second call to mmap() results in error

Steven Bardwell SBardwell@lbmsys.com
Thu Jan 30 13:20:00 GMT 2014


> On Jan 30 07:25, Steven Bardwell wrote:
> > > > On 29/01/2014 19:12, Corinna Vinschen wrote:
> > > > >On Jan 29 09:00, Steven Bardwell wrote:
> > > > >>My application needs several areas of shared memory, and I am
> getting
> > > an
> > > > >>error ("No such device") on the second call to mmap(). The first call
> > > works
> > > > >>fine.
> > >
> > > Sorry guys, but it still works fine for me.  I tried your testcase on W7
> > > 32, W7 64 in 32 and 64 bit, and on Windows 8.1 64 in 32 and 64 bit.  I
> > > tried it with Cygwin 1.7.27 and with the latest snapshot.  I'm always
> > > getting the output "Shared memory initialized" and no error at all.
> > >
> > >
> > > Any chance one of you guys could debug this further, by stepping
> through
> > > the Cygwin mmap64 function, preferredly using the latest snapshot or,
> > > a self-built Cygwin DLL from?
> > >
> > >
> > > Corinna
> >
> > I reinstalled Cygwin, rebooted and the error persisted. Running 'gdb' and
> > stepping through the program showed that the call to mmap() fails for
> /block1
> > also -- it is returning an invalid address. This simplification of the program
> > shows that error on my machine ('Bus error (core dumped)' ) occurs
> > when it tries to do the memcpy() to the mapped address.
> The reason is that ftruncate is defined with the second argument being
> off_t, which is 8 byte.  524304 is an int (4 byte) only, though.  Since
> ftruncate is declared in unistd.h, but you didn't include unistd.h, the
> 2nd parameter to ftruncate is auto-propagated to int, which results in
> an invalid new file length, and which makes ftruncate fail.  Since you
> missed to check ftruncate's return value... you get the idea.
> 
> 
> Corinna

Thanks so much for wasting your time debugging my program. 

Adding the <unistd.h> fixes the problem in all its iterations -- the original mmap() 
test works (without the call to ftruncate()), as well as the second one. 

You all run a great technical support operation.

Steve


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