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docs/pdds/pdd16_native_call.pod - NCI conventions and definitions


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.

Function signatures

The following list are the valid letters in the function signatures for Parrot's NCI. Note that only letters and numbers are valid, and each letter represents a single parameter passed into the NCI. Note that the letters 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 letters.

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

  • Void. As a return type indicates that there is no return type. As a parameter indicates that there are no parameters. Can't be mixed with other parameter types.
  • 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.
  • short. An integer type, taken from or put into an I register. It is always signed, not unsigned.
  • int. An integer type. It is always signed, not unsigned.
  • long. An integer type. You know the drill. It is always signed, not unsigned.
  • float. F register denizen.
  • double. F register, double-precision floating point type
  • A PMC register.
  • PMC thingie. A generic pointer, taken from or stuck into a PMC's data pointer. If this is a return type, parrot will create a new UnManagedStruct PMC type, which is just a generic "pointer to some damn thing or other" PMC type which Parrot does no management of.
  • 2
  • A pointer to a short, taken from an P register of an int-like PMC. On return from NCI, the PMC_int_val will hold the new value.

  • 3
  • A pointer to an int, taken from an P register of an int-like PMC. On return from NCI, the PMC_int_val will hold the new value.

  • 4
  • A pointer to a long, taken from an P register of an int-like PMC. On return from NCI, the PMC_int_val will hold the new value.

  • string pointer. Taken from, or stuck into, a string register. (Converted to a null-terminated C string before passing in)
  • This parameter is used for passing user data to a callback creation. More explanation in the callbacks section.

Note that not all types are valid as return types.


Most of the function parameters are reasonably self-evident. Some, however, merit additional explanation. The


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
  • Create a callback function.

      new_callback CB_PMC, CB_SUB, USER_DATA, "signature"
  • Step 2
  • Register the callback

      dlfunc C_FUNCTION, "function_name", "signature"

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

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

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.




"pmc/nci.t" in t, "nci_test.c" in src



    Maintainer: Dan Sugalski
    Class: Internals
    PDD Number: 16
    Version: 1.1
    Status: Developing
    Last Modified: Oct 12, 2004
    PDD Format: 1
    Language: English


  • version 1.1
  • Changed callback section to reflect current status.

  • version 1
  • None. First version


  • version 1.1
  • Changed callback section to reflect current status.

  • Version 1.0
  • None. First version