Re: Mmap problem

[Earnie Boyd <earnie_boyd at yahoo dot com>]

--- Steve Jorgensen <steve@khoral.com> wrote:
> 
> 	I'm trying to port Khoros Pro 2001 to NT using cygwin, and
> 	am having a problem with the mmap call.
> 
> 	Khoros has a transport abstraction that makes using mmap, tcpip,
> 	file, pipes, or memory all look like a standard unix file, allowing
> 	users to use our k* file i/o calls to read and write to each
> 	different kind of transport.  Anyway, the mmap transport, in order
> 	to avoid excessive resizing, forces the underlying mmap'ed file
> 	to grow more than the write call needs.  At close time, we
> 	ftruncate the file back to the size it's supposed to be.  The
> 	problem is that ftruncate is returning with an access error.
> 	This works on all the unix systems we've ported to.  
> 
> 	I've tried the ftruncate both before and after the munmap call
> 	with the same results.  Any ideas what else I can try, or am I
> 	just out of luck?
> 

This is an issue with the underlying file system.  Assuming that it can be
done, you must have all accesses to the file closed.  Win32 will not allow such
changes to an open file, even if you are the one that has it open.  I think I
remember that there is coding in cygwin to queue operations on a file that fail
due to an open fd to the file, so this could be a bug in that related coding.

I'm not in a position to test theories or grab my documentation currently, but
this should get you started in finding a fix/workaround.  I'd suggest that you
look to see if the ftruncate function exists in the VC++ runtime manuals to see
what caveats they place on the use of the function.  Yea, I know that the two
runtimes aren't remotely the same but functions dealing with the underlying
file system must be close in nature as you're not going to be able to change
the behavior of the file system with a new runtime.

=====
Earnie Boyd <mailto:earnie_boyd@yahoo.com>

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