Mobile Data, Dynamic Allocation and Zero Aliasing: an occam Experiment

Traditional imperative languages (such as C) and modern object-oriented languages are plagued by uncontrolled resource aliasing problems. Add in concur- rency and the problems compound exponentially. Improperly synchronised access to shared (i.e. aliased) resources leads to problems of race-hazard, deadlock, livelock and starvation. This paper describes the binding into occam (a concurrent processing language based on CSP) of a secure, dynamic and efficient way of sharing data between parallel processes with minimal synchronisation overheads. The key new facilities provided are: a movement semantics for assignment and communication, strict zero-aliasing, apparently dynamic memory allocation and automatic zero-or-very-small-unit-time garbage collection. The implementation of this mechanism is also presented, along with some initial performance figures (e.g. 80ns for mobile communication on an 800 MHz Pentium 3). With occam becoming available on a variety of microprocessors for GUI building, internet services and small-memory-footprint embedded products, these capabilities are timely. Lessons are drawn for concurrency back in OO languages - and especially for the JCSP (CSP for Java) package library.

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