Shared memory and IPC

[Rich Salz]

> MEMCTX_SHARED returns an optionally-provided shared allocator

Well at least it's optional. :)

> What if they had prior knowledge of the existence of the shared memory
> region?

Both mmap() and shmat() let you specify a "hint", but things can fail
depending on the initial process size, amount of sbrk() [malloc] use,
etc.  It's hard to see how a single pointer can be valid for both of
these, for example:
; size /bin/rc /usr/local/bin/htmldoc
   text    data     bss     dec     hex filename
  61207    8960   12184   82351   141af /bin/rc
 133889   11532 6220864 6366285  61244d /usr/local/bin/htmldoc
; file /bin/rc /usr/local/bin/htmldoc 
/bin/rc:                ELF 32-bit LSB executable, Intel 80386, version
1, dynamically linked (uses shared libs), not stripped
/usr/local/bin/htmldoc: ELF 32-bit LSB executable, Intel 80386, version
1, dynamically linked (uses shared libs), not stripped

(htmldoc does HTML to PDF; rc is my login shell, a clone of the Plan9
shell)
	/r$
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