7.5.1 Asynchronous operation

GnuTLS can be used with asynchronous socket or event-driven programming. During a TLS protocol session GnuTLS does not block for anything except calculations. The only blocking operations are due to the transport layer (sockets) functions. Those, however, in an asynchronous scenario are typically set to non-blocking mode, which forces them to return EAGAIN error code instead of blocking. In that case GnuTLS functions will return the GNUTLS_E_AGAIN error code and can be resumed the same way as a system call would. The only exception is gnutls_record_send, which if interrupted subsequent calls need not to include the data to be sent (can be called with NULL argument).

The select system call can also be used in combination with the GnuTLS functions. select allows monitoring of sockets and notifies on them being ready for reading or writing data. Note however that this system call cannot notify on data present in GnuTLS read buffers, it is only applicable to the kernel sockets API. Thus if you are using it for reading from a GnuTLS session, make sure the session is read completely. That can be achieved by checking there are no data waiting to be read (using gnutls_record_check_pending), either before the select system call, or after a call to gnutls_record_recv. GnuTLS does not keep a write buffer, thus when writing select need only to be consulted.

In the DTLS, however, GnuTLS might block due to timers required by the protocol. To prevent those timers from blocking a DTLS handshake, the gnutls_init should be called with the GNUTLS_NONBLOCK flag (see Session initialization).

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