What is `scanf` supposed to do with incomplete exponent-part?

Question

Take for example rc = scanf("%f", &flt); with the input 42ex. An implementation of scanf would read 42e thinking it would encounter a digit or sign after that and realize first when reading x that it didn't get that. Should it at this point push back both x and e? Or should it only push back the x.

The reason I ask is that GNU's libc will on a subsequent call to gets return ex indicating they've pushed back both x and e, but the standard says:

An input item is read from the stream, unless the specification includes an n specifier. An input item is defined as the longest sequence of input characters which does not exceed any specified field width and which is, or is a prefix of, a matching input sequence[245] The first character, if any, after the input item remains unread. If the length of the input item is zero, the execution of the directive fails; this condition is a matching failure unless end-of-file, an encoding error, or a read error prevented input from the stream, in which case it is an input failure.

I interpret this as since 42e is a prefix of a matching input sequence (since for example 42e1 would be a matching input sequence), which should mean that it would consider 42e as a input item that should be read leaving only x unread. That would also be more convenient to implement if the stream only supports single character push back.

Answer 1

Your interpretation of the standard is correct. There's even an example further down in the C standard which says that 100ergs of energy shouldn't match %f%20s of %20s because 100e fails to match %f.

But most C libraries seem to implement this differently, probably due to historical reasons. I just checked the C library on macOS and it behaves like glibc. The corresponding glibc bug was closed as WONTFIX with the following explanation from Ulrich Drepper:

This is stupidity on the ISO C committee side which goes against existing practice. Any change can break existing code.