Session nao funciona

Eu fiz este codigo para teste

1.php

<?phpsession_start();$obra=1;$_SESSION['obra']=$obra;header("location: 2.php")?>

2.php

<?phpsession_start();if(isset($_SESSION["obra"]))$obra=@$_SESSION["obra"];echo"$obra";?>

e da esse erro aqui

Warning: session_start(): Cannot send session cookie - headers already sent by (output started at c:\arquivos de programas\easyphp1-8\www\meusite\tmp7f2ygxxj1s.php:8) in c:\arquivos de programas\easyphp1-8\www\meusite\tmp7f2ygxxj1s.php on line 9Warning: session_start(): Cannot send session cache limiter - headers already sent (output started at c:\arquivos de programas\easyphp1-8\www\meusite\tmp7f2ygxxj1s.php:8) in c:\arquivos de programas\easyphp1-8\www\meusite\tmp7f2ygxxj1s.php on line 9Warning: Cannot modify header information - headers already sent by (output started at c:\arquivos de programas\easyphp1-8\www\meusite\tmp7f2ygxxj1s.php:8) in c:\arquivos de programas\easyphp1-8\www\meusite\tmp7f2ygxxj1s.php on line 12