The operation did not complete; the same TLS/SSL I/O function
should be called again later. If, by then, the underlying BIO has
data available for reading (if the result code is SSL_SSL_ERROR_WANT_READ) or
allows writing data (SSL_ERROR_WANT_WRITE), then some TLS/SSL protocol
progress will take place, i.e. at least part of an TLS/SSL record
will be read or written. Note that the retry may again lead to a
SSL_ERROR_WANT_READ or SSL_ERROR_WANT_WRITE condition. There is
no fixed upper limit for the number of iterations that may be necessary
until progress becomes visible at application protocol level.
For socket BIOs (e.g. when SSL_set_fd() was used), select()
or poll() on the underlying socket can be used to find out when
the TLS/SSL I/O function should be retried.
Caveat: Any TLS/SSL I/O function can lead to either of SSL_ERROR_WANT_READ
and SSL_ERROR_WANT_WRITE . In particular, SSL_read() or SSL_peek()
may want to write data and SSL_write() may want to read data. This
is mainly because TLS/SSL handshakes may occur at any time during
the protocol (initiated by either the client or the server); SSL_read(),
SSL_peek(), and SSL_write() will handle any pending handshakes.