For the conditions under which the fwscanf() functions shall fail
and may fail, refer to fgetwc(3p).
In addition, the fwscanf() function shall fail if:
EILSEQ
Input byte sequence does not form a valid character.
ENOMEM
Insufficient storage space is available.
In addition, the fwscanf() function may fail if:
EINVAL
There are insufficient arguments.
The following sections are informative.