nghttp2_select_alpn
===================

Synopsis
--------

*#include <nghttp2/nghttp2.h>*

.. function:: int nghttp2_select_alpn(const unsigned char **out, unsigned char *outlen, const unsigned char *in, unsigned int inlen)

    
    A helper function for dealing with ALPN in server side.  The *in*
    contains peer's protocol list in preferable order.  The format of
    *in* is length-prefixed and not null-terminated.  For example,
    ``h2`` and ``http/1.1`` stored in *in* like this::
    
        in[0] = 2
        in[1..2] = "h2"
        in[3] = 8
        in[4..11] = "http/1.1"
        inlen = 12
    
    The selection algorithm is as follows:
    
    1. If peer's list contains HTTP/2 protocol the library supports,
       it is selected and returns 1. The following step is not taken.
    
    2. If peer's list contains ``http/1.1``, this function selects
       ``http/1.1`` and returns 0.  The following step is not taken.
    
    3. This function selects nothing and returns -1 (So called
       non-overlap case).  In this case, *out* and *outlen* are left
       untouched.
    
    Selecting ``h2`` means that ``h2`` is written into *\*out* and its
    length (which is 2) is assigned to *\*outlen*.
    
    For ALPN, refer to https://tools.ietf.org/html/rfc7301
    
    To use this method you should do something like::
    
        static int alpn_select_proto_cb(SSL* ssl,
                                        const unsigned char **out,
                                        unsigned char *outlen,
                                        const unsigned char *in,
                                        unsigned int inlen,
                                        void *arg)
        {
            int rv;
            rv = nghttp2_select_alpn(out, outlen, in, inlen);
            if (rv == -1) {
                return SSL_TLSEXT_ERR_NOACK;
            }
            if (rv == 1) {
                ((MyType*)arg)->http2_selected = 1;
            }
            return SSL_TLSEXT_ERR_OK;
        }
        ...
        SSL_CTX_set_alpn_select_cb(ssl_ctx, alpn_select_proto_cb, my_obj);