called by the SSL/TLS library for a protocol message, the function arguments have the following meaning:
write_p
This flag is 0 when a protocol message has been received and 1 when a protocol message has been sent.
version
The protocol version according to which the protocol message is interpreted by the library. Currently, this is one of SSL2_VERSION, SSL3_VERSION and TLS1_VERSION (for SSL 2.0, SSL 3.0 and TLS 1.0, respectively).
content_type
In the case of SSL 2.0, this is always 0. In the case of SSL 3.0 or TLS 1.0, this is one of the ContentType values defined in the protocol specification (change_cipher_spec(20), alert(21), handshake(22); but never application_data(23) because the callback will only be called for protocol messages).
buf, len
buf points to a buffer containing the protocol message, which consists of len bytes. The buffer is no longer valid after the callback function has returned.
ssl
The SSL object that received or sent the message.
arg
The user-defined argument optionally defined by SSL_CTX_set_msg_callback_arg() or SSL_set_msg_callback_arg().