possible problem with memory allocation using calloc/mmap/munmap

Corinna Vinschen corinna-cygwin@cygwin.com
Mon Jun 3 11:55:00 GMT 2019


On May  3 13:33, Stanislav Kascak wrote:
> Hello cygwin team,
> 
> I came across a problem with memory allocation/deallocation when
> trying to compile and run tests of openldap under cygwin.
> 
> I created a test program to simulate sequence of actions. First a
> bigger chunk of memory (>~262kB) is allocated using calloc(), then
> mmap() is called with requested memory size much bigger than actual
> file size. Subsequently,  munmap() to free previous mmap(), and at the
> end an attempt to access memory allocated with calloc(). That last
> memory access is causing Segmentation fault.
> 
> It seems that when mmap() is called with length argument exceeding
> size of file, only memory to fit that file is allocated. munmap()
> however frees the full specified length. Since (at least on my
> computer) big chunk of memory allocated by calloc() is located after
> mmap() allocation, munmap() frees even memory of that calloc().

Ken's right.  Due to the differences between mapping files on Windows
vs. Unix, Cygwin can't map beyond the file size + the remainder of the
last page.  Cygwin tries to workaround that on 32 bit by allocating
an anonymous mapping following the file mapping to keep the range free
from other mappings.  But on 64 bit this workaround doesn't work anymore
because the OS is missing an (undocumented) flag which allows to
create mappings on 4K boundaries, rather than just on 64K boundaries.

I know this situation is unsatisfying, but I have no easy workaround
to allow this.  Cygwin could add the anonymous mapping on the next
64K boundary on 64 bit, but that would result in a hole in the mapping
which seemed like a rather bad idea when porting mmap to 64 bit.

Ken's also right that munmap is doing the right thing here.  If
anything's wrong, it's mmap's workaround for mappings beyond the file
length.  If only 64 bit would allow 4K-aligned mappings :(


Corinna

-- 
Corinna Vinschen
Cygwin Maintainer
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