C++/VB - setting LFH (low fragmentation heap) on handles returned by getprocessheaps()

Asked By ted

16-Feb-08 04:49 AM

I'm running this simple C program (see below, compiled with cl.exe
version 14.00, XP SP2) and see the following  output:

Number of heaps returned by GetProcessHeaps() = 4
Heap 0: Succeeded (1) setting LFH
Heap 1: Succeeded (1) setting LFH
Heap 2: Failed (0) setting LFH, error = 31
Heap 3: Succeeded (1) setting LFH

Q: What are these 4 heaps? Do they have names (and how can I get the
names)?

Q: Which one corresponds to the one used by malloc()?

Q: Why does Heap 2 fail?



int main()
{
HANDLE aHeap[101] ;
ULONG HeapFragValue = 2 ;
BOOL retval ;
DWORD ii, nHeaps ;

nHeaps =(int)GetProcessHeaps( 100, aHeap ) ;     printf( "\nNumber
of heaps returned by GetProcessHeaps() = %d", nHeaps ) ;
for ( ii = 0 ; ii < nHeaps ; ii++ )
{
retval = HeapSetInformation( aHeap[ii],
HeapCompatibilityInformation,
&HeapFragValue,
sizeof(HeapFragValue) ) ;
if ( retval != 0 )
{
printf( "\nHeap %d: Succeeded (%d) setting LFH",
ii, retval ) ;
}
else
{
printf( "\nHeap %d: Failed (%d) setting LFH, error
= %d", ii, retval, GetLastError() ) ;
}
}

return 0 ;
}

15-Feb-08 05:18 PM

I don't know what are these heaps, but heaps don't have names,
AFAIK.


The one with the handle equal to the value returned by
`_get_heap_handle' CRT function.


Probably it has some features incopatible with LFH. For example,
it could be created with `HEAP_NO_SERIALIZE' flag, as mentioned in
MSDN.

Alex

17-Feb-08 10:19 AM

HeapQueryInformation gives info about what the specific heap is. See
here - http://msdn2.microsoft.com/en-us/library/aa366703(VS.85).aspx
HeapInformation could be 0, 1 or 2. See details at the above page.


The standard heap (for which HeapInformation is 0) would be the one
where from you get the allocations, but I believe it depends on the
runtime to decide which one would be better from performance point of
view but I am sure of this. To allocate from specific heaps you would
make calls to HeapAlloc (and functions of that family).



To understand why heap 2 failed to set up as LFH, you would need to
know what heap that is and if setting it as LFH makes sense.

16-Feb-08 01:32 PM

This is incorrect. HeapInformation = 0 does not automatically mean
that the heap is used by CRT. You should get CRT heap's handle by
calling `_get_heap_handle'.

Alex

17-Feb-08 10:20 AM

Actually, I missed a "not" before sure. :) Thanks for correcting me.
But, isn't the standard heap the one from which the allocations would
be done? I mean, one could change the heap info of standard heap to an
LFH but unless that is done, _get_heap_handle should return the same
heap as GetProcessHeap and that would be the same as the heap with HI
as 0 returned from GetProcessHeaps (considering heap info has not been
played around with). No?

17-Feb-08 07:01 AM

Actually, I don't know whether there exists "standard" heap at
all. I think that CRT is free to choose a heap to use or to create
its own. I tried the code form this post:

http://groups.google.com/group/microsoft.public.win32.programmer.kernel/msg/b290154476a4425c

It reveals that there are at least two heaps with HeapInformation
= 0. One of them is indeed CRT heap.


I can't neither confirm nor refute that since I never heard about
this requirement. I always thought that CRT can use any heap it
sees fit.

Alex