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Re: Get the order of HTTP request headers

by Haarg (Chaplain)
on May 05, 2010 at 15:43 UTC ( #838519=note: print w/ replies, xml ) Need Help??


in reply to Get the order of HTTP request headers

This isn't possible with CGI. The CGI RFC doesn't define any mechanism for determining the order of the headers. Additionally, environment variables in general don't have a defined order, and Perl's %ENV hash is randomized. The only way to do this would be to have a different mechanism of communicating with the server software such as ig's mod_perl example.

It wouldn't be possible to do this in PHP for scripts run as CGI either. Assuming you meant PHP's getallheaders function, it is even documented as only working when running as an Apache module.


Comment on Re: Get the order of HTTP request headers
Re^2: Get the order of HTTP request headers
by ikegami (Pope) on May 06, 2010 at 05:18 UTC

    Perl's %ENV hash is randomized.

    First, the order of values returned by hashes isn't random, at least not in any formal sense of the word. There's definitely no attempt to make them random as implied by saying the order is "randomized".

    Second, %ENV is magical. It's not really a hash (although it might use one), so the properties of hash don't necessarily apply.

    This is just a nit as your point stands without the quoted bit.

      First, the order of values returned by hashes isn't random, at least not in any formal sense of the word. There's definitely no attempt to make them random as implied by saying the order is "randomized".

      I always assumed that the order is indeed "randomized". Well, I guess I misunderstood perlsec and perlrun. Or maybe not.

        The perlsec passage to which you linked says the order shouldn't be assumed to be random. The perlrun passage to which you link doesn't explicitly mention changing key orderings at all. It is not the purpose of the feature to alter the key ordering, much less making it random.

        Two problems with your assumption:

        • The feature in question only kicks in when needed.

        • Even if you could reliably trigger the feature, it's purpose is to salt the hashing algorithm (i.e. perturb bucket selection). While this has the side effect of affecting key ordering, it hasn't been shown to produce random key orderings.

        But enough theory. Let's look at a practical example:

        for (1..100) { my %h; $h{$_} = 1 for 'a'..'z'; print(join('', keys(%h)), "\n"); }
        wraxdjyukhgftienvmslcpqbzo wraxdjyukhgftienvmslpcqbzo wraxdjyukhgftienvmslpcqbzo wraxdjyukhgftienvmslpcqbzo wraxdjyukhgftienvmslpcqbzo wraxdjyukhgftienvmslpcqbzo ... wraxdjyukhgftienvmslpcqbzo wraxdjyukhgftienvmslpcqbzo

        And the same if I run the program again.

      You are correct. I had meant it in the sense that hashes are unordered and not returned in the same order they are entered, but saying it is random isn't correct.

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