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According to Corinna Vinschen on 11/9/2009 7:05 AM:
> This part of the testcase
>
> data2 = (char *) malloc (2 * pagesize);
> if (!data2)
> return 1;
> data2 += (pagesize - ((long int) data2 & (pagesize - 1))) & (pagesize - 1);
> if (data2 != mmap (data2, pagesize, PROT_READ | PROT_WRITE,
> MAP_PRIVATE | MAP_FIXED, fd, 0L))
> return 1;
>
> is bad. The chance that the address of data2 is not usable for mmap on
> Windows/Cygwin is 100%.
But in testing this further, I discovered that you CAN do:
data2 = mmap(...);
munmap (data2,...);
mmap (data2, ... MAP_FIXED)
and get success on cygwin. So I will be updating autoconf accordingly,
based on the STD below. Unfortunately, it looks like I also found a hole
in cygwin. Consider this (borrowing heavily from the autoconf test that I
am fixing):
#include <stdio.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <stdlib.h>
#include <string.h>
#include <unistd.h>
#include <fcntl.h>
#include <sys/mman.h>
int
main (int argc, char **argv)
{
char *data, *data2, *data3;
int i, pagesize;
int fd, fd2;
pagesize = getpagesize ();
/* First, make a file with some known garbage in it. */
data = (char *) malloc (pagesize);
if (!data)
return 1;
for (i = 0; i < pagesize; ++i)
*(data + i) = rand ();
umask (0);
fd = creat ("conftest.mmap", 0600);
if (fd < 0)
return 2;
if (write (fd, data, pagesize) != pagesize)
return 3;
close (fd);
/* Next, check that a page is zero-filled if not backed by a file. */
fd2 = open ("conftest.txt", O_RDWR | O_CREAT | O_TRUNC, 0600);
if (fd2 < 0)
return 11;
data2 = "";
if (write (fd2, data2, 1) != 1)
return 12;
else
/* We expect mmap to succeed, but reads to give SIGBUS, since mapped
region is an entire page beyond bounds of mapped file. */
;
data2 = mmap (0, pagesize, PROT_READ | PROT_WRITE, MAP_SHARED, fd2, 0L);
if (data2 == MAP_FAILED)
return 14;
printf ("mapped %p\n", data2);
for (i = 0; i < pagesize; ++i)
if (*(data2 + i))
{
printf ("%p, %x\n", data2 + i, *(data2 + i));
return 15;
}
close (fd2);
if (argc > 1)
munmap (data2, pagesize);
/* Next, try to mmap the file at a fixed address which already has
something else allocated at it. If we can, also make sure that
we see the same garbage. */
fd = open ("conftest.mmap", O_RDWR);
if (fd < 0)
return 4;
if (data2 != mmap (data2, pagesize, PROT_READ | PROT_WRITE,
MAP_PRIVATE | MAP_FIXED, fd, 0L))
return 6;
for (i = 0; i < pagesize; ++i)
if (*(data + i) != *(data2 + i))
{
printf ("%p, exp %x, got %x\n", data2 + i, *(data + i), *(data2 + i));
return 7;
}
/* Finally, make sure that changes to the mapped area do not
percolate back to the file as seen by read(). (This is a bug on
some variants of i386 svr4.0.) */
for (i = 0; i < pagesize; ++i)
*(data2 + i) = *(data2 + i) + 1;
data3 = (char *) malloc (pagesize);
if (!data3)
return 8;
if (read (fd, data3, pagesize) != pagesize)
return 9;
for (i = 0; i < pagesize; ++i)
if (*(data + i) != *(data3 + i))
return 10;
close (fd);
return 0;
}
This test behaves differently on Linux than on cygwin; on Linux, both
'./foo' and './foo 1' give status 0, but on cygwin, './foo' gives status
6, and only './foo 1' succeeds. In other words, the second mmap fails if
there is no intermediate munmap.
POSIX apparently allows cygwin's behavior:
"If MAP_FIXED is set, mmap() may return MAP_FAILED and set errno to
[EINVAL]. If a MAP_FIXED request is successful, the mapping established by
mmap() replaces any previous mappings for the pages in the range
[pa,pa+len) of the process."
However, since we already have to maintain a list of mappings in order to
implement fork(), it seems like it would be easy to fix cygwin to
implicitly munmap anything that would otherwise be in the way of a
subsequent MAP_FIXED request, rather than blindly calling
NtMapViewOfSection and failing because of the overlap, so that we could be
even more like Linux behavior.
> That's why I think we need at least two tests in autoconf, a generic
> mmap test and a mmap test for the "mmap private/shared fixed at
> somewhere already mapped" case, if an application actually insists on
> using that.
In the case of the autoconf test, I think a single test is still
sufficient, once it is fixed to be portable to what POSIX requires.
gnulib provides a more interesting test, for whether MMAP_ANON works.
http://git.savannah.gnu.org/cgit/gnulib.git/tree/m4/mmap-anon.m4
- --
Don't work too hard, make some time for fun as well!
Eric Blake e...@byu.net
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