Re: [ns] NS2 problem with Packet::datalen() function (packet.h)

[Brian Adamson <adamson@itd.nrl.navy.mil>]

I had code in the "recv(Packet* p, Handler* h)" method of my derived 
Agent class which _was_ originally calling:

int len = p->datalen();

The returned value of that was "8" while the code:

int len = ((PacketData*)(p->userdata()))->size();

returns the proper length of of the PackData my sender agent had 
attached to the Packet  (512 bytes)

This was with ns-2.1b7a compiled on Redhat 7.1 with gcc 2.96 ...

I suspect the compiler isn't properly setting up the 
AppData/PacketData virtual function tables as a result of all of the 
inline functions?  Historically with the evolution of C++ compilers, 
I think inline virtual functions are an area with which to be 
cautious.






At 3:19 PM -0700 5/21/01, Haobo Yu wrote:
>  > The current problem I have is that the AppData base class
>>  implementation of returns
>>
>>  "sizeof(AppData)"
>>
>>  So the current "int Packet::datalen()" function always returns
>>  "sizeof(AppData)" instead of the packet's "datalen()" ... which I
>
>I don't understand.
>
>         inline int datalen() const { return data_ ? data_->size() : 0; }
>
>Here is Packet::datalen(), and data_ is:
>
>         AppData* data_;         // variable size buffer for 'data'
>
>And here is AppData::size():
>
>         virtual int size() const { return sizeof(AppData); }
>
>Sicne it's a virtual function, when you call it from a pointer to the base
>class it'll automatically return whatever the actual value is of the
>derived class.
>
>I'm curious how did you test your code to know it always returns
>sizeof(AppData)?
>
>>  believe was the intention of the "datalen()" method of the "Packet"
>>  class.  There are probably alternative (better) ways to remedy this
>>  situation ... Perhaps the type of the "data_" member of the Packet
>>  class should be of type PacketData instead of AppData? ...  Any
>>  suggestions for a better fix ... My current work-around in my own
>>  code is to access the Packet "data_" pointer (via the "userdata()
>>  method), cast is as type "PacketData" and get the PacketData size()
>>  ... This seems a faily non-intuitive approach as compared to using
>>  the Packet::datalen() method given some fix for it ...
>
>I'm very confused. Since the size() function is virtual you do not need
>this explict typecast, which is exactly what a virtual method is intented
>to be...

-- 
Brian
__________________________________
Brian Adamson
<http://manimac.itd.nrl.navy.mil>
<mailto:adamson@itd.nrl.navy.mil>