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Christopher Faylor 2001-09-22 01:37:47 +00:00
parent e0197c5ea5
commit 99426172cc
1 changed files with 26 additions and 10 deletions
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winsup/cygwin/how-cygheap-works.txt
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@ -91,14 +91,30 @@ Previously, we let the "OS" choose where to allocate the cygwin heap in
the initial cygwin process and attempted to use this same location in
subsequent cygwin processes started from this parent.
This was basically done to accomodate Windows XP, although there were
sporadic complaints of cygwin heap failures in other pathological
situations with both NT and 9x. In Windows XP, Microsoft made the
allocation of memory less deterministic. This is certainly their right.
Cygwin was previously relying on undocumented and "iffy" behavior before.
The reason for putting cygheap at a fixed, known location is that we
need to put this information at a fixed location since it incorporates
pointers to entities within itself. So, when a process forks or execs,
the memory referred to by the pointers has to exist at the same place in
both the parent or the child.
We're not exactly on completely firm ground now, though. We are assuming
that there is sufficient space after the cygwin DLL for the allocation
of the cygwin heap. So far this assumption has proved workable but there
is no guarantee that newer versions of Windows won't break this assumption
too.
(It "might be nice" to used something like Microsoft's "based pointers"
for the cygheap. Unfortunately gcc does not support that feature, as of
this writing.)
The reason for choosing a fixed, arbitrary location is to accomodate
Windows XP, although there were sporadic complaints of cygwin heap
failures in other pathological situations with both NT and 9x. In
Windows XP, Microsoft made the allocation of memory less deterministic.
This is certainly their right. Cygwin was previously relying on
undocumented and "iffy" behavior before. So, now we always allocate
space immediately after the dll in the theory that there is not going
to be anything else living there.
Recent (2001-09-20) cygwin email threads have indicated that we're not
exactly on completely firm ground now, though. We are assuming that
there is sufficient space after the cygwin DLL for the allocation of the
cygwin heap. Unfortunately the ld option '--enable-auto-image-base'
has a tendency to allocate DLLs immediately after cygwin1.dll. This
causes the dreaded "Couldn't reserve space for cygwin's heap" message.
Solutions for this behavior are currently in the musing state.