#include <nghttp2/nghttp2.h>

int nghttp2_select_next_protocol(unsigned char **out, unsigned char *outlen, const unsigned char *in, unsigned int inlen)

A helper function for dealing with NPN in client side or 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

See for more details about NPN.

For NPN, to use this method you should do something like:

static int select_next_proto_cb(SSL* ssl,
                                unsigned char **out,
                                unsigned char *outlen,
                                const unsigned char *in,
                                unsigned int inlen,
                                void *arg)
    int rv;
    rv = nghttp2_select_next_protocol(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_next_proto_select_cb(ssl_ctx, select_next_proto_cb, my_obj);