当SSL_read（）返回WANT_READ时调用SSL_write（）

Question

I'm working on a client-application that uses OpenSSL for TLS using non-blocking I/O. Both sides of the connection may write at any time, indicating some state-change. So I need to check the socket regularly for new data to become available. If nothing has been received my code should immediately continue with the next task: sending new data from the client to the server.

But I'm not sure, how I can do this with the OpenSSL-API: Calling SSL_read() on a socket where no data has been received (as the other side did not send anything) always results in an error-code of SSL_ERROR_WANT_READ (or even SSL_ERROR_WANT_WRITE in case of a renegotiation). In that case may I just continue with the next task and call SSL_write() to write other data to server?

The man-page doesn't talk about this in detail. It just mentions that the arguments of the repeated read-call need to be exactly the same as before. The book Network Security with OpenSSL by Viega et. al. contains an example of non-blocking I/O. But the author waits for SSL_read() to succeed (reading 1 or more bytes) and only then calls SSL_write() to send data to the other side, which means that no write-operation is possible until data from the other side has been received. For my purpose this would not work.

In a nutshell: If SSL_read() returns with SSL_ERROR_WANT_WRITE or SSL_ERROR_WANT_READ may I just call SSL_write() with other data before I repeat the read? And if not, how can I achieve that read-if-available-and-write-thing?

Answer 1

SSL_ERROR_WANT_WRITE or SSL_ERROR_WANT_READ does not necessarily mean an error condition. It signifies that in order to proceed further, Read or write must be done first.

Hence, if your SSL_read is returning either one of these. If you have to wait the condition to be fulfilled just the example in the book that you have mentioned. Untill then calling any other SSL API including SSL_write will not work.

Perhaps, you can use ,if not used, select/poll/epoll or whatever to know any network activity on your socket