Like Tree3Likes
  • 1 Post By iMalc
  • 1 Post By tabstop
  • 1 Post By grumpy

Good way to construct packets

This is a discussion on Good way to construct packets within the C++ Programming forums, part of the General Programming Boards category; A lot of communication is done by creating packets (for sockets/IPC etc). Often you end up with variable sized packets ...

  1. #1
    Registered User
    Join Date
    Dec 2010
    Posts
    10

    Good way to construct packets

    A lot of communication is done by creating packets (for sockets/IPC etc). Often you end up with variable sized packets for example strings in it. For example.

    Code:
    struct StringPacketBase
    {
    protected:
    
    int stringLen;
    };
    Then the name comes after the struct. If you do a class of this

    Code:
    class StringPacket : public StringPacketBase
    {
    public:
    
    uint getLength { return stringLen; } char *GetString { return (char*)this + sizeof(StringPacketBase) }
    };
    I wonder if there is a better way to automate packet creation classes. Also a problem is that the string will never show up in debugging information.

    I've seen this:


    Code:
    struct StringPacketBase
    {
    protected:
    
    int stringLen; char string[];
    };
    Now string is a part of the struct but what happens if you want two strings after each other? Then this method only work for the first string. Also if you use sizeof with StringPacketBase you have to be more careful now.

    The question is how you can create packets so that it becomes easy to write. Right now you do a lot of calculations with offsets and I wonder if there is a method make writing all these packets (which is a lot packets) more conveniently.

  2. #2
    C++まいる!Cをこわせ! Elysia's Avatar
    Join Date
    Oct 2007
    Posts
    22,555
    So what is it that you really need to do in order for you to have packet classes?
    Typical network protocols (well, TCP, at least) will chunk up the data for you.
    Quote Originally Posted by Adak View Post
    io.h certainly IS included in some modern compilers. It is no longer part of the standard for C, but it is nevertheless, included in the very latest Pelles C versions.
    Quote Originally Posted by Salem View Post
    You mean it's included as a crutch to help ancient programmers limp along without them having to relearn too much.

    Outside of your DOS world, your header file is meaningless.

  3. #3
    Algorithm Dissector iMalc's Avatar
    Join Date
    Dec 2005
    Location
    New Zealand
    Posts
    6,302
    It's simple - you don't create a structure to represent the data that is sent over the wire. Look into serialisation / deserialisation instead.
    phantomotap likes this.
    My homepage
    Advice: Take only as directed - If symptoms persist, please see your debugger

    Linus Torvalds: "But it clearly is the only right way. The fact that everybody else does it some other way only means that they are wrong"

  4. #4
    and the Hat of Guessing tabstop's Avatar
    Join Date
    Nov 2007
    Posts
    14,185
    I think I'm missing the plot here; are you deliberately not storing the data in your Packet class, but instead relying on declaring it directly after and hoping that the compiler puts those two variables next to each other and in the right order? Why not -- and this is crazy I'm sure -- why not put the string inside the class, and then provide a create_packet method that creates a new string of length+actual data (on the assumption that that is the point of the exercise) for you to send, or write a function that takes an object and then sends first the length then the string?

    I also don't know what you mean when you say: "what happens if you want two strings after each other?" In what way is this not covered by what you have? (Either you want two \0-terminated strings, in which case there is no alternative to making two packets, or you don't want the \0 in the middle, in which case you have one string with a larger length. And sizeof is irrelevant, because sizeof has not worked with any of your samples yet.) Your empty-array-trick was legalized in C99, but I don't think it made it into C++, even the brand-new C++.

    And why not use a std::string instead of char[]?
    phantomotap likes this.

  5. #5
    Registered User
    Join Date
    Dec 2010
    Posts
    10
    Quote Originally Posted by iMalc View Post
    It's simple - you don't create a structure to represent the data that is sent over the wire. Look into serialisation / deserialisation instead.
    I cannot use that in this case for performance reasons. Otherwise it would have been a good idea. I must send all the data in packet at once and I need to know the size from the beginning. The receiver side must be able to do random access on the members.

  6. #6
    Registered User
    Join Date
    Aug 2010
    Location
    Poland
    Posts
    682
    Quote Originally Posted by TotalTurd View Post
    I cannot use that in this case for performance reasons.
    That is a completely different story then. Do you want to say that you have already tried serialization/deserialization? What were the results then? Premature optimizations may often lead to a less readable, less scalable, and sometimes the worst - less efficient code. Not to mention, that constructing/analysing packet is usually way faster than transmitting it through the network.
    I never put signature, but I decided to make an exception.

  7. #7
    Registered User
    Join Date
    Dec 2010
    Posts
    10
    Quote Originally Posted by kmdv View Post
    That is a completely different story then. Do you want to say that you have already tried serialization/deserialization? What were the results then? Premature optimizations may often lead to a less readable, less scalable, and sometimes the worst - less efficient code. Not to mention, that constructing/analysing packet is usually way faster than transmitting it through the network.
    This is for Mach OS/QNX type message IPC and I cannot just serialize like that. I realize the socket word in the original text was misleading. The structures are in memory and will end up in just like before they were sent, there will be no serialization.

  8. #8
    Registered User
    Join Date
    Aug 2010
    Location
    Poland
    Posts
    682
    Quote Originally Posted by TotalTurd View Post
    This is for Mach OS/QNX type message IPC and I cannot just serialize like that.
    So, all in all, you have not used serialization before? Is your software ready to test (can send packets etc)? Profile it if it is.

