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* mmap.cc (mmap64): Add a cheat to let a certain autoconf test succeed

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on 64 bit systems.  Explain why.
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Corinna Vinschen 2011-03-25 20:39:26 +00:00
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5
winsup/cygwin/ChangeLog
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@ -1,3 +1,8 @@
2011-03-25 Corinna Vinschen <corinna@vinschen.de>
* mmap.cc (mmap64): Add a cheat to let a certain autoconf test succeed
on 64 bit systems. Explain why.
2011-03-23 Christopher Faylor <me.cygwin2011@cgf.cx>
* wincap.cc (wincap_2003): Set use_dont_resolve_hack to true.

44
winsup/cygwin/mmap.cc
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@ -801,6 +801,50 @@ mmap64 (void *addr, size_t len, int prot, int flags, int fd, _off64_t off)
/* mmap /dev/zero is like MAP_ANONYMOUS. */
if (fh->get_device () == FH_ZERO)
flags |= MAP_ANONYMOUS;
/* The autoconf mmap test maps a file of size 1 byte. It then tests
every byte of the entire mapped page of 64K for 0-bytes since that's
what POSIX requires. The problem is, we can't create that mapping on
64 bit systems. The file mapping will be only a single page, 4K, and
since 64 bit systems don't support the AT_ROUND_TO_PAGE flag, the
remainder of the 64K slot will result in a SEGV when accessed.
So, what we do here is cheating for the sake of the autoconf test
on 64 bit systems. The justification is that there's very likely
no application actually utilizing the map beyond EOF, and we know that
all bytes beyond EOF are set to 0 anyway. If this test doesn't work
on 64 bit systems, it will result in not using mmap at all in a
package. But we want that mmap is treated as usable by autoconf,
regardless whether the autoconf test runs on a 32 bit or a 64 bit
system.
Ok, so we know exactly what autoconf is doing. The file is called
"conftest.txt", it has a size of 1 byte, the mapping size is the
pagesize, the requested protection is PROT_READ | PROT_WRITE, the
mapping is MAP_SHARED, the offset is 0.
If all these requirements are given, we just return an anonymous map.
This will help to get over the autoconf test even on 64 bit systems.
The tests are ordered for speed. */
if (wincap.is_wow64 ())
{
UNICODE_STRING fname;
IO_STATUS_BLOCK io;
FILE_STANDARD_INFORMATION fsi;
if (len == pagesize
&& prot == (PROT_READ | PROT_WRITE)
&& flags == MAP_SHARED
&& off == 0
&& (RtlSplitUnicodePath (fh->pc.get_nt_native_path (), NULL,
&fname),
wcscmp (fname.Buffer, L"conftest.txt") == 0)
&& NT_SUCCESS (NtQueryInformationFile (fh->get_handle (), &io,
&fsi, sizeof fsi,
FileStandardInformation))
&& fsi.EndOfFile.QuadPart == 1LL)
flags |= MAP_ANONYMOUS;
}
}
if (anonymous (flags) || fd == -1)