# Copyright (C) 2001-2015, Parrot Foundation.

=head1 [DRAFT] PDD 16: Native Call Interface (NCI)

=head2 Abstract

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

=head2 Description

The NCI is designed to allow Parrot to interface to I<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.

=head2 Implementation

=head3 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 I<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.

=over 4

=item v

Void. As a return type it indicates that there I<is> no return type.

Not valid as a parameter type.

=item c

Char. This is an integer type, taken from (or put into) an I register. NOTE:
it might be signed or unsigned because that is how an unadorned C 'char'
works.

=item s

short. An integer type, taken from or put into an I register. It is always
signed, not unsigned.

=item i

int. An integer type. It is always signed, not unsigned.

=item l

long. An integer type. You know the drill. It is always signed, not unsigned.

=item f

float. F register denizen.

=item d

double. F register, double-precision floating point type

=item P

A PMC register.

=item p

PMC object. A generic pointer, taken from a PMC by using its
get_pointer vtable function, or NULL for a PMCNULL.
If this is a return type and the value is NULL, PMCNULL is returned,
otherwise Parrot will create a new UnManagedStruct PMC type, which
is just a generic "pointer to something" PMC type which Parrot does
I<no> management of.

=item U

This parameter is used for passing user data to a callback creation. More
explanation in the L<callbacks> section.

=item 2

A pointer to a short, taken from an P register of an int-like PMC.

=item 3

A pointer to an int, taken from an P register of an int-like PMC.

=item 4

A pointer to a long, taken from an P register of an int-like PMC.

=item t

string pointer.

=back

Note that not all types are valid as return types.

=head3 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


=head3 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 C<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 C<library_data> is normally coming from C code and can be
any C type that Parrot supports as NCI value.

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

  cb_PMC = new_callback cb_Sub, user_data, "SU"

Given a Parrot function C<cb_Sub>, and a C<user_data> PMC, this creates a
callback PMC C<cb_PMC>, which expects the user data as the second argument.
The information returned by the callback (C<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 C<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 C<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 I<parrot> subroutine which is called by the callback
should be:

   void parrotsub(PMC user_data, <type> external_data)

The sequence for this is:

=over 4

=item Step 1

Create a callback function.

  new_callback CB_PMC, CB_SUB, USER_DATA, "signature"

=item Step 2

Register the callback

  dlfunc C_FUNCTION, "function_name", "signature"
  C_FUNCTION(CP_PMC, USER_DATA)

=back

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

=begin PIR

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

=end PIR

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.

=head3 Example Callback

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

=begin PIR

  .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, "vSU"

    # 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

=end PIR

The C code contains the function to be invoked through NCI. In the function
C<sayhello> a function call is done to a Parrot subroutine. The C<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 C<include> directory so F<parrot/parrot.h> can be found.)

=head2 References

F<pdd06_pasm.pod>

=head2 See Also

F<t/pmc/nci.t>, F<src/nci_test.c>

=cut

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