SSL_read() returning SSL_ERROR_SYSCALL with errno 11 EAGAIN

Viktor Dukhovni openssl-users at
Thu May 2 14:55:55 UTC 2019

> On May 2, 2019, at 5:56 AM, John Unsworth <John.Unsworth at> wrote:
> Create a non-blocking TCP socket 
> 	socket() for a sock_stream.
> 	connect().

Do you wait for the non-blocking connect to complete at this point?

> 	SSL_new(), SSL_set_fd(), SSL_connect().
> The application sends LDAP operations from many threads.

Are multiple threads writing to the same SSL connection?  How do
you ensure orderly use of the SSL connection?  Sharing connections
across threads without application level synchronization is not
supported in OpenSSL.

> We have just one thread that reads LDAP results.

How are further requests locked out when you're performing reads?
What is the granularity of the relevant locks?

> If an operation is outstanding then the result thread does (simplified):
> SSL_read()
> If > 0 return data.

At this point you'd be calling SSL_get_error(), is there a lock that
prevents writes between SSL_read() and SSL_read() and SSL_get_error()?

> Else if SSL_ERROR_WANT_READ or SSL_ERROR_WANT_WRITE then poll(); back to SSL_read() when data available.
> Else return error and disconnect.
> Don't know what protocol was negotiated or what the server does in terms of returned data. TCP/OpenSSL handle that.
> Both ends OpenSSL 1.1.0h.
> Problem seems to occur at random - only reproducable on customer site and after a long time running their soak test.

It would be helpful if the customer could gather more diagnostic information
from that "soak test".  With 1.1.0, presumably they negotiate TLS 1.2, since
TLS 1.3 is not available, while 1.2 is available on both ends.

IIRC OpenSSL will normally send the record layer header in the same segment
as the payload, so running into EAGAIN is unlikely after the initial 5 bytes
of record header, unless the TCP receive window was nearly full.

I gather the protocol is full-duplex and multiple outstanding requests can be
written before the corresponding replies are read?  Or is it strict half-duplex


More information about the openssl-users mailing list