E-code Language Specification

In our design, plugins can be specified either as pre-compiled machine code, or in E-code - a language akin to `C [19] and developed as part of the ECho high-performance event-delivery middleware [10]. E-code is a fairly complete subset of the C language that compiles to native machine code at runtime using a dynamic code generator that processes one function at a time. A more detailed list of E-code capabilities follows:

• Datatypes: E-code supports the following basic types: char, int, float, double, and boolean. It also supports structures, pointers (including pointers to structures), and pointer arithmetic.

• Variables: Global variables are allocated on the heap, which is a per-runtime pool of ring-1 memory persistent across plugin invocations. Local variables are allocated on the plugin stack.

• Function calls: Plugins are allowed to perform function calls only to other functions or callbacks registered within their runtime. Appropriate trap or trampoline code for the invocation is generated automatically and transparently.

• Function prototypes: Plugin functions must conform to a prototype convention - their first argument must be a `runtime_t $*$' to provide linkage back to their runtime. Furthermore, their result type is restricted to long, however, that is not a severe restriction since most basic datatypes are easily cast to a long value, with the notable exception of the class of floating point numbers, which must be passed back by reference.

• Language: E-code supports the C operators, for loops, if, and return statements.

Currently, E-code does not support while loops, switch statements, unions, and function pointers, though they do not pose conceptual difficulties and can be implemented if needed in the future.