And yet another scanf interpretation question

[Chris Torek]

The draft says that scanf's %[efg] formats match `an optionally
signed floating-point number, whose format is the same as expected
for the subject string of the |strtod| function.'  This in turn
is defined as an optional sign, followed by a non-optional digit
sequence, followed by an optional exponent.  The exponent, if
present, has the form: `e' or `E', followed by an optional sign,
followed by a non-optional digit sequence.

Thus, the question that applies to strtol and strtoul (as to whether
a sign followed by no digits is acceptable) does not apply.  A
different question then rears its ugly head:

If the number `looks right' up to a point, but then fails to match
the constraints imposed on it, what is to happen?  We have the following
possible sequences we can feed scanf() when it is matching %[efg]:

	.e10			[missing mantissa digits]
	+1.2345e		[missing exponent digits]
	-e			[missing both digits]

This much is clear:  These can only be considered a matching failure.
The draft goes on to say, however, that `If conversion terminates on
a conflicting input character, the offending input character is left
unread in the input stream.'  This can only be meant to imply `conflicting
with a literal character from the format string', not `conflicting with
the format required by a conversion such as %f'.  Alas, the draft does
*not* say what input character(s) are left unread in the case of a
matching failure.  This question arises only for numeric formats
(and perhaps only for floating-point, depending upon whether `%d' should
accept bare `-' and `+').

Note that the most useful answer---that the entire malformed floating
point number remains unconsumed---requires mandating an arbitrary amount
of pushback (or, equivalently, lookahead): the `floating point number'

    1.111111111111111111111111111111111111111111111111111111111e-

looks just fine until the lack of a digit following the `-' shows up.

The question, then, can be stated as follows:

  What is the condition of the input stream when a matching failure
  occurs `deep inside' a conversion?

(We intend to allow an arbitrary amount of pushback, so whatever the
answer to this question, it is easy for me to handle; but I want to know
what the standard intends.)
-- 
In-Real-Life: Chris Torek, Univ of MD Comp Sci Dept (+1 301 454 7163)
Domain:	chris at cs.umd.edu	Path:	uunet!mimsy!chris