[XML-DEV Mailing List Archive Home] [By Thread] [By Date] [Recent Entries] [Reply To This Message] Poor interoperability due to ambiguity of which has precedence: XML enco
Hi Folks, The encoding of an XML document can be specified: 1. In the XML declaration 2. In an HTTP charset header field (if the XML document is being transported via HTTP) 3. In the BOM Suppose the BOM of an XML document indicates that each character in the XML document uses 2 bytes but the XML declaration specifies UTF-8. Applications must resolve this ambiguity. The problem is that every application resolves it differently. Hence, there is poor interoperability. How should this ambiguity be resolved? Here is one approach: In cases where conflicting information is supplied (from charset param, BOM and/or XML encoding declaration) give the BOM, if present, authoritative status. Martin J. D�� responded: I'm a bit uneasy about the fact that we now have BOM (internal) - charset (external) - encoding (internal), i.e. internal-external-internal, but I guess there is lots of experience in HTML 5 for giving the BOM precedence. Also, it will be extremely rare to have something that looks like a BOM but isn't, and this combined with the fact that XML balks on encoding errors should make things quite robust. This recommendation has been proposed: Recommend against the use of UTF-32. Martin J. D�� responded: UTF-32 has some (limited) appeal for internal representation, but none really on the network, and media types are for network interchange, so this should be fine, too. Leif Halvard Silli responded to Martin J. D��: > I'm a bit uneasy about the fact that we now have BOM (internal) - > charset (external) - encoding (internal), i.e. > internal-external-internal, A better way of looking at would be that we now get External-Internal, where external is subdivided in charset parameter and encoding signature [BOM]. And internal is subdivided in encoding declaration and default/fallback encoding. Yeah, it might be that a lack of clear classification of the BOM as an external method is quite directly linked to the lack of interoperability. Previously we had External-Limbo-Internal. However, per XML, both BOM and charset param are external.[1] > but I guess there is lots of experience > in HTML 5 for giving the BOM precedence. Sorry for focusing on XML rather than XML media types, but I think both of them should be edited. The way of looking at it that I propose above also incorporates the fact that XML-capable Web browsers (the HTML 5 browsers) give precedence to the BOM, and without fatal error if there is a (conflicting) XML encoding declaration. (BTW, I find it very odd that, up until now, the *charset* parameter could override the encoding declaration, but if the BOM does the same [that is: overrides the encoding declaration], *then* it is a fatal error ...) It makes sense to treat all external encoding declaration methods the same. Currently only the external *transport* protocol may override the internal mechanism. But the BOM should have the same "right". Therefore I would suggest that the other spec, XML 1.0, section 4.3.3 [2] does this (see the <INS> element): In the absence of information provided by an external transport protocol (e.g. HTTP or MIME) <INS>OR BY THE BYTE ORDER MARK</INS>, it is a fatal error for an entity including an encoding declaration to be presented to the XML processor in an encoding other than that named in the declaration. It should still be an error, but not a fatal error, if the XML encoding declaration conflicts with the external method - BOM or HTTP. QUESTION How do you recommend dealing with the situation where the BOM, charset, and XML encoding are conflicting? /Roger [1] http://www.w3.org/TR/REC-xml/#NT-document [2] http://www.w3.org/TR/REC-xml/#charencoding [Date Prev] | [Thread Prev] | [Thread Next] | [Date Next] -- [Date Index] | [Thread Index] |
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