On Mon, 25 Apr 2011 13:54:20 -0400 Bob Hall <rjh...@gmail.com> wrote:
> On Mon, Apr 25, 2011 at 05:46:33PM +0200, C. P. Ghost wrote: > > On Mon, Apr 25, 2011 at 5:15 PM, Bob Hall <rjh...@gmail.com> wrote: > > > On Mon, Apr 25, 2011 at 03:18:46PM +0100, RW wrote: > > >> I don't believe the heap is allocated zeroed pages. The kernel > > >> does allocate such pages to the BSS segment, but that's because > > >> it holds zeroed data such as C static variables. > > > > > > According to McKusick and Neville-Neil's book on FreeBSD, sbrk > > > extends the uninitialized data segment with zero-filled pages. > > > Since malloc() is an interface to sbrk, it does the same thing. > > > > True, except that malloc(3) now uses both sbrk(2) and mmap(2) > > allocators, depending on the user-settable flags > > in /etc/malloc.conf, MALLOC_OPTIONS and the global variable > > _malloc_options. So you have to look into mmap(2) too. > > Good point. From the man page: > "Any such extension beyond the end of the mapped object will be > zero-filled." > and > "A successful mmap deletes any previous mapping in the allocated > address range." The above quote refers to zeroing the fraction of a page that's left over when "len" isn't a multiple of the page size. However, there's a comment in malloc.c about mmap'ed regions being zeroed, so I guess they are, but it doesn't seem to be mentioned at all in mmap(2). The reason I thought that heap memory isn't zeroed is from the discussion of pre-zeroed pages in this article: http://www.freebsd.org/doc/en_US.ISO8859-1/articles/vm-design/prefault-optimizations.html It reads as if the BSS region is the only significant user of zeroed pages. _______________________________________________ freebsd-questions@freebsd.org mailing list http://lists.freebsd.org/mailman/listinfo/freebsd-questions To unsubscribe, send any mail to "freebsd-questions-unsubscr...@freebsd.org"
