SSL_shutdown返回值的研究（2）

1.Analyze：
According to the conclusion（1）of return value of SSL_shutdown：
a.Return code 0 indicates that the application issued the SSL_shutdown function first. Continue issuing the SSL_shutdown function until you receive return code 1, which indicates the remote application has also shut down.

    b.SSL_shutdown() supports both uni- and bidirectional shutdown by its 2
step behaviour.
    c.When the application is the first party to send the "close notify"
alert, SSL_shutdown() will only send the alert and the set the SSL_SENT_SHUTDOWN 
flag (so that the session is considered good and willbe kept in cache). SSL_shutdown()
will then return with 0. If a unidi-rectional shutdown is enough (the underlying 
connection shall be closed anyway), this first call to SSL_shutdown() is sufficient.
In order to complete the bidirectional shutdown handshake, SSL_shutdown() must be
called again. The second call will make SSL_shutdown() wait for the peer's "close 
notify" shutdown alert. On success, the second call to SSL_shutdown() will return with 1.

2.sample:
ret = SSL_shutdown(ssl);/*First call*/
printf("line=%d,ret=%d\n",__LINE__,ret);
if(ret != 1)
SSL_shutdown(ssl);/*Call again*/
done:
/*end socket */
close(sockfd);
SSL_free(ssl);
SSL_CTX_free(ctx);
ERR_free_strings();