How to determine if a connection used encryption/SSL from apache2 httpd module?

In the context of an Apache2 module written in C, how can I determine if a request (request_rec *r) is encrypted? I've seen code that seems to do that, but I either haven't been able to figure out how to implement it or it always return 'true', even on unencrypted connections.

In PHP, $_SERVER['HTTPS'] gets set to 'on' if HTTPS is used.

In your own module, define

#include <mod_ssl.h>

static int is_https(conn_rec *c)
{
    int (*is_https_conn)(conn_rec *c);
    is_https_conn = APR_RETRIEVE_OPTIONAL_FN(ssl_is_https);
    if (is_https_conn)
        return is_https_conn(c);
    return 0;
}

so you can test if request request_rec *r uses an SSL/TLS connection, using

if (is_https(r->connection)) {
   /* Yes, request is via an SSL/TLS-encrypted connection */
} else {
   /* No, the request is not via an encrypted connection */
}

The logic is based on the fact that the SSL/TLS module uses APR_REGISTER_OPTIONAL_FN(ssl_is_https) to register the function ssl_is_https() in modules/ssl/ssl_engine_vars.c:ssl_var_register(). This function takes a connection (conn_rec *), and returns nonzero if the connection uses SSL/TLS encryption (HTTPS).

We use APR_RETRIEVE_OPTIONAL_FN(ssl_is_https) to retrieve the pointer to said function. If it is not registered, this returns NULL (and obviously no connection can then be encrypted). If it returns a non-NULL pointer, we call the function. It will return nonzero if the connection is HTTPS.