[#SSL_CTX_npn_select_cb_func]
= SSL_CTX_npn_select_cb_func
:mrdocs:

NPN client callback that selects a protocol from the server's list.

== Synopsis

Declared in `<openssl/ssl.h>`

[source,cpp,subs="verbatim,replacements,macros,-callouts"]
----
typedef int(* SSL_CTX_npn_select_cb_func)(xref:SSL.adoc[SSL]*, unsigned char**, unsigned char*, unsigned char const*, unsigned int, void*);
----

== Return Value

SSL_TLSEXT_ERR_OK on success; other values abort the connection.

== Parameters

[cols="1,4"]
|===
| Name| Description
| *s*
| SSL connection performing NPN.
| *out*
| On success, set to the selected protocol (may point into `in).`
| *outlen*
| Receives the length of the selected protocol name.
| *in*
| Server‐advertised protocols in wire list format.
| *inlen*
| Length of `in` in bytes.
| *arg*
| User argument registered with SSL_CTX_set_next_proto_select_cb.
|===


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