Xref: utzoo comp.lang.c++:6737 gnu.g++:699
Path: utzoo!utgpu!jarvis.csri.toronto.edu!cs.utexas.edu!swrinde!zaphod.mps.ohio-state.edu!usc!ucsd!ucbvax!decwrl!shelby!csli!keith
From: keith@csli.Stanford.EDU (Keith Nishihara)
Newsgroups: comp.lang.c++,gnu.g++
Subject: Recovering when out of memory.
Keywords: new_handler
Message-ID: <12586@csli.Stanford.EDU>
Date: 8 Mar 90 02:09:37 GMT
Sender: keith@csli.Stanford.EDU (Keith Nishihara)
Reply-To: Neil Hunt <Neil%teleos.com@ai.sri.com>
Followup-To: comp.lang.c++
Organization: Center for the Study of Language and Information, Stanford U.
Lines: 62

Can anyone send me an example of a new_handler function which re-
covers so that the new, which exhausted memory, is able to return
with a valid pointer.  The kind of thing I  have  in  mind  is  a
new_handler  which  initially  allocates  a chunk of memory large
enough to allocate the largest objects which might be needed, and
holds  it  in reserve.  When a new failure occurs, the reserve is
deleted back into the free store, allowing the new to return nor-
mally.  Of course, warning flags and messages will be set and is-
sued, so that the user knows to do something about the situation.

Ideally, a solution portable to cfront 2.0 and g++ is desired!

I tried the obvious thing in g++, just deleting  a  large  object
from an installed new_handler.  Although the delete call is made,
no new memory is returned:

static char *large = 0;

void
my_new_handler()
{
    if(large)
    {
        fprintf(stderr, "Freeing large\n");
        delete large;
        large = 0;
    }
    else
    {
        fprintf(stderr, "Handler fails\n");
        exit(-1);
    }
}

main()
{
    __new_handler = my_new_handler;

    large = new char[1000000];

    for( ; ; )
        fprintf(stderr, "new at %x\n", new char[100000]);
}

new at 12751000
new at 12772000
new at 12793000
new at 127b4000
new at 127d5000
out of swap space, pid 8655, proc k
Freeing large
new at 0
out of swap space, pid 8655, proc k
Handler fails

Note that although I deleted 1M of space, (`Freeing large'),  the
call to new which invoked the new_handler returns 0 (`new at 0'),
and the next call to allocate 100k of space still finds no memory
and  the new_handler is called again, this time exiting (`Handler
fails').

Neil/.          Neil%teleos.com@ai.sri.com