Difference between scanf() and strtol() / strtod() in parsing numbers

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Communication with Fred J. Tydeman, Vice-char of PL22.11 (ANSI “C”), on comp.std.c shed some light on this:

fscanf

An input item is defined as the
longest sequence of input characters
[…] which is, or is a prefix of, a
matching input sequence. (7.19.6.2 P9)

This makes “0x” the longest sequence that is a prefix of a matching input sequence. (Even with %i conversion, as the hex “0x” is a longer sequence than the decimal “0”.)

The first character, if any, after the
input item remains unread. (7.19.6.2 P9)

This makes fscanf read the “z”, and put it back as not-matching (honoring the one-character pushback limit of footnote 251)).

If the input item is not a matching
sequence, the execution of the
directive fails: this condition is a
matching failure. (7.19.6.2 P10)

This makes “0x” fail to match, i.e. fscanf should assign no value, return zero (if the %x or %i was the first conv. specifier), and leave “z” as the first unread character in the input stream.

strtol

The definition of strtol (and strtoul) differs in one crucial point:

The subject sequence is defined as the
longest initial subsequence of the
input string, starting with the first
non-white-space character, that is of
the expected form. (7.20.1.4 P4, emphasis mine)

Which means that strtol should look for the longest valid sequence, in this case the “0”. It should point endptr to the “x”, and return zero as result.

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