Question about SSL_ERROR_WANT_WRITE

[Kurt Roeckx]

On Thu, Dec 10, 2020 at 05:14:00PM +0200, Cosmin Apreutesei wrote:
> Hello,
> 
> I have a question regarding SSL_write() and returning SSL_ERROR_WANT_WRITE
> from the write callback.
> 
> _After_ SSL_write() returns with SSL_ERROR_WANT_WRITE (because my write
> callback returned  SSL_ERROR_WANT_WRITE), can I _then_ send the data given
> to the calback and then call SSL_write() again (with the same arguments)
> and then in the second call to the callback return the number of bytes
> written? Is that a supported use of the API? (I'm asking because that's the
> only way I can use the API, I can't send the data inside the callback, I
> need to send it outside the callback, see below for why).
> 
> In other words, is it guaranteed that on that second call to  SSL_write(),
> SSL will want to send the exact same data that it tried before when it
> failed, and not change its mind about it wants to send? Because
> technically, since SSL_ERROR_WANT_WRITE implies that "no data was sent",
> the state machine might as well advance and send something different at a
> later time (because it received data or something inside expired or
> whatever).

I don't fully understand your question, but the manpage says:

WARNINGS
       When a write function call has to be repeated because
SSL_get_error(3) returned SSL_ERROR_WANT_READ or
SSL_ERROR_WANT_WRITE, it must be repeated with the same arguments.
The data that was passed might have been partially processed.
When SSL_MODE_ACCEPT_MOVING_WRITE_BUFFER was set using
SSL_CTX_set_mode(3) the pointer can be different, but the data and
length should still be the same.

Does that answer your question?


Kurt