    It is just a mechanism. Noones tells you to use a general-purpose serialization library. You can do all the serialization/deserialization on your own (providing two methods and a buffer). Get rid of references and advanced structures, and the whole process will be similar to plain copying (that is what you probably want to hear).

    Random access of a variable-length packet will result in analysing. I (and probably anybody here) do not know what answer you want, since you seem to know all the internals.
    I never put signature, but I decided to make an exception.

  9. #9
    Master Apprentice phantomotap's Avatar
    Join Date
    Jan 2008
    Posts
    4,231
    Code:
    char *GetString { return (char*)this + sizeof(StringPacketBase) }
    Now string is a part of the struct but what happens if you want two strings after each other?
    Then this method only work for the first string. Also if you use sizeof with StringPacketBase you have to be more careful now.
    O_o

    I cannot use that in this case for performance reasons.
    o_O

    Right, well, when you get tired of bashing your head against the wall be sure to look around and see how proper serialization of C++ classes is done for targetting "IPC".

    You'll be amazed at how proper primitives to serialize types like `std::string' relieves those headaches.

    Soma

  10. #10
    Registered User
    Join Date
    Dec 2010
    Posts
    10
    Quote Originally Posted by kmdv View Post
    So, all in all, you have not used serialization before? Is your software ready to test (can send packets etc)? Profile it if it is.

    It is just a mechanism. Noones tells you to use a general-purpose serialization library. You can do all the serialization/deserialization on your own (providing two methods and a buffer). Get rid of references and advanced structures, and the whole process will be similar to plain copying (that is what you probably want to hear).

    Random access of a variable-length packet will result in analysing. I (and probably anybody here) do not know what answer you want, since you seem to know all the internals.
    No I have not used serialization and maybe that's what I'm looking for. On receiver side I guess this require parsing the entire packet once first in order to find the members, basically unwinding it. Doesn't lead to that I must access the members twice in some cases, for example packets with several strings. First the strings must be parsed in order to find the pointers to the first characters. Then they must be accessed again when they are actually used.

    Anyway, I will look at serialization of structures. Thank you for your help.
    Last edited by TotalTurd; 06-19-2011 at 04:53 PM.

  11. #11
    Registered User
    Join Date
    Jun 2005
    Posts
    6,266
    Quote Originally Posted by TotalTurd View Post
    The structures are in memory and will end up in just like before they were sent, there will be no serialization.
    That's not true.

    You have a specification of your data structure (the struct or class definition). You have not specified how it is to be transmitted over a network. Serialization is the necessary step of converting data into a form that it can be stored or (applicable in your case) transmitted over a network. Once the data has been transmitted over the network, the process needs to be reversed. That means a deserialization step, in order to faithfully recreate a representation of the original structure in memory that can be used by the recipient.

    These things don't happen by magic.
    Salem likes this.
    Right 98% of the time, and don't care about the other 3%.

  12. #12
    Algorithm Dissector iMalc's Avatar
    Join Date
    Dec 2005
    Location
    New Zealand
    Posts
    6,302
    Quote Originally Posted by TotalTurd View Post
    I cannot use that in this case for performance reasons. Otherwise it would have been a good idea. I must send all the data in packet at once and I need to know the size from the beginning. The receiver side must be able to do random access on the members.
    That's a misconception. This is not an issue of performance.
    The process is serialisation / deserialisation whether you try and fudge it by dreaming up an impossible and unusable struct to dump verbatim onto the wire and call it by a different name, or whether you plan out a sensible on-the-wire representation and explicitly write code to serialise and deserialise it.
    My homepage
    Advice: Take only as directed - If symptoms persist, please see your debugger

    Linus Torvalds: "But it clearly is the only right way. The fact that everybody else does it some other way only means that they are wrong"

  13. #13
    Frequently Quite Prolix dwks's Avatar
    Join Date
    Apr 2005
    Location
    Canada
    Posts
    8,046
    See Serializing classes (especially Salem's link).
    dwk

    Seek and ye shall find. quaere et invenies.

    "Simplicity does not precede complexity, but follows it." -- Alan Perlis
    "Testing can only prove the presence of bugs, not their absence." -- Edsger Dijkstra
    "The only real mistake is the one from which we learn nothing." -- John Powell


    Other boards: DaniWeb, TPS
    Unofficial Wiki FAQ: cpwiki.sf.net

    My website: http://dwks.theprogrammingsite.com/
    Projects: codeform, xuni, atlantis, nort, etc.

Popular pages Recent additions subscribe to a feed

Similar Threads

  1. Is this loop construct only available using goto's???
    By Muster Mark in forum C Programming
    Replies: 2
    Last Post: 04-01-2011, 03:48 AM
  2. Construct a std::string from char*
    By C_ntua in forum C++ Programming
    Replies: 17
    Last Post: 06-18-2010, 02:10 AM
  3. is there some language construct to do this?
    By m37h0d in forum C++ Programming
    Replies: 1
    Last Post: 03-17-2010, 11:43 AM
  4. Idea for new language construct
    By Magos in forum A Brief History of Cprogramming.com
    Replies: 34
    Last Post: 09-24-2006, 06:26 AM
  5. foreach construct vs for
    By subdene in forum A Brief History of Cprogramming.com
    Replies: 9
    Last Post: 10-18-2003, 06:10 PM

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21