Re: ASN.1 decoder for h323-conntrack-nat

[Alexander Nasonov <alnsn-mycop@yandex.ru>]

Jozsef Kadlecsik wrote:

> That would be great! We cannot use code written in C++.

I'm using C++ simply because C++ has inline functions and if you put all 
these functions (with bodies) into the class scopr you don't need to 
worry about the order in which they should be placed. I can rewrite the 
generator to generate C code (the generator itself will be written in 
C++ anyway).

>[Actually, I have just discovered that the source code of the most recent
>ethereal contains ASN.1 PER/H225/H245 decoders written in C.]
>
This tool helped me a lot to understand what PER is. Hmm, I thought that 
ethereal is based on openh323 ASN.1 generator which is C++-based. I'll 
take a look at the code later. Is there any free ASN.1 decoder?
I wish I had a better code. I could implement more optimization then. 
For example, if a type after extention mark (...) doesn't have callbacks 
it could be just skiped as open type. I could also try to simplify  
parsing of fixed-length types. May be later I'll find more time to 
rewrite the code.
I forgot to mention that my code is based on 
http://iiiasn1.sourceforge.net (this project AFAIK is derived from 
openh323 decoder/encoder).

--
Alexander Nasonov

PS. Example of generated code:

  void f_MSC__MultimediaSystemControlMessage()
  {
      // CHOICE { < 4 field(s) > ,... }
      size_t len;
      unsigned char* saved_byte;
      unsigned int choice = GetBits3();
      switch(choice)
      {
        case 0:
          f_MSC__MultimediaSystemControlMessage__request();
          break;
        case 1:
          f_MSC__MultimediaSystemControlMessage__response();
          break;
        case 2:
          f_MSC__MultimediaSystemControlMessage__command();
          break;
        case 3:
          f_MSC__MultimediaSystemControlMessage__indication();
          break;
        case 4: case 5: // small number bit is 0
          choice = GetBits5() + ((choice - 4) << 5);
          SkipDataWithSemiConstrainedLength();
          break;
        default: // small number bit is 1
          UngetBits(1);
          SkipDataWithSemiConstrainedLength();
          SkipDataWithSemiConstrainedLength();
          break;
      }
  }