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Re: sysconf(_SC_PAGESIZE) set to 64k
On Jun 15 15:09, Dave Korn wrote:
> On 15 June 2006 14:56, Ehren Jarosek wrote:
> > I don't know if this is something I am doing wrong or an issue.
> > When compiling under cygwin sysconf(_SC_PAGESIZE) returns 65536 (64k)
> > memory page size. My understanding is that:
> > sysconf(_SC_PAGESIZE) * sysconf(_SC_PHYS_PAGES)
> > should yield the total physical memory size of the machine. However,
> > when I do this it yields a very large number (actually overflows my
> > long). However, if I multiply sysconf(_SC_PHYS_PAGES) * 4096 it yields
> > the correct size.
> Alas there is a problem with the definition of sysconf: it is
> supposed to be the size of the unit of granularity of mmap'ing, but
> it is also supposed to be the size of a single pageframe of memory.
_SC_PAGESIZE is only for indicating the page size as used in calls to
mmap(2). POSIX does not demand that _SC_PAGESIZE is actually the
physical page size.
Two quotes from the Linux man pages:
$ man getpagesize
The function getpagesize() returns the number of bytes in a page,
where a "page" is the thing used where it says in the description of
mmap(2) that files are mapped in page-sized units.
The size of the kind of pages that mmap uses, is found using
long sz = sysconf(_SC_PAGESIZE);
$ man sysconf
These values also exist, but may not be standard.
The number of pages of physical memory. Note that it is possi-
ble for the product of this value and the value of
_SC_PAGE_SIZE to overflow.
So, actually Ehren's application works on Linux just coincidentally,
since it make invalid assumptions.
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