On 08/09/2017 02:59 AM, Markus Armbruster wrote: > Eric Blake <ebl...@redhat.com> writes: > >> qobject_from_jsonv() was unusual in that it took a va_list*, instead >> of the more typical va_list; this was so that callers could pass >> NULL to avoid % interpolation. While this works under the hood, it >> is awkward for callers, so move the magic into qjson.c rather than >> in the public interface, and finally improve the documentation of >> qobject_from_jsonf(). >>
>> - /* Going through qobject ensures we escape strings properly. >> - * This seemingly unnecessary copy is required in case va_list >> - * is an array type. >> - */ >> - va_copy(ap_copy, ap); >> - qobj = qobject_from_jsonv(fmt, &ap_copy, &error_abort); >> - va_end(ap_copy); >> + /* Going through qobject ensures we escape strings properly. */ >> + qobj = qobject_from_jsonv(fmt, ap); >> qstr = qobject_to_json(qobj); >> >> /* > > Wait! Oh, the va_copy() moves iinto qobject_from_jsonv(). Okay, I > guess. >> + >> + /* >> + * This seemingly unnecessary copy is required in case va_list >> + * is an array type. >> + */ > > --verbose? Code motion. But if the comment needs to be more verbose in the destination than it was on the source, the rationale is that C99/POSIX allows 'typedef something va_list[]' (that is, where va_list is an array of some other type), although I don't know of any modern OS that actually defines it like that. Based on C pointer-decay rules, '&ap' has a different type based on whether va_list was a struct/pointer or an array type, when 'va_list ap' was passed as a parameter; so we can't portably use qobject_from_json_internal(string, &ap, &error_abort). The va_copy() is what lets us guarantee that &ap_list is a pointer to a va_list regardless of the type of va_list (because va_copy was declared locally, rather than in a parameter list, and is therefore not subject to pointer decay), and NOT an accidental pointer to first element of the va_list array on platforms where va_list is an array. -- Eric Blake, Principal Software Engineer Red Hat, Inc. +1-919-301-3266 Virtualization: qemu.org | libvirt.org
signature.asc
Description: OpenPGP digital signature
