121 lines
5.8 KiB
Plaintext
121 lines
5.8 KiB
Plaintext
Contributed by Christopher Faylor
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Cygwin has recently adopted something called the "cygwin heap". This is
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an internal heap that is inherited by forked/execed children. It
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consists of process specific information that should be inherited. So
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things like the file descriptor table, the current working directory,
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and the chroot value live there.
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The cygheap is also used to pass argv information to a child process.
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There is a problem here, though. If you allocate space for argv on the
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heap and then exec a process the child process (1) will happily use the
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space in the heap. But what happens when that process execs another
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process (2)? The space used by child process (1) still is being used in
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child process (2) but it is basically just a memory leak.
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To rectify this problem, memory used by child process 1 is tagged in
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such a way that child process 2 will know to delete it. This is in
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cygheap_fixup_in_child.
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The cygheap memory allocation functions are adapted from memory
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allocators developed by DJ Delorie. They are similar to early BSD
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malloc and are intended to be relatively lightweight and relatively
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fast.
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How is the cygheap propagated to the child?
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Well, it depends if you are running on Windows 9x or Windows NT.
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On NT and 9x, just before CreateProcess is about to be called in
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fork or exec, a shared memory region is prepared for copying of the
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cygwin heap. This is in cygheap_setup_for_child. The handle to this
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shared memory region is passed to the new process in the 'child_info'
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structure.
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If there are no handles that need "fixing up" prior to starting another
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process, cygheap_setup_for_child will also copy the contents of the
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cygwin heap to the shared memory region.
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If there are any handles that need "fixing up" prior to invoking
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another process (i.e., sockets) then the creation of the shared
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memory region and copying of the current cygwin heap is a two
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step process.
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First the shared memory region is created and the process is started
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in a "CREATE_SUSPENDED" state, inheriting the handle. After the
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process is created, the fixup_before_*() functions are called. These
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set information in the heap and duplicate handles in the child, essentially
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ensuring that the child's fd table is correct.
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(Note that it is vital that the cygwin heap should not grow during this
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process. Currently, there is no guard against this happening so this
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operation is not thread safe.)
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Meanwhile, back in fork_parent, the function
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cygheap_setup_for_child_cleanup is called. In the simple "one step"
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case above, all that happens is that the shared memory is ummapped and
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the handle referring to it is closed.
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In the two step process, the cygheap is now copied to the shared memory
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region, complete with new fdtab info (the child process will see the
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updated information as soon as it starts). Then the memory is unmapped,
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the handle is closed, and upon return the child process is started.
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It is in the child process that the difference between Windows 9x and
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Windows NT becomes evident.
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Under Windows NT, the operation is simple. The shared memory handle is
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used to map the information that the parent has set up into the cygheap
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location in the child. This means that the child has a copy of the
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cygwin heap existing in "shared memory" but the only process with a view
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to this "shared memory" is the child.
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Under Windows 9x, due to address limitations, we can't just map the
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shared memory region into the cygheap position. So, instead, the memory
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is mapped whereever Windows wants to put it, a new heap region is
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allocated at the same place as in the parent, the contents of the shared
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memory is *copied* to the new heap, and the shared memory is unmapped.
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Simple, huh?
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Why do we go to these contortions? Previous versions (<1.3.3) of cygwin
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used to block when creating a child so that the child could copy the
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parent's cygheap. The problem with this was that when a cygwin process
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invoked a non-cygwin child, it would block forever waiting for the child
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to let it know that it was done copying the heap. That caused
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understandable complaints from people who wanted to run non-cygwin
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applications "in the background".
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In Cygwin 1.3.3 (and presumably beyond) the location of the cygwin heap
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has been fixed to be at the end of the cygwin1.dll address space.
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Previously, we let the "OS" choose where to allocate the cygwin heap in
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the initial cygwin process and attempted to use this same location in
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subsequent cygwin processes started from this parent.
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The reason for putting cygheap at a fixed, known location is that we
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need to put this information at a fixed location since it incorporates
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pointers to entities within itself. So, when a process forks or execs,
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the memory referred to by the pointers has to exist at the same place in
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both the parent or the child.
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(It "might be nice" to used something like Microsoft's "based pointers"
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for the cygheap. Unfortunately gcc does not support that feature, as of
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this writing.)
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The reason for choosing a fixed, arbitrary location is to accommodate
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Windows XP, although there were sporadic complaints of cygwin heap
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failures in other pathological situations with both NT and 9x. In
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Windows XP, Microsoft made the allocation of memory less deterministic.
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This is certainly their right. Cygwin was previously relying on
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undocumented and "iffy" behavior before. So, now we always allocate
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space immediately after the dll in the theory that there is not going
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to be anything else living there.
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Recent (2001-09-20) cygwin email threads have indicated that we're not
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exactly on completely firm ground now, though. We are assuming that
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there is sufficient space after the cygwin DLL for the allocation of the
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cygwin heap. Unfortunately the ld option '--enable-auto-image-base'
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has a tendency to allocate DLLs immediately after cygwin1.dll. This
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causes the dreaded "Couldn't reserve space for cygwin's heap" message.
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Solutions for this behavior are currently in the musing state.
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