: <201408131936.s7DJaA1r089174@idle.juniper.net> References: <201408131936.s7DJaA1r089174@idle.juniper.net>
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On Aug 13, 2014, at 12:36 PM, Phil Shafer <phil@juniper.net> wrote:
>
> I've a related topic: when an app goes to run a child command, how
> can it determine whether that binary supports libxo-based encoding
> requests? This should be known before the binary is run, since
> there's no means of auto-detecting the supported output after the
> fact.
>
> For example, say I want to make a JSON-based API for my server. I
> can setenv("LIBXO_OPTIONS", "json") to get JSON output, but I won't
> know if the binary supports this or if the output needs to be wrapped
> and escaped.
Aside:
Using environment variables can be handy, but isn't always.
What do you think about calling a libxo init function from
main() and giving it argc and argv so that libxo options
are parsed and removed just like what xlib does?
> I know ELF "Note" elements can be used to carry vendor-specific
> data, but have no experience with them. Would it be reasonable to
> use them as a means of communicating this information to other bits
> of software? Is FreeBSD using Notes for other information currently?
Notes are used to tag the binary as a FreeBSD one (note is
consumed by the kernel) or in core files for meta data
(consumed by the debugger).
A note section is definitely possible and reasonable.
Especially if it's a note section for listing features. A
special utility that consumes the note section to list
features and returns whether a feature is supported is then
very reasonable because it's generic. libxo would be the
first feature we can check for. The question is: do we have
more features we want to check for this way? If not, then
such a scheme could be perceived as "heavy handed".
Alternatives include looking for a particular symbol or
possibly even running the utility with a libxo option that
has predictable output. The last suggestion has some issues
with handling the behaviour when libxo isn't supported
therefore, a passive way to check seems better than having
to run the utility.
BTW: this is pretty powerful stuff! I feel FreeBSD is
maturing :-)
--
Marcel Moolenaar
marcel@xcllnt.net
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