This function returns the system path name for the SGML public identifier string pubid. If the identifier is not found in any of the catalog files in the search path given by the set catalogpath option, the public_id_path function returns a null string.

The second argument, unused, is no longer used. It should be the null string.

The catalogpath argument specifies a list of directories to search for the catalog file. If it is not specified, the path list specified in the set catalogpath option, which is initialized from the environment variable APTCATPATH, is searched.