CGI error

I'm getting this when access the cgi:

The specified CGI application misbehaved by not returning a complete set of
HTTP headers. The headers it did return are:

Here's the cgi code:

int main()
{
  printf("Content-Type: %s\n\n", contentType);

  conn = PQconnectdb("dbname=template1");
  if (PQstatus(conn) == CONNECTION_BAD)
  {
     printf("Connection to database failed.\n");
     printf("%s", PQerrorMessage(conn));
     return(1);
  }

  strcpy(query_string,
          "SELECT * FROM cities");

  res = PQexec(conn, query_string);

  if (PQresultStatus(res) != PGRES_TUPLES_OK)
  {
    printf("SELECT query failed.\n");
    PQclear(res);
    PQfinish(conn);
    return(1);
  }

  for (i = 0; i < PQntuples(res); i++)
     printf("%s\n", PQgetvalue(res, i, 0));
  PQclear(res);
  PQfinish(conn);
return(0);
}