|parrotcode: NCI conventions and definitions|
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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.
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 Parrot_callback_C(void *external_data, PMC *callback_info);
void Parrot_callback_D(PMC *callback_info, void *external_data);
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, corresponding to callback function signature letters C and D, respectively. If the callback function is supposed to look like:
void (function *)(void *library_data, void *your_data);
then use type C, where if the function signature is:
void (function *)(void *your_data, void *library_data);
then use a type of D. The actual pointer types for the parameters don't have to be void *, but they must be pointers of some sort.
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
your_data pointer, which contains a custom PMC holding all the information that Parrