[openssl-users] OpenSSL non-blocking epoll hanging on data receiving

Matt Caswell matt at openssl.org
Fri May 27 08:53:24 UTC 2016

On 27/05/16 07:32, counterpoint wrote:
> Hmm, some progress, but still puzzled. When my code is acting as the client,
> it seems that the problem can be overcome by calling SSL_set_read_ahead with
> a zero parameter, to turn off reading ahead. This is done just before
> calling SSL_connect. The application now seems able to read megabytes of
> data from the server without hanging.

read_ahead and SSL_pending() do not play nicely together unfortunately.
See the master (1.1.0) version of the SSL_pending() documentation which
discusses this issue and introduced the new function SSL_has_pending()
which addresses it:


> However, adding a similar call just before SSL_accept does not solve the
> problem when the application is the server, and is reading a lot of data
> from the client. It looks as if the data is read (certainly hundreds of KB
> in packets of 16384 bytes) but the application then hangs. Presumably for
> lack of any trigger (such as EPOLLIN) to generate any further activity.

So it sounds like this is during reading of application data? Has
SSL_accept() returned successfully, and you are now wanting to call


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