MLton 20210117

# ForeignFunctionInterfaceTypes

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.)

In addition, there is an MLBasis file, $(SML_LIB)/basis/c-types.mlb, which provides structure aliases for various C types:

C type | Structure | Signature
char | C_Char | INTEGER
signed char | C_SChar | INTEGER
unsigned char | C_UChar | WORD
short | C_Short | INTEGER
signed short | C_SShort | INTEGER
unsigned short | C_UShort | WORD
int | C_Int | INTEGER
signed int | C_SInt | INTEGER
unsigned int | C_UInt | WORD
long | C_Long | INTEGER
signed long | C_SLong | INTEGER
unsigned long | C_ULong | WORD
long long | C_LongLong | INTEGER
signed long long | C_SLongLong | INTEGER
unsigned long long | C_ULongLong | WORD
float | C_Float | REAL
double | C_Double | REAL
size_t | C_Size | WORD
ptrdiff_t | C_Ptrdiff | INTEGER
intmax_t | C_Intmax | INTEGER
uintmax_t | C_UIntmax | WORD
intptr_t | C_Intptr | INTEGER
uintptr_t | C_UIntptr | WORD
void * | C_Pointer | WORD

These aliases depend on the configuration of the C compiler for the target architecture, and are independent of the configuration of MLton (including the -default-type type compiler option).