MLton's ForeignFunctionInterface only allows values of certain SML types to be passed between SML and C. The following types are allowed: bool, char, int, real, word. All of the different sizes of (fixed-sized) integers, reals, and words are supported as well: Int8.int, Int16.int, Int32.int, Int64.int, Real32.real, Real64.real, Word8.word, Word16.word, Word32.word, Word64.word. There is a special type, MLton.Pointer.t, for passing C pointers -- see MLtonPointer for details.

Arrays, refs, and vectors of the above types are also allowed. Because in MLton monomorphic arrays and vectors are exactly the same as their polymorphic counterpart, these are also allowed. Hence, string, char vector, and CharVector.vector are also allowed. Strings are not null terminated, unless you manually do so from the SML side.

Unfortunately, passing tuples or datatypes is not allowed because that would interfere with representation optimizations.

The C header file that -export-header generates includes typedefs for the C types corresponding to the SML types. Here is the mapping between SML types and C types.

Because MLton assumes that vectors and strings are read-only (and will perform optimizations that, for instance, cause them to share space), you must not modify the data pointed to by the char * in C code.

Although the C type of an array, ref, or vector is always Pointer, in reality, the object has the natural C representation. Your C code should cast to the appropriate C type if you want to keep the C compiler from complaining.

When calling an imported C function from SML that returns an array, ref, or vector result or when calling an exported SML function from C that takes an array, ref, or string argument, then the object must be an ML object allocated on the ML heap. (Although an array, ref, or vector object has the natural C representation, the object also has an additional header used by the SML runtime system.)