A Portable C Interface for Standard ML of New Jersey

This paper describes the design and implementation of an interface to C for the SML/NJ ML compiler. The interface supplies ML datatypes with which programmers specify C types and C data. An ML program uses these datatypes to register a foreign C function with the interface and to build speci cations of structured C data. The interface automatically instantiates C function arguments from C data speci cations upon foreign function application. Most C types, including aggregate and functionpointer types, are supported. A runtime code generation technique converts ML closures to C-callable function pointers. Function pointers allow C programs to call ML programs. We solve the problems due to di erences in data representation, function calling conventions, and storage management by copying data between the ML and C heaps, converting representations and changing calling conventions in the process. We nd that this copying strategy provides adequate performance in practice. The interface is portable in the sense that its implementation does not require changes to the SML/NJ compiler proper; it is isolated in a pair of libraries (ML and C). The interface has already found use in several non-trivial applications.