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);
}