My Location table:
<pre>

+-------------+---------+---------+
| Location_Id | Address | Country |
+-------------+---------+---------+
|           1 | London  |  London |
|           2 | US      |    null |
+-------------+---------+---------+
</pre>

My code:
/*connect to db*/
void connect()
{
    /* Connect to ORACLE. */
    strcpy(username, "hr");
    strcpy(password, "hr");

    EXEC SQL DECLARE DB_NAME DATABASE;
    EXEC SQL CONNECT : username IDENTIFIED BY : password;
    printf("nConnected to ORACLE as user: %sn", username);
}

void testNull()
{
    short state_ind = -1;
    char address[20];
    char *state_province_input = "state_province";
    EXEC SQL SELECT STREET_ADDRESS INTO : address FROM LOCATIONS
    WHERE : state_province_input INDICATOR : state_ind is null;
    printf("address :%s, state: %sn", address, state_province_input);
    printf("indicator: %dn", state_ind);
}

void main()
{
    connect();
    testNull();
}