alpn_select_cb
if (sctx->ext.alpn_select_cb == NULL)
SSL_CTX_set_alpn_select_cb(srv->ctx, alpn_select_cb, srv);
ctx->ext.alpn_select_cb = cb;
int (*alpn_select_cb)(SSL *s,
if (sctx->ext.alpn_select_cb != NULL && s->s3.alpn_proposed != NULL) {
int r = sctx->ext.alpn_select_cb(SSL_CONNECTION_GET_USER_SSL(s),
SSL_CTX_set_alpn_select_cb(sctx, alpn_select_cb, NULL);
SSL_CTX_set_alpn_select_cb(sctx, alpn_select_cb, NULL);