Re: C++ exception handling question [solved]

[hendrik@topoi.pooq.com]

On Sun, Nov 12, 2006 at 08:31:42PM -0700, Paul E Condon wrote:
> On Sun, Nov 12, 2006 at 09:55:52AM -0800, John L Fjellstad wrote:
> > Paul E Condon <pecondon@mesanetworks.net> writes:
> > 
> > > Thanks for stack profiling info. I wonder if this _is_ a reportable
> > > bug. After all, there is a lot of information on the 'bad_alloc'
> > > exception in various sources. If GNU C++ library doesn't try to throw
> > > this exception until it is too late for the throw to succeed, what
> > > good is it? And, how was its operation verified during testing on a
> > > Debain box? At the point where it kill happens, I think the program is
> > > allocating space for the contents of large STL vector of vectors.  It
> > > doesn't seem to me that this should be done on the stack, but I
> > > haven't thought deeply on the issue.
> 
> All my speculation as to the quality of GNU C++ compiler is
> inoperative. The problem is in the kernel, as pointed out by John F.:
> 
> > 
> > One thing you might not be aware of is that Linux overcommit the
> > memory.  That is, even if there is no memory left, it might tell the
> > program that it has enough, since it expect that by the time you use the
> > memory, it will be available. It's controlled by
> > /proc/sys/vm/overcommit_memory. 
> > 
> > Check the man page for malloc (look under BUGS), and do some search in
> > your favorite search engine.
> > 
> 
> This is really interesting. man malloc calls the default behavior
> of the kernel a really bad bug, with which I agree. kernel-docs
> sysctl/vm.txt justifies the default by claiming it is helpful to
> programmers who 'malloc() huge amounts of memory "just-in-case" and
> don't use much of it.' I had thought such programmers were rare in the
> Linux world, as they are all happily writing garbage code for
> BillG. But, oh well. Now I know how to make my code work under Linux. 

It is reasonable to reserve large blocks of address space, if you need 
unknown amounts of contiguous space -- and then populate it later.  This 
happens, for example, with some memory allocation algorithms.  FOr 
specific applications, such techniques may be more efficient or 
otherwise appropriate than using malloc.  But I do believe that if such 
a feature is provided, it should *not* be called malloc, which has other 
important uses.

In any case, it sould be possible to ask for storage allocation in such 
a way that the allocation request fails if the storage is not going to 
be available when it is needed,

-- hendrik