How to retrieve CLOB data when NLS_CHARACTERSET=AL32UTF8

Question

I have written a c++ code to write and read clob data to/from oracle database. When nls database parameters like below this runs perfectly.

NLS_NCHAR_CHARACTERSET =    AL16UTF16
NLS_CHARACTERSET =  WE8MSWIN1252
(Using connection string = "oracle://service=abc user=abc password=abc charset=178 ncharset=1000")

However, when database nls parameters are like these code return and soci error: Unknown error code while executing "select s from long_string_test where id = 1

NLS_NCHAR_CHARACTERSET =    AL16UTF16
NLS_CHARACTERSET =  AL32UTF8


    std::string DbConnectionString = "oracle://service=abc user=abc password=abc charset=873 ncharset=1000";
soci::session dbCon;
dbCon.open(DbConnectionString.c_str());


int id = 1;
soci::xml_type xml;

xml.value = "<file>";
for (int i = 0; i != 200; ++i)
{
xml.value += "ABCDEFGHIJKLMNOPQRSTUVWXYZ";
}
xml.value += "</file>";

// additional test for empty and non-empty long_string

dbCon << "create table long_string_test (id integer, s clob)";

soci::long_string s1; // empty
dbCon << "insert into long_string_test(id, s) values (1, :s)", use(s1);

soci::long_string s2;
s2.value = "test value";
dbCon << "select s from long_string_test where id = 1", into(s2);


s1.value = xml.value;

dbCon << "update long_string_test set s = :s where id = 1", use(s1);

dbCon << "select s from long_string_test where id = 1", into(s2);