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).