Pathname identifying location of buffer.
An external format spec.
A buffer whose contents are examined.
Length (an integer) up to which buffer should be examined.
Default external format spec created by merging ef-spec with the encoding that was found to be valid.
specific-valid-file-encoding tests each element of *specific-valid-file-encodings* to see if it is valid for the contents of buffer, bounded by length. The first valid encoding is returned. For input files, buffer will contain the start of the file, so it is assumed that it contains a representative sample. For output files, buffer will have length 0, so the first element of *specific-valid-file-encodings* will always be returned.
You need to set *specific-valid-file-encodings* for
specific-valid-file-encoding to have any effect. The default value of *specific-valid-file-encodings* is
nil, which causes
specific-valid-file-encoding to return ef-spec unchanged.
LispWorks User Guide and Reference Manual - 20 Sep 2017