How to handle OpenSSL SSL_ERROR_WANT_READ / WANT_WRITE on non-blocking sockets

Question

The OpenSSL library allows to read from an underlying socket with SSL_read and write to it with SSL_write. These functions maybe return with SSL_ERROR_WANT_READ or SSL_ERROR

Answer 1

With non-blocking sockets, SSL_WANT_READ means "wait for the socket to be readable, then call this function again."; conversely, SSL_WANT_WRITE means "wait for the socket to be writeable, then call this function again.". You can get either SSL_WANT_WRITE or SSL_WANT_READ from both an SSL_read() or SSL_write() call.