Re: Segmentation violation after reading from file

[Michael Meeks <michael ximian com>]

Hi Peter,

On Wed, 2003-10-08 at 16:38, Peter Van Osta wrote:
> I am currently implementing the sorage and retrieval of CORBA structures 
> and I am getting segmentation violations when retrieving the objects 

	Hmm,

> (example se below). Wrting them to disk works as it doesn't give any 
> errors

	That's also an interestin assertion.

> Everything looks fine until the actual transfer back to the client.
> The structures contain "strings", should I copy those subelements 
> intdividualy with "CORBA_string_dup" or allocate them before sensidng 
> them back ?

	How do you think 'write' works ? show me how you could possibly
implement fwrite (void *, size_t, size_t, FILE *) to load pointer
dereferenced strings, and CORBA allocate them into memory ? how would
the code look ? - perhaps here is the pseudo code ;->

size_t
fwrite (void *mem, size_t size, size_t nmemb, FILE *stream)
{
	1. ... guess C type of void * ...
	2. ... guess C type is a CORBA type ...
	3. ... generically walk C type, dereferencing pointers ...
	4. ... keep cache of dereferenced pointers to avoid loops ...
	5. ... assign each element a global id ...
	6. ... start serialising id / element pairs to disk with
	       hierarchical information ...
}

	The problem with this is - that fwrite writes raw bytes straight to
disk (as you might expect), steps 1 -> 3 are (unfortunately) impossible.

> The error only occurs in those structures where I have changed the 
> content of a "string" element prior to wrting them to disk

	Grief - well - it's possible that owing to some incredible fluke, the
2nd time you run the program the strings are allocated at the same place
- and thus reading a pointer value in _might_ work.

> What am I doing wrong in retrieving these structures from disk and 
> sending them back from the server to the client. On both the client and 
> the server the objects and their string subelementts are allocated at 
> startup.

	You need to carefully hand-marshal each of the elements individually to
disk in your binary file format; and then hand marshal them back. I
would recommend using XML for this.

	I would suggest though that you use ghex2 on your existing file first
to see what's actually happening.

	Regards,

		Michael.

-- 
 michael@ximian.com  <><, Pseudo Engineer, itinerant idiot