[cffi-devel] protect C frames from Lisp non-local exits in callbacks
A brief discussion in #lisp pointed me to the EXIT-EXTENT:MINIMAL ANSI issue[1]. The salient point is that there is no way to portably stop all unwinding for a non-local exit at a particular point, due to this case of undefined behavior: ;;returns 2 under MEDIUM, is error under MINIMAL (block a (block b (unwind-protect (return-from a 1) (return-from b 2)))) A CL implementation that implements MEDIUM should still be fully compliant with the MINIMAL decision. Therefore, I would like to hear from those with particular interest in any particular implementation running CFFI: does implementation X implement EXIT-EXTENT:MEDIUM, and/or does its callback definition form as exploited by cffi-sys:%defcallback have some "magic" unwind stopping? As an aside: would it be better for the defcallback semantics to simply say "don't exit non-locally through a C frame"? It would probably make callbacks (and perhaps callouts, for that matter) cheaper to call, on those without "magic" unwind stopping anyway. [1] http://www.lisp.org/HyperSpec/Issues/iss152.html -- Stephen Compall http://scompall.nocandysw.com/blog
On Thu, 05 Jan 2006 13:10:59 -0600, Stephen Compall <s11@member.fsf.org> said:
Stephen> A brief discussion in #lisp pointed me to the EXIT-EXTENT:MINIMAL ANSI Stephen> issue[1]. The salient point is that there is no way to portably stop Stephen> all unwinding for a non-local exit at a particular point, due to this Stephen> case of undefined behavior: Stephen> ;;returns 2 under MEDIUM, is error under MINIMAL Stephen> (block a Stephen> (block b Stephen> (unwind-protect (return-from a 1) Stephen> (return-from b 2)))) Stephen> A CL implementation that implements MEDIUM should still be fully Stephen> compliant with the MINIMAL decision. Therefore, I would like to hear Stephen> from those with particular interest in any particular implementation Stephen> running CFFI: does implementation X implement EXIT-EXTENT:MEDIUM, and/or Stephen> does its callback definition form as exploited by cffi-sys:%defcallback Stephen> have some "magic" unwind stopping? LispWorks implements MEDIUM. -- Martin Simmons Email: martin@lispworks.com LispWorks Ltd, St John's Innovation Centre TEL: +44 1223 421860 Cowley Road, Cambridge CB4 0WS, England. FAX: +44 870 2206189
participants (2)
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Martin Simmons -
Stephen Compall