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It needs topack user data into an array, which is then included in the user dataarea of an packet by an Agent (this is not supported by currentAgents. User must derive new agents to accept user data fromapplications, or use an wrapper like TcpApp. We'll discuss thislater). <P>Compared with Packet, ADU provides this functionality in a differentway. In Packet, a common area is allocated for all packet headers; anoffset is used to access different headers in this area. In ADU thisis not applicable, because some ADU allocates their space dynamicallyaccording the the availability of user data. For example, if we wantto deliver an OTcl script between applications, the size of the scriptis undetermined beforehand. Therefore, we choose a less efficient butmore flexible method. Each ADU defines its own data members, andprovides methods to serialize them (i.e., pack data into an array andextract them from an array). For example, in the abstract base classof all ADU, AppData, we have:<P><PRE> class AppData { private: AppDataType type_; // ADU type public: struct hdr { AppDataType type_; }; public: AppData(char* b) { assert(b != NULL); type_ = ((hdr *)b)-\>type_; } virtual void pack(char* buf) const; }</PRE><P>Here <TT>pack(char* buf)</TT> is used to write an AppData objectinto an array, and <TT>AppData(char* b)</TT> is used to build a newAppData from a ``serialized'' copy of the object in an array.<P>When deriving new ADU from the base class, users may add more data,but at the same time a new <TT>pack(char *b)</TT> and a new constructor should be provided to write and read those new data members from anarray. For an example as how to derive from an ADU, look at /webcache/http-aux.h.<P><HR><!--Navigation Panel--><A NAME="tex2html8992" HREF="node493.html" tppabs="http://www.isi.edu/nsnam/ns/doc/node493.html"><IMG WIDTH="37" HEIGHT="24" ALIGN="BOTTOM" BORDER="0" ALT="next" SRC="file:/usr/share/latex2html/icons/next.png"></A> <A NAME="tex2html8986" HREF="node491.html" tppabs="http://www.isi.edu/nsnam/ns/doc/node491.html"><IMG WIDTH="26" HEIGHT="24" ALIGN="BOTTOM" BORDER="0" ALT="up" SRC="file:/usr/share/latex2html/icons/up.png"></A> <A NAME="tex2html8980" HREF="node491.html" tppabs="http://www.isi.edu/nsnam/ns/doc/node491.html"><IMG WIDTH="63" HEIGHT="24" ALIGN="BOTTOM" BORDER="0" ALT="previous" SRC="file:/usr/share/latex2html/icons/prev.png"></A> <A NAME="tex2html8988" HREF="node1.html" tppabs="http://www.isi.edu/nsnam/ns/doc/node1.html"><IMG WIDTH="65" HEIGHT="24" ALIGN="BOTTOM" BORDER="0" ALT="contents" SRC="file:/usr/share/latex2html/icons/contents.png"></A> <A NAME="tex2html8990" HREF="node590.html" tppabs="http://www.isi.edu/nsnam/ns/doc/node590.html"><IMG WIDTH="43" HEIGHT="24" ALIGN="BOTTOM" BORDER="0" ALT="index" SRC="file:/usr/share/latex2html/icons/index.png"></A> <BR><B> Next:</B> <A NAME="tex2html8993" HREF="node493.html" tppabs="http://www.isi.edu/nsnam/ns/doc/node493.html">36.1.2 Passing data between</A><B> Up:</B> <A NAME="tex2html8987" HREF="node491.html" tppabs="http://www.isi.edu/nsnam/ns/doc/node491.html">36.1 Using application-level data</A><B> Previous:</B> <A NAME="tex2html8981" HREF="node491.html" tppabs="http://www.isi.edu/nsnam/ns/doc/node491.html">36.1 Using application-level data</A>   <B> <A NAME="tex2html8989" HREF="node1.html" tppabs="http://www.isi.edu/nsnam/ns/doc/node1.html">Contents</A></B>   <B> <A NAME="tex2html8991" HREF="node590.html" tppabs="http://www.isi.edu/nsnam/ns/doc/node590.html">Index</A></B> <!--End of Navigation Panel--><ADDRESS>2003-09-23</ADDRESS></BODY></HTML>⌨️ 快捷键说明
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