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Subject: |
23.3; malloc initialization should (sometimes) happen at runtime |
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Date: |
Tue, 09 Aug 2011 16:09:48 -0400 |
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User-agent: |
Mozilla/5.0 (Windows; U; Windows NT 6.1; en-US; rv:1.9.2.18) Gecko/20110616 Thunderbird/3.1.11 |
The code in src/gmalloc.c makes assumptions about how a system maintains
its memory that are not necessarily valid. In particular, they will not
be valid on Cygwin starting with version 1.7.10 (which will almost
certainly be released before emacs 24.1). The problem is that malloc
initialization is done by temacs, and the results are dumped into emacs.
This includes the setting __malloc_initialized = 1, so no malloc
initialization is done when emacs is run. But the dumped value of
_heapbase, while appropriate for temacs, may not point to the beginning
of the runtime heap for emacs. This causes all code that uses the BLOCK
and ADDRESS macros to be invalid.
Here's what happens on Cygwin. temacs (on Cygwin) uses a static buffer
as its heap and a function bss_sbrk that simulates sbrk. (See
src/sheap.c.) The data in this buffer, including malloc information,
are then dumped into emacs.exe as initialized data. But when the dumped
emacs is run, it uses Cygwin's sbrk, which allocates memory on a heap
that won't (as of Cygwin 1.7.10) be contiguous with the static heap.
The saved value of _heapbase, which points into the static heap, is
never changed, but it will mess up later calculations as soon as sbrk is
called for the first time.
All of this is described in detail on the Cygwin mailing list in the
thread starting at
http://cygwin.com/ml/cygwin/2011-08/msg00153.html
See especially
http://cygwin.com/ml/cygwin/2011-08/msg00193.html
which contains a gdb session illustrating the problem. The context for
that session is that, as a result of the problem I'm reporting,
morecore_nolock went into an infinite loop. I attached gdb to that
looping process.
Maybe the solution is for emacs to do malloc initialization, including
the assignment of _heapbase, every time it starts, at least on systems
that use gmalloc.c. I made one naive attempt to do this, but it didn't
work (and it was Cygwin specific). Namely, I made unexec (for Cygwin)
set _malloc_initialized = 0 before dumping. The resulting emacs aborted
as soon as it was started. I haven't figured out what went wrong, but
I'm not sure that's the right answer anyway.
Ken
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