Re: Heap memory available (W32 console app in Visual C++)

"Igor Tandetnik" <>
Tue, 26 Aug 2008 13:38:32 -0400
CriCri <> wrote:

int wanted1, wanted2;
void *p1, *p2;

// program state 1

if ( ! ( p1 = malloc( wanted1 ) )
  // switch to alternative method

// process first block
// --> program state 2

if ( ! ( p2 = malloc( wanted2 ) )
  // handle failure - i.e. exit()
  // because it's no longer easy to switch to alternative method

// process second block

Why not

p1 = malloc(wanted1);
if (p1) p2 = malloc(wanted2);
if (p1 && p2) {
    // Process two blocks
} else {
    if (p1) free(p1);
    // switch to alternative method

Now as you said other processes could have allocated and freed memory
between the _memavl() and the two malloc()'s (obviously perfectly
true). So that means that the _memval() and the two malloc()'s need
to be in a critical section.

You seem to misunderstand how critical sections work. All threads
accessing the same shared data must do so under the same critical
section, otherwise it won't help any. You can't enforce proper access
synchronization from just one thread - all threads must cooperate.

Now, how do you plan to get all other processes in the system to use
your critical section whenever they allocate memory?

But according to the results of Alex's research my VM process should
have been allocated a fixed 2GB (less the system overhead), so _my_
heap size shouldn't be affected.

You have 2GB of available address space. That doesn't mean you actually
have 2GB of available memory.

Your assumption is that all (user?) processes share a global heap (but
certainly not a global stack...).

Of course not. But all processes do share the same physical memory chips
installed on the motherboard, and the same swap file on the same
physical hard drive. You don't really believe that you can start 1000
processes and each of them would magically have 2GB of free memory
available to it? We are not yet at the stage where every computer
routinely has 2TB of storage attached.
With best wishes,
    Igor Tandetnik

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