This function is used to calculate the space needed to hold an
uncompressed copy of a Resource Record encoded
p pointer references the lwIP
packet buffer containing the original string structure, with
index specifying the starting offset within
the lwIP packet for the string.
raw flag is used to control the
style of uncompressed string for which a length is being calculated.
The return value depends on the supplied
the return value is the size needed for an uncompressed
“raw” copy of the string still using the mDNS encoding
style (length byte followed by UTF-8 character data).
specified then the function returns the size needed to hold a final
string representation using '.' to separate fields, and also including the
NUL character. If invalid parameters,
or data, are given then the error limit
0xFFFF is returned.