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UDP - recvfrom() call

Posted on 2000-04-04
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Last Modified: 2008-03-17
There are two simple programs: A, and B communicating through UDP (socket() called with SOCK_DGRAM). A just bounces all datagrams.
B sends a datagram to A, calls recvfrom and then
examines the sockaddr *from field.

Depedning on:
a) sockaddr_in declaration or allocation
b) recvfrom calling point
c) B's compilation host
I'm getting different results:

Compiled on my machine (RH 6.1):

sockaddr_in  | recvfrom()     | result
-----------------------------
main()          | main()           | A's address
answer0()     | answer0()      | EINVAL
B.c              | answer1()      | 0.0.0.0
malloc()        | answer2()     | EINVAL

Compiled on my university host:

sockaddr_in  | recvfrom()     | result
--------------------------
main()          | main()           | A's address
answer0()     | answer0()      | 0.0.0.0
B.c              | answer1()      | 0.0.0.0
malloc()        | answer2()     | 0.0.0.0

I don't like the idea of putting all my code
in main(), so hope there's someone who has
at least the slightest idea about what's going
on and how to make it work. I'll provide any
additional information, if it is necessary.

Sources of A and B are here: http://rainbow.mimuw.edu.pl/~robson/AB.tgz 
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Question by:Robson
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by:munsie
ID: 2685755
the code would be very helpful, if you don't mind... i have a few ideas, but I would like to see what A and B are doing.

thanks,
dennis
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by:Robson
ID: 2686743
I've put sources of A and B at http://rainbow.mimuw.edu.pl/~robson/AB.tgz
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by:Robson
ID: 2686751
Edited text of question.
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Accepted Solution

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munsie earned 300 total points
ID: 2687360
in your get_answer2() routine, you didn't set the length field to be the sizeof(answer).  Actually, you didn't do this in multiple places...

set length before calling recvfrom() will fix your problem.

good luck,
dennis
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Author Comment

by:Robson
ID: 2687763
Thank's a lot, dennis!
This requirement isn't mentioned either in man page, nor in Steven's book (I 've nearly broken its cover in anger). Also in example programs given by my network-programming teacher (he's going to score some, too) initialization isn't made.

Last, but not least:
I MISSED GREAT SCI-FI CONVENTION BECAUSE OF THIS EFFING BUG!
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Expert Comment

by:munsie
ID: 2687971
glad it helped... sorry about the sci-fi convention, though

dennis
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by:Robson
ID: 2688162
NOTE: I was wrong about Stevens' "Un*x Network Programming" - it explains this problem. I promise to read it carefully and with respect - and no more violence, of course :-).  
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