SSL_connect - initiate the TLS/SSL handshake with an TLS/SSL server
#include <openssl/ssl.h> int SSL_connect(SSL *ssl);
SSL_connect() initiates the TLS/SSL handshake with a server. The communication channel must already have been set and assigned to the ssl by setting an underlying BIO.
The behaviour of SSL_connect() depends on the underlying BIO.
If the underlying BIO is blocking, SSL_connect() will only return once the handshake has been finished or an error occurred.
If the underlying BIO is nonblocking, SSL_connect() will also return when the underlying BIO could not satisfy the needs of SSL_connect() to continue the handshake, indicating the problem by the return value -1. In this case a call to SSL_get_error() with the return value of SSL_connect() will yield SSL_ERROR_WANT_READ or SSL_ERROR_WANT_WRITE. The calling process then must repeat the call after taking appropriate action to satisfy the needs of SSL_connect(). The action depends on the underlying BIO. When using a nonblocking socket, nothing is to be done, but select() can be used to check for the required condition. When using a buffering BIO, like a BIO pair, data must be written into or retrieved out of the BIO before being able to continue.
Many systems implement Nagle's algorithm by default which means that it will buffer outgoing TCP data if a TCP packet has already been sent for which no corresponding ACK has been received yet from the peer. This can have performance impacts after a successful TLSv1.3 handshake or a successful TLSv1.2 (or below) resumption handshake, because the last peer to communicate in the handshake is the client. If the client is also the first to send application data (as is typical for many protocols) then this data could be buffered until an ACK has been received for the final handshake message.
The TCP_NODELAY socket option is often available to disable Nagle's algorithm. If an application opts to disable Nagle's algorithm consideration should be given to turning it back on again later if appropriate. The helper function BIO_set_tcp_ndelay() can be used to turn on or off the TCP_NODELAY option.
The following return values can occur:
The TLS/SSL handshake was not successful but was shut down controlled and by the specifications of the TLS/SSL protocol. Call SSL_get_error() with the return value ret to find out the reason.
The TLS/SSL handshake was successfully completed, a TLS/SSL connection has been established.
The TLS/SSL handshake was not successful, because a fatal error occurred either at the protocol level or a connection failure occurred. The shutdown was not clean. It can also occur if action is needed to continue the operation for nonblocking BIOs. Call SSL_get_error() with the return value ret to find out the reason.
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