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  • Linux- passwordless ssh from system (root) script

    - by redmoskito
    What's the easiest way to have a system script (running as root) execute remote commands over ssh? I've written some scripts that execute commands remotely via ssh, and they work great when I run them as myself, as I've set up ssh-agent and keys for passwordless login. I'd like to call these when my laptop docks and undocks. I've been successful at running arbitrary scripts when docking/undocking, but since the ACPI event scripts run as root, trying to run my ssh script fails during authentication. I tried using sudo with the -u and -i flags to simulate running the script as my user, e.g.: sudo -u redmoskito -i /home/redmoskito/bin/remote_command which successfully finds my private key and tries to use it, but the ssh-agent credentials are still missing, so it still needs my passphrase.

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  • Remote offscreen rendering

    - by redmoskito
    My research lab recently added a server that has a beefy NVIDIA graphics card, which we would like to use to do scientific computations. Since it isn't a workstation, we'll have to run our jobs remotely, over an ssh connection. Most of our applications require doing opengl rendering to an offscreen buffer, then doing image analysis on the result in CUDA. My initial investigation suggests that X11 forwarding is a bad idea, because opengl rendering will occur on the client machine (or rather the X11 server--what a confusing naming convention!) and will suffer network bottlenecks when sending our massive textures. We will never need to display the output, so it seems like X11 forwarding shouldn't be necessary, but Opengl needs the $DISPLAY to be set to something valid or our applications won't run. I'm sure render farms exist that do this, but how is it accomplished? I think this is probably a simple X11 configuration issue, but I'm too unfamiliar with it to know where to start. We're running Ubuntu server 10.04, with no gdm, gnome, etc installed. However, xserver-xorg package is installed.

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  • boost::serialization with mutable members

    - by redmoskito
    Using boost::serialization, what's the "best" way to serialize an object that contains cached, derived values in mutable members, such that cached members aren't serialized, but on deserialization, they are initialized the their appropriate default. A definition of "best" follows later, but first an example: class Example { public: Example(float n) : num(n), sqrt_num(-1.0) {} float get_num() const { return num; } // compute and cache sqrt on first read float get_sqrt() const { if(sqrt_num < 0) sqrt_num = sqrt(num); return sqrt_num; } template <class Archive> void serialize(Archive& ar, unsigned int version) { ... } private: float num; mutable float sqrt_num; }; On serialization, only the "num" member should be saved. On deserialization, the sqrt_num member must be initialized to its sentinel value indicating it needs to be computed. What is the most elegant way to implement this? In my mind, an elegant solution would avoid splitting serialize() into separate save() and load() methods (which introduces maintenance problems). One possible implementation of serialize: template <class Archive> void serialize(Archive& ar, unsigned int version) { ar & num; sqrt_num = -1.0; } This handles the deserialization case, but in the serialization case, the cached value is killed and must be recomputed. Also, I've never seen an example of boost::serialize that explicitly sets members inside of serialize(), so I wonder if this is generally not recommended. Some might suggest that the default constructor handles this, for example: int main() { Example e; { std::ifstream ifs("filename"); boost::archive::text_iarchive ia(ifs); ia >> e; } cout << e.get_sqrt() << endl; return 0; } which works in this case, but I think fails if the object receiving the deserialized data has already been initialized, as in the example below: int main() { Example ex1(4); Example ex2(9); cout << ex1.get_sqrt() << endl; // outputs 2; cout << ex2.get_sqrt() << endl; // outputs 3; // the following two blocks should implement ex2 = ex1; // save ex1 to archive { std::ofstream ofs("filename"); boost::archive::text_oarchive oa(ofs); oa << ex1; } // read it back into ex2 { std::ifstream ifs("filename"); boost::archive::text_iarchive ia(ifs); ia >> ex2; } // these should be equal now, but aren't, // since Example::serialize() doesn't modify num_sqrt cout << ex1.get_sqrt() << endl; // outputs 2; cout << ex2.get_sqrt() << endl; // outputs 3; return 0; } I'm sure this issue has come up with others, but I have struggled to find any documentation on this particular scenario. Thanks!

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  • C++ -- typedef "inside" template arguments?

    - by redmoskito
    Imagine I have a template function like this: template<Iterator> void myfunc(Iterator a, Iterator::value_type b) { ... } Is there a way to declare a typedef for Iterator::valuetype that I can use in the function signature? For example: template< typename Iterator, typedef Iterator::value_type type> void myfunc(Iterator a, type b) { ... } Thus far, I've resorted to using default template arguments and Boost concept checking to ensure the default is always used: template< typename Iterator, typename type = Iterator::value_type > void myfunc(Iterator a, type b) { BOOST_STATIC_ASSERT(( boost::type_traits::is_same< typename Iterator::value_type, type >::value )); ... } ...but it would be nice if there was support in the language for this type of thing.

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  • Yaml Emitter in C++

    - by redmoskito
    Is there a C++ library for emitting YAML? Wikipedia mentions a c++ wrapper for libyaml, but the link is broken. The official YAML site only offers yaml-cpp, which was also suggested in this SO question, but cpp-yaml is only a parser, not an emitter. Am I out of luck? Edit: I'm looking for an object oriented interface, hence the C++ requirement. I know I could use libyaml's C interface in C++ code, but that's less than ideal.

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