Re: VirtualAlloc v.s. malloc

From:

"Alexander Nickolov" <agnickolov@mvps.org>

Newsgroups:

microsoft.public.vc.language

Date:

Mon, 5 Nov 2007 09:44:39 -0800

Message-ID:

<e3RxsN9HIHA.4880@TK2MSFTNGP03.phx.gbl>

I'd add:

4. If you need to allocate memory to write executable code in,
like ATL's windowing thunks.

--
=====================================
Alexander Nickolov
Microsoft MVP [VC], MCSD
email: agnickolov@mvps.org
MVP VC FAQ: http://vcfaq.mvps.org
=====================================

"Alexander Grigoriev" <alegr@earthlink.net> wrote in message
news:eeBSxOvHIHA.2480@TK2MSFTNGP05.phx.gbl...

VirtualAlloc sould be used only:

1. I you need allocation aligned on page boundary (usually for
non-buffered I/O).
2. You don't want to commit full amount of memory (sparse allocation).
3. You want to allocate very large amount at once. There is limit on a
single allocation in HeapAlloc.

"George" <George@discussions.microsoft.com> wrote in message
news:43D41846-2B89-4F12-B445-FF123319EA9D@microsoft.com...

Thanks Alex,

They are great learning resources and I have read them for 2 hours. I
have
found almost all of my questions are answered except this one,

when should be use VirtualAlloc and when should we use HeapAlloc (the
specific advantages and disadvantages of the two functions), could you
share
some experiences please?

regards,
George

"Alex Blekhman" wrote:

"George" wrote:

I have read some documents in MSDN about VirtualAlloc API
and HeapAlloc API,
but I am still confused about what is the differences
between VirtualAlloc
and HeapAlloc? Seems they allocate memory from different
places?

I'd suggest you to follow Scott's advice: Don't use low
level OS calls unless you're absolutely required to.

`VirtualAlloc' allocates memory directly from OS. That's why
it has OS specific limitations like allocating in 4K chunks,
address alignment etc.

`HeapAlloc' calls `VirtualAlloc' internally. It returns you
aleady pre-allocated memory from process' heap. It is
somewhat similar to CRT's `malloc'.

If you want to understand the differences, then you should
read at least following articles:

"Managing Virtual Memory in Win32"
http://msdn2.microsoft.com/en-us/library/ms810627.aspx

"Managing Heap Memory in Win32"
http://msdn2.microsoft.com/en-us/library/ms810603.aspx

They are quite old, but mostly relevant even today. For
comprehensive material on Win32 memory management I'd
suggest to refer to good book:

"rogramming Applications for Microsoft Windows" by Jeffrey
Richter
http://www.microsoft.com/MSPress/books/2345.aspx

HTH
Alex