Re: Distributed objects: can't send simple C struct by value
Re: Distributed objects: can't send simple C struct by value
- Subject: Re: Distributed objects: can't send simple C struct by value
- From: Michael Heins <email@hidden>
- Date: Sun, 31 Oct 2004 09:53:06 -0700
Thanks to Wade and Jim for helping. I have concluded that this feature
(passing structs by value to a remote object) is indeed broken for
simple C structs of more than 32 bytes. I have filed bug report
3860011 for it.
The workaround is pretty benign: Simply pass a pointer to the struct
instead; because of the way the DO software must work, the struct
contents must be transfered anyway, which is what I was after in the
first place.
On Oct 30, 2004, at 9:21 AM, Wade Tregaskis wrote:
Here is a dump of the struct in the sender just before sending:
BFFFE990: AAAAAAAA 54686973 20697320 6D792074 ....This is my t
BFFFE9A0: 65737420 73747269 6E672077 68696368 est string which
BFFFE9B0: 20676574 73207061 72746C79 20776970 gets partly wip
BFFFE9C0: 6564206F 75742E00 00000000 00000000 ed out..........
BFFFE9D0: 00000000 BBBBBBBB ........
Here it is in the receiver:
BFFFE650: AAAAAAAA 54686973 20697320 6D792074 ....This is my t
BFFFE660: 65737420 73747269 6E672077 A0A03CD4 est string w..<.
BFFFE670: 00528BB0 BFFFE6F0 A0A01234 909FA88C .R.........4....
BFFFE680: BFFFEB00 24022244 909FABF0 02800000 ....$."D........
BFFFE690: 00000020 00000018 ... ....
If I set up authenticationDataForComponents / authenticateComponents
delegates on both ends and look at the encoded data in the sender
just after encoding the corruption has already happened. I suspect
that this has something to do with the way structures are passed by
value by the compiler -- I believe that the first part of structs are
passed in registers, and the remainder on the stack. I'm guessing
that the encoder is not properly taking this into account.
I'm pretty sure gcc doesn't pass values in registers unless
specifically instructed to at compile time, so unless you're doing so
with your own code, I doubt it. Nonetheless, your suspicion does seem
plausible given the values that are being substituted... are these
values in any way reliable, or do they appear to be random?
For a definitive test of your theory, check the contents of the
registers at some stage before the data is actually sent down the
wire, and compare it to what you receive... if your theory is correct
(and you check the register contents close enough to where the data is
actually sent) you should see some clear correlation.
I'd be very surprised, nonetheless, if this were the issue you
postulate... it's hard to imagine no one else would have struck this
at any point, even if it is limited to DO. I would imagine the struct
is treated simply as a character array of length sizeof(_Msg), so if
it were in any way broken, nothing would work (since ultimately
everything has to be treated as a character array at one point or
another).
I can't think of any alternatives possibilities, though. I'd be very
interested to hear of any progress you make, and of course the final
answer if you can find it.
Wade Tregaskis (AIM, Yahoo & Skype: wadetregaskis, ICQ: 40056898, MSN
& email: email@hidden, Jabber:
email@hidden
-- Sed quis custodiet ipsos custodes?
Michael Heins AA7XY
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