The declaration

val $id$ = _import "$symbol$" : $type$

is statically resolved to a C function having the name $symbol$. When SML# compiles a source file containing this declaration, the compiler generates an object file containing $symbol$ as an external name. When an executable program is build from these objects files, the system linker links these object files with C functions. This is also true in interactive mode, which is implemented by a simple iteration that performs separate compilation, linking and loading.

However for a library that is only available at runtime, such static resolution is impossible. To cope with those situation, SML# provide dynamic linking through the following module.

structure DynamicLink : sig
  type lib
  type codeptr
  datatype scope = LOCAL | GLOBAL
  datatype mode = LAZY | NOW
  val dlopen : string -> lib
  val dlopen’ : string * scope * mode -> lib
  val dlsym : lib * string -> codeptr
  val dlclose : lib -> unit
end

These functions provide the system services of the same names provide in a Unix-family OS.

• •

  dlopen opens a shared library.

• •

  dlopen’ takes one of those parameters to control dlopen. RTLD_LOCAL, RTLD_GLOBAL and RTLD_LAZY, RTLD_NOW. For its details, consult OS manual on dlopen.

• •

  dlsym takes a library handle obtained by dlopen and a function name, and returns a C pointer to the function.

• •

  dlclose closes the shared library.

The C pointer returned by dlsym can be converted to SML# function by the following expression.

$exp$ : _import $type$

$exp$ is a SML# expression of type codeptr. $type$ specifies the type of C function as in _import declaration.

Figure 10.2 shows an example.