Joerg-Cyril, I'm not sure I fully understand your example or concern. I agree that sharing and copying do not have exactly the same semantics. For a wide array of applications, the outcome of "copy the CL array to a foreign array, pass the foreign array to a function, and copy it back to the CL array" and "pass this pointer to my CL array directly to C and let it operate" will be the same.
Obviously, if my program is multithreaded or the C program needs to maintain the state across calls or I don't know what else, then the semantics become very different. It seems to me that a universal implementable "shared-vector" API will handle those cases where copying semantics are sufficient for correctness, and we want to share primarily for efficiency reasons. Since this seems to cover a lot of cases, why not provide this, with documentation caveats that it's not going to DTRT across all platforms if you require true sharing? rif > Hi, > > there's an IMHO compelling reason why with-pointer-to-vector-data CANNOT be > implemented using copying. As a result, the shareable vector API must be > provided as a strictly optional, API on implementations that support it. The > API's service would then merely be unification of various vendor APIs, a > valuable goal for CFFI. > > The reason is as follows: Sharing produces problems similar to those of > multiprocessor machines with separate caches. AFAIK, the 68060 processor can > sniff the bus while "idle" and invalidate (or possibly reload) its cache. > CFFI cannot do this. > > Trying to copy in one or the other direction does not implement sharing. It > instead implements "with exclusive access from Lisp (resp. foreign) side > within the scope of the body" -- depending on the direction of the copying. > > Here's a testcase. Try to imagine the following within the body of a macro > that surrounds it with copying the block in one direction or the other. > > (setq/defparameter shared-array (make-shareable-byte-vector 2000)) > (fill shared-array 2) > > (with-pointer-to-vector-data (ptr shared-array) > (loop for i below (length shared-array) > when (oddp i) do (setf (aref shared-array i) (mod i 256)) > else do (setf (mem-aref ptr :ubyte i) 17))) > ;2nd loop to verify result on shared-array not shown, but trivial. > > So what's the feature's name? > #+cffi-feature:shareable-vector > #+cffi-feature:shared-vector > > Regards, > Jorg Hohle. > > _______________________________________________ > cffi-devel mailing list > cffi-devel@common-lisp.net > http://common-lisp.net/cgi-bin/mailman/listinfo/cffi-devel _______________________________________________ cffi-devel mailing list cffi-devel@common-lisp.net http://common-lisp.net/cgi-bin/mailman/listinfo/cffi-devel
