MLton 20070826 ForeignFunctionInterfaceTypes
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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.

SML type C typedef C type Note
array Pointer unsigned char *
bool Bool int32_t
char Char8 uint8_t
Int8.int Int8 int8_t
Int16.int Int16 int16_t
Int32.int Int32 int32_t
Int64.int Int64 int64_t
int Int32 int32_t (default)
MLton.Pointer.t Pointer unsigned char *
Real32.real Real32 float
Real64.real Real64 double
real Real64 double (default)
ref Pointer unsigned char *
string Pointer unsigned char * (read only)
vector Pointer unsigned char * (read only)
Word8.word Word8 uint8_t
Word16.word Word16 uint16_t
Word32.word Word32 uint32_t
Word64.word Word64 uint64_t
word Word32 uint32_t (default)

Note (default): The default int, real, and word types may be set by the -default-type type compiler option. The given C typedef and C types correspond to the default behavior.

Note (read only): 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 unsigned 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.)


Last edited on 2007-08-16 01:14:48 by MatthewFluet.