Using C to modify response in VCL

The varnish docs say that we can include C snippets inside a VCL file, like

sub vcl_hash {
    C{
            int i = /* Some logic to get a number */
    }C
}

But now how can I use the value of the integer i to set a response header, or cookie

See varnish.vcc

And the functions:

• VRT_SetHdr
• VRT_GetHdr

in varnish 4 there is ctx variabl defined for the context (as opposed to sp in varnish 3 ) (source)

example:

sub vcl_hash {
    C{
        const char *hash = calc_hash(...);
        const struct gethdr_s hdr = { 
            HDR_BERESP,
            "\010X-Hash:" // length prefixed string, in octal 
        };
        VRT_SetHdr(ctx, &hdr, hash, vrt_magic_string_end);
    }C
}

see here for another example