On 24 July 2013 09:20, Nevin Liber <nevin_at_[hidden]> wrote:

> On 24 July 2013 06:58, Gabriel Dos Reis <gdr_at_[hidden]> wrote:
>
>> The "union hack" is treacherous and needs far more investigation
>> than it appears.
>
>
> So treacherous that people have been managing to use it for the last 30
> years or so?
>
>
>> Are we going to mandate that the reintepretation of
>> the bits cannot possibly yield trap representation?
>
>
> I wouldn't. This is for non-portable machine-specific code. If you blow
> a trap representation or an alignment access, you are on your own.
>
> If I wanted to do this via copying, I'd just use Java.
>

Assuming no alignment, packing, endian or malformed packet issues, here is
an example of what people want to do:

enum class Protocol : char
{
    UDP = 17,
    // ...
};

struct IPHeader
{
    //...
    Protocol protocol;
    // ...
};

struct UDPHeader : IPHeader
{
    //...
    uint16_t length;
    //...
    char data[1];
};

union Header
{
    IPHeader ip;
    UDPHeader udp;
    //...
};

// Deliver payload to f without copying
Payload(dynarray<char> const& packet, std::function<void(char const*, char
const*)> const& f)
{
    // Want to overlay Header on packet.data()
    auto& header = *static_cast<Header const*>(static_cast<void
const*>(packet.data()));

    switch (header.ip.protocol)
    {
        case Protocol::UDP:
            f(&header.udp.data[0], &header.udp.data[header.udp.length - 8]);
            break;
        //...
    }

}

Q1: How many places has undefined behavior been invoked in the above?
Q2: What is the correct way to write this code?

-- 
 Nevin ":-)" Liber  <mailto:nevin_at_[hidden]>  (847) 691-1404