On 2/08/2013 7:30 AM, Paul Gilmartin wrote:
On Thu, 1 Aug 2013 17:50:10 -0400, John Gilmore wrote:
What seems to have fallen through the cracks in this [tangential]
discussion is that the use of code-modification schemes, ALTER and the
like, in a routine makes it non-reentrant.
This in my view is the crucial objection to their use.
One might imagine restoring reentrancy by making the alterable
fragments pointers into obtained storage.
Almost as bad as code-modification is the use in C of function pointers
in structs. BTDT.
What's wrong with function pointers in structs? That's the only way you
can do polymorphism in C. I would argue
that they are rather useful. The interface for Unix device drivers is
function pointers in structs.
In PostScript, subroutines are first-class data objects. Something
similar applies to Lisp. Sometimes the interpreter helps with
the debugging.
And there was a version of the PDP-8 with the OS in ROM with all
modifiable locations replaced with pointers to locations in RAM.
PSA makes z/OS partly reentrant. Exploited by multiple CPUs in any
LPAR, but not, AFAIK, by VM or PR/SM.
CMS has a reentrant kernel (nucleus?), precious for virtualization.
I believe Linux has not.
-- gil
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