On Thu, Jan 21, 2016 at 07:21:49AM +0100, Markus Armbruster wrote: > Eric Blake <ebl...@redhat.com> writes: > > > On 01/20/2016 02:02 AM, Markus Armbruster wrote: > > > >>> @@ -519,6 +519,8 @@ static QObject *parse_literal(JSONParserContext *ctxt) > >>> } > >>> case JSON_FLOAT: > >>> /* FIXME dependent on locale */ > >>> + /* FIXME our lexer matches RFC7159 in forbidding Inf or NaN, > >> > >> For what it's worth, the RFC spells this "RFC 7159". > > > > Looks like we use space more often than not, but that we're > > inconsistent. For example: > > > > slirp/tcp.h: * Per RFC 793, September, 1981. > > slirp/tcp.h: * Per RFC793, September, 1981. > > > > Will fix if I need to respin, otherwise I assume you can do it. > > Okay. > > >>> + /* FIXME: snprintf() is locale dependent; but JSON requires > >>> + * numbers to be formatted as if in the C locale. */ > >> > >> The new FIXME matches the existing one in parse_literal(), visible > >> above. > >> > >> However, dependence on C locale is a pervasive issue in QEMU. These two > >> comments could give readers the idea that it's an issue just here. > >> Suggest to add something like "Dependence on C locale is a pervasive > >> issue in QEMU." > > > > Good idea. > > > >> > >>> + /* FIXME: This risks printing Inf or NaN, which are not valid > >>> + * JSON values. */ > >>> + /* FIXME: the default precision of %f may be insufficient to > >>> + * tell this number apart from others. */ > >> > >> Yup. > >> > >> The easy way to avoid loss of precision is %a, but of course that's way > >> too much sophistication for JSON. > >> > >> Avoiding loss of precision with a decimal format is non-trivial; see > >> Steele, Jr., Guy L., and White, Jon L. How to print floating-point > >> numbers accurately, SIGPLAN ’90, and later improvements collected here: > >> http://stackoverflow.com/questions/7153979/algorithm-to-convert-an-ieee-754-double-to-a-string > > > > Ah, memories. I read and implemented that paper when working on the > > jikes compiler for the Java language back in the late nineties, as it is > > the Java language specification which had the very neat property of > > requiring the shortest decimal string that will unambiguously round back > > to the same floating point pattern. > > > > One alternative is to always output a guaranteed unambiguous decimal > > string (although not necessarily the shortest), by using %.17f, using > > <float.h> DBL_DECIMAL_DIG. (Note that DBL_DIG of 15 is NOT sufficient - > > it is the lower limit that says that a decimal->float->decimal will not > > change the decimal; but we want the converse where a > > float->decimal->float will not change the float. There are stretches of > > numbers where the pigeonhole principle applies; you can think of it this > > way: there is no way to map all possible 2^10 (1024) binary values > > inside 2^3 (1000) decimal digits without at least 24 of them needing one > > more decimal digit. But by the same arguments, DBL_DECIMAL_DIG is an > > upper limit and usually more than you need.) > > > > So, the question is whether we want to always output 17 digits, or > > whether we want to do the poor-man's truncation scheme (easy to > > understand, but not optimal use of the processor), or go all the way to > > the algorithm of that paper (faster but lots harder to understand). For > > reference, here's the poor-man's algorithm in pseudocode: > > I don't think we want to implement floating-point formatting ourselves.
FWIW in libvirt we use per-thread locale switching, so that we can flip into the C locale temporarily when formatting doubles. If those are not available we have a bit of a nasty hack to post-process the locale dependant string back into what we (hope) is C-locale formatting. We don't do anything aobut the Inf/NaN problem though. http://libvirt.org/git/?p=libvirt.git;a=blob;f=src/util/virutil.c;h=bb9604a0c1ffb9c99e454e84878a8c376f773046;hb=HEAD#l454 Regards, Daniel -- |: http://berrange.com -o- http://www.flickr.com/photos/dberrange/ :| |: http://libvirt.org -o- http://virt-manager.org :| |: http://autobuild.org -o- http://search.cpan.org/~danberr/ :| |: http://entangle-photo.org -o- http://live.gnome.org/gtk-vnc :|