SML# - Language/FFI/Ffiapply Diff

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This is a low level syntax for calling a C function.
It has the following syntax.
_ffiapply <exp> (<arg>, ..., <arg>) : <ty>
* ''<exp>'' is an expression of type ''unit ptr'' denoting a C function pointer.
* ''<arg>'' is an argument to the C function.
* ''<ty>'' is the return type.
Arguments to C function can be given using the following syntax.
<arg> ::= <exp> : <ty> | _sizeof(<ty>)
* ''<exp> : <ty>'' is an argument expression and its type.
* ''_sizeof(<ty>)'' is a reference to the size attribute of the type ''ty'' which the SML# type-directed compiler maintain. This special form is automatically substituted with the term that denotes the size of the type.
* ''<ty>'' can be any [[interoperable type|Language/FFI/Types]].

A C library function '''qsort(3)'' can be called as follows.
  val libc = DynamicLink.dlopen "/lib/"
  val c_qsort = DynamicLink.dlsym (libc, "qsort")
  fun qsort (a, f) =
      _ffiapply c_qsort (a : 'a array,
                         Array.length a : int,
                         f : ('a ptr, 'a ptr) -> int) : unit
qsort has two arguments: an array '''a''' and a callback function
In the function body, it passes the size of the data element through
the special term '''_sizeof('a)''' to the C function.
This allows the C function to traverse the array by advancing the
The above code yields the following expected declaration.
val qsort : ['a. 'a array * ('a ptr * 'a ptr -> int) -> unit]

Conceptually, _ffiapply construct can be understood as an ''inlined typed intermediate code''
that is to be plugged-in the intermediate code, SML# type-directed compiler generates.
No static typechecking is performed for this construct.
Therefore, a special care must be taken to ensure that the number and the types
of arguments agree with those of C function.

As far as these two conditions are met, the rest of the system is
guaranteed to be type safe. This allow type safe usage of low-level
efficient polymorphic code such qsort above as a typed polymorphic function
in SML# program.