[SG16] Draft proposal: Clarify guidance for use of a BOM as a UTF-8 encoding signature

Shawn Steele Shawn.Steele at microsoft.com
Tue Oct 13 15:42:37 CDT 2020

My assertion is that if the application cannot change to UTF-8 due to legacy considerations, that the subtleties of whether to use a BOM or not also cannot be prescribed.  If the application could follow best practices, it would use UTF-8.  Since it cannot use UTF-8, therefore it can’t follow any prescribed behavior.  Therefore anything beyond “Use Unicode!” is merely suggestions.  Terminology like “require” implies a false sense of rigor that these applications can’t follow in practice.

Eg:  Presume I have a text editor that has been used in some context for some time.  If I’m told “use UTF-8”, that’s cool, I could try to do that, but if I cannot, then I’m in an exceptional path.  Unicode could suggest that I consider behavior for BOMs (such as ignoring them if present), however I’m already stuck in my legacy behavior, so there’s a limit to what my application can do.

However, if Unicode says “if you see a BOM, then you must use UTF-8”, then users of my legacy application that is difficult to change, may have expectations of the application that don’t match reality.  They could even enter bugs like “The app isn’t recognizing data being tagged with BOMs.”  Or “your system isn’t compliant, so we can’t license it.”  If the app could properly handle UTF-8, we’d have been captured in the first requirements and wouldn’t even be having this part of the conversation.  Since they can’t handle UTF-8, trying to enforce it through the BOM isn’t going to add much.

IMO it’s better that everyone involved understand that this legacy app that can’t handle UTF-8 by default isn’t necessarily going to behave per any set expectations and likely has legacy behaviors that users may need to deal with.

Granted, the difference between “requiring,” and “suggesting” or “recommending”, may be subtle, however those subtleties can sometimes cause unnecessary pain.

I don’t mind mandating UTF-8 without BOM if possible.  I don’t really mind mandating that BOMs be ignored if “without BOM” isn’t reasonable to mandate.

After that though, it’s trying to create a higher order protocol for codepage detection.  BOM isn’t a great way to identify UTF-8 data.  (It’s probably more effective to decode it as UTF-8.  If it decodes properly, then it’s likely UTF-8.  With a certainty of about as many “nines” as you have bytes of input.  Linguistically appropriate strings that fail that test are rare.)


From: Tom Honermann <tom at honermann.net>
Sent: Tuesday, October 13, 2020 1:04 PM
To: Shawn Steele <Shawn.Steele at microsoft.com>; Alisdair Meredith <alisdairm at me.com>
Cc: sg16 at lists.isocpp.org; Unicode Mail List <unicode at unicode.org>
Subject: Re: [SG16] Draft proposal: Clarify guidance for use of a BOM as a UTF-8 encoding signature

On 10/12/20 4:54 PM, Shawn Steele wrote:
I’m having trouble with the attempt to be this prescriptive.

These make sense:  “Use Unicode!”

  *   If possible, mandate use of UTF-8 without a BOM; diagnose the presence of a BOM in consumed text as an error, and produce text without a BOM.
  *   Alternatively, swallow the BOM if present.
After that the situation is clearly hopeless.  Applications should Use Unicode, eg: UTF-8, and clearly there are cases happening where that isn’t happening.  Trying to prescribe that negotiation should therefore happen, or that BOMs should be interpreted or whatever is fairly meaningless at that point.  Given that the higher-order guidance of “Use Unicode” has already been ignored, at this point it’s garbage-in, garbage-out.  Clearly the app/whatever is ignoring the “use unicode” guidance for some legacy reason.  If they could adapt, it should be to use UTF-8.   It *might* be helpful to say something about a BOM likely indicating UTF-8 text in otherwise unspecified data, but prescriptive stuff is pointless, it’s legacy stuff that behaves in a legacy fashion for a reason and saying they should have done it differently 20 years ago isn’t going to help 😊

There are applications that, for legacy reasons, are unable to change their default encoding to UTF-8, but that also need to handle UTF-8 text.  It is not clear to me that such situations are hopeless or that they cannot be improved.

The prescription offered follows what you suggest.  The first three cases are are all of the "use Unicode!" variety.  The distinction between the third and the fourth is to relegate use of a BOM as an encoding signature to the last resort option.  The intent is to make it clear, with stronger motivation than is currently present in the Unicode standard, that use of a BOM in UTF-8 is not a best practice today.


From: Unicode <unicode-bounces at unicode.org><mailto:unicode-bounces at unicode.org> On Behalf Of Tom Honermann via Unicode
Sent: Monday, October 12, 2020 7:03 AM
To: Alisdair Meredith <alisdairm at me.com><mailto:alisdairm at me.com>
Cc: sg16 at lists.isocpp.org<mailto:sg16 at lists.isocpp.org>; Unicode List <unicode at unicode.org><mailto:unicode at unicode.org>
Subject: Re: [SG16] Draft proposal: Clarify guidance for use of a BOM as a UTF-8 encoding signature

Great, here is the change I'm making to address this:
Protocol designers:

  *   If possible, mandate use of UTF-8 without a BOM; diagnose the presence of a BOM in consumed text as an error, and produce text without a BOM.
  *   Otherwise, if possible, mandate use of UTF-8 with or without a BOM; accept and discard a BOM in consumed text, and produce text without a BOM.
  *   Otherwise, if possible, use UTF-8 as the default encoding with use of other encodings negotiated using information other than a BOM; accept and discard a BOM in consumed text, and produce text without a BOM.
  *   Otherwise, require the presence of a BOM to differentiate UTF-8 encoded text in both consumed and produced text unless the absence of a BOM would result in the text being interpreted as an ASCII-based encoding and the UTF-8 text contains no non-ASCII characters (the exception is intended to avoid the addition of a BOM to ASCII text thus rendering such text as non-ASCII). This approach should be reserved for scenarios in which UTF-8 cannot be adopted as a default due to backward compatibility concerns.

On 10/12/20 8:40 AM, Alisdair Meredith wrote:
That addresses my main concern.  Essentially, best practice (for UTF-8) would be no BOM unless the document contains code points that require multiple code units to express.


On Oct 11, 2020, at 23:22, Tom Honermann <tom at honermann.net<mailto:tom at honermann.net>> wrote:

On 10/10/20 7:58 PM, Alisdair Meredith via SG16 wrote:
One concern I have, that might lead into rationale for the current discouragement,
is that I would hate to see a best practice that pushes a BOM into ASCII files.
One of the nice properties of UTF-8 is that a valid ASCII file (still very common) is
also a valid UTF-8 file.  Changing best practice would encourage updating those
files to be no longer ASCII.
Thanks, Alisdair.  I think that concern is implicitly addressed by the suggested resolutions, but perhaps that can be made more clear.  One possibility would be to modify the "protocol designer" guidelines to address the case where a protocol's default encoding is ASCII based and to specify that a BOM is only required for UTF-8 text that contains non-ASCII characters.  Would that be helpful?


On Oct 10, 2020, at 14:54, Tom Honermann via SG16 <sg16 at lists.isocpp.org<mailto:sg16 at lists.isocpp.org>> wrote:

Attached is a draft proposal for the Unicode standard that intends to clarify the current recommendation regarding use of a BOM in UTF-8 text.  This is follow up to discussion on the Unicode mailing list<https://corp.unicode.org/pipermail/unicode/2020-June/008713.html> back in June.
Feedback is welcome.  I plan to submit<https://www.unicode.org/pending/docsubmit.html> this to the UTC in a week or so pending review feedback.
SG16 mailing list
SG16 at lists.isocpp.org<mailto:SG16 at lists.isocpp.org>

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