Re: 64 bit C++ and OS defined types
* Ian Collins:
Alf P. Steinbach wrote:
* Ian Collins:
Alf P. Steinbach wrote:
* gw7rib@aol.com:
Why do you want a signed type to indicate a quantity, which can't be
negative? Aren't you wasting half its potential values?
No, there's no waste except for the case of a single byte array
that's more than half the size of addressable memory, which on a
modern system you simply will not ever have.
Why not?
Just try to come up with a /concrete/ example... :-)
32bit Solaris?
Any 32 bit OS with memory mapped files and large file support?
And is the concrete example then of mapping a 2 GB file to memory under Solaris
and using indexing instead of pointer arithmetic to access it?
Well, I grant that it's possible, and so "will not ever have" was too strong.
But we have to really search to come up with such special cases, and they're not
problematic for the convention of using signed sizes. For when you're doing a
over-half-memory file mapping you're on the edge of things and have to deal with
much more serious problems. By comparision, applying the right types for the
special case shouldn't be hard, and anyway shouldn't influence the choice of
types for more normal code: the marginal gain for the special case doesn't
outweight the serious problems elsewhere (unless Marketing gets into it :-) ).
Cheers, & hth.,
- Alf
PS-
I don't have Solaris (James has) but I think the following C code, just grabbed
from the net, would do something like that:
#include <stdio.h>
#include <stdlib.h>
#include <sys/types.h>
#include <sys/mman.h>
#include <fcntl.h>
#include <unistd.h>
int main(void)
{
int fd, i;
char *mmap_space;
long pagesize = sysconf(_SC_PAGESIZE);
unsigned long mmap_size = 3200000000;
mmap_size = ALLOCSIZE - malloc_size;
if ((fd = open("/dev/zero", O_RDWR)) == -1)
perror("open"), exit(-1);
mmap_space = (void*)mmap((caddr_t) 0,
mmap_size,
(PROT_READ | PROT_WRITE),
MAP_PRIVATE,
fd,
(off_t)0);
if (mmap_space == MAP_FAILED)
perror("mmap"), exit(-1);
(void)close(fd);
(void)fprintf(stderr, "mmap'd %lu bytes\n", mmap_size);
/*
* Just to be thorough, test evey page
*/
(void)fprintf(stderr, "Testing the %lu mmap'd bytes ...\n", mmap_size);
for (i=0; i<mmap_size; i+=pagesize)
mmap_space[i] = i;
(void)fprintf(stderr, "done\n");
return 0;
}
Have you tried it?
-DS
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