Error code for database down

HI...
    When i executed a query using PQexec..It returned error.    Is there any way to check this error is because of
Databaseis down.
 
    like    PGRES_EMPTY_QUERY -- The string sent to the backend was empty.    PGRES_COMMAND_OK -- Successful completion
ofa command returning no
 
data    PGRES_TUPLES_OK -- The query successfully executed               etc......
    Like this is there any value which will specify the database
failure?????

Thanks in advance-Naren.