[DRAFT] PDD 16: Native Call Interface (NCI)

Abstract

This PDD describes the native call interface, and the function signatures used to describe those functions.

The NCI is designed to allow Parrot to interface to most of the functions in a C library without having to write any C code for that interface. It isn't designed to be a universal C-less interface--there will always be libraries that have some bizarre parameter list that requires that some C be written. It should, however, handle all the simple cases.

Using the NCI, parrot automatically wraps the C functions and presents them as prototyped subroutines that follow normal parrot calling conventions, and can be called like any other parrot subroutine.

The NCI uses the platform native dynamic by-name function loading mechanism (dlopen/dlsym on unix and LoadLibrary/GetProcAddress on Win32, for example) to get the function pointer, then dynamically generates the wrapper based on the signature of that function.

As there is no good platform-independent way to determine function signatures (C header files are not always available (certainly not for libraries not designed for access from C) and not always reasonably parseable anyway, and there is no generic way to query a function for its signature) the signature must be passed in when the linkage between the C function and parrot is made.

Implementation

Function signatures

The following list are the valid symbols in the function signatures for Parrot's NCI. Note that only letters and numbers are valid, and each symbol represents a single parameter passed into the NCI. Note that the symbols are case-sensitive, and must be within the base 7-bit ASCII character set.

At some point punctuation may be used as modifiers on the function parameters, in which case each parameter may be represented by multiple symbols.

In no case should the signature symbols be separated by whitespace. This restriction may be lifted in the future, but for now remains as an avenue for adding additional functionality.

v
c
s
i
l
f
d
P
p
2
3
4
t
U

Note that not all types are valid as return types.

Example NCI call

This section describes the simplest example for NCI possible. To every NCI invocation, there are two parts: the native function to be invoked, and the PIR code to do the invocation.

First the native function, to be written in C. On Windows, it is necessary to do a DLL export specification of the NCI function:

  /* foo.c */

  /* specify the function prototype */
  #ifdef __WIN32
  __declspec(dllexport) void foo(void);
  #else
  void foo(void);
  #endif

  void foo(void) {
    printf("Hello Parrot!\n");
  }

Then, after having compiled the file as a shared library, the PIR code looks like this:

  .sub main :main
     .local pmc lib, func

     # load the shared library
     lib = loadlib "hello" # no extension, .so or .dll is assumed

     # get a reference to the function from the library just
     # loaded, called "foo", and signature "void" (and no arguments)
     func = dlfunc lib, "foo", "v"

     # invoke
     func()

  .end

Callbacks

Some libraries, particularly ones implementing more complex functionality such as databases or GUIs, want callbacks, that is ways to call a function under the control of the library rather than under control of the interpreter. These functions must be C functions, and generally are passed parameters to indicate what should be done.

Unfortunately there's no good way to generically describe all possible callback parameter sets, so in some cases hand-written C will be necessary. However, many callback functions share a common signature, and parrot provides some ready-made functions for this purpose that should serve for most of the callback uses.

There are two callback functions, Parrot_callback_C and Parrot_callback_D, which differ if the passed in user_data is second or first respectively:

   void (function *)(void *library_data, void *user_data);

   void (function *)(void *user_data, void *library_data);

The information library_data is normally coming from C code and can be any C type that Parrot supports as NCI value.

The position of the user_data is specified with the U function signature, when creating the callback PMC:

  cb_PMC = new_callback cb_Sub, user_data, "tU"

Given a Parrot function cb_Sub, and a user_data PMC, this creates a callback PMC cb_PMC, which expects the user data as the second argument. The information returned by the callback (library_data) is a C string.

Since parrot needs more than just a pointer to a generic function to figure out what to do, it stuffs all the extra information into the user_data pointer, which contains a custom PMC holding all the information that Parrot needs. This also implies that the C function that installs the callback, must not make any assumptions on the user_data argument. This argument must be handled transparently by the C code.

The callback function takes care of wrapping the external data pointer into an UnManagedStruct PMC, the same as if it were a p return type of a normal NCI function.

The signature of the parrot subroutine which is called by the callback should be:

   void parrotsub(PMC user_data, <type> external_data)

The sequence for this is:

Step 1
Step 2

When the callback function is invoked by the external library, the function itself should look like:

  .sub _my_callback
    .param pmc my_data
    .param pmc library_data   # type depends on signature
    # Do something with the passed in data
  .end

Parrot itself handles all the nasty bits involved in collecting up the interpreter pointer, creating the wrapping PMCs, stuffing data various places, and generally dealing with the bookkeeping.

Example Callback

This section contains an example to register a callback function and have it call back into Parrot.

  .sub main

    # set up callback

    .local pmc sub, userdata
    sub = get_global "foo_callback"  # get the sub to act as a callback sub

    userdata = new 'Integer'         # set up some userdata
    userdata = 42

    .local pmc callback_sub
    callback_sub = new_callback sub, userdata, "vtU"

    # set up NCI

    .local pmc lib, fun
    lib = loadlib "hello"
    fun = dlfunc lib, "sayhello", "vpP"

    # do the NCI call, foo_callback is invoked from C
    fun(callback_sub, userdata)

  .end

  .sub foo_callback
    .param pmc result
    .param pmc udata
    print "Foo callback\n"
  .end

The C code contains the function to be invoked through NCI. In the function sayhello a function call is done to a Parrot subroutine. The sayhello function gets a reference to this callback function, so its signature needs to be known.

  #include <stdio.h>
  #include <parrot/parrot.h>

  /* declare the function signature of the Parrot sub that will be invoked */
  typedef void (*callbackfun)(const char*, void*);

  #ifdef __WIN32
  __declspec(dllexport) void sayhello(callbackfun cb, void *userdata);
  #else
  void sayhello(callbackfun cb, void *userdata);
  #endif

  void sayhello(callbackfun cb, void* userdata) {
      const char *result = "succeeded";

      /* invoke the callback synchronously */
      cb(result, userdata);
  }

The file containing this C code should be compiled as a shared library (specifying the include directory so parrot/parrot.h can be found.)

References

pdd06_pasm.pod