I am trying to debug a problem in DBD::ODBC. DBD::ODBC which has an attribute named odbc_err_handler which when set to a subroutine reference gets called whenever there is an error. Unfortunately, when odbc_err_handler is reset to undef, the error handler still gets called. The actual C code fragment is: 
int
   dbd_db_STORE_attrib(dbh, imp_dbh, keysv, valuesv)
   SV *dbh;
imp_dbh_t *imp_dbh;
SV *keysv;
SV *valuesv;
{
   switch(pars->fOption)
   {
      case ODBC_ERR_HANDLER:
     bSetSQLConnectionOption = FALSE;

     /* This was taken from DBD::Sybase 0.21 */
     if(valuesv == &sv_undef) {
        imp_dbh->odbc_err_handler = NULL;
         } else if(imp_dbh->odbc_err_handler == (SV*)NULL) {
        imp_dbh->odbc_err_handler = newSVsv(valuesv);
     } else {
        sv_setsv(imp_dbh->odbc_err_handler, valuesv);
     }
     break;

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I have verified that when I set $database_handle->{odbc_error_handler} it gets into the code above but always ends up in the else statement. The problem is that DBD::ODBC uses (if imp_dbh->odbc_error_handler) to test whether it should call the error handler and even after setting to undef, it is set. If I add the following to the start of the if it works: 
     if(!SvOK(valuesv)) {
        imp_dbh->odbc_err_handler = NULL;
         }

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My problem is that I do not understand what the first 2 original if tests and following statements do and I though &sv_undef should be &PL_sv_undef anyway (not that this helps).

In reply to testing for undef in perl XS code (in DBD::ODBC) by mje

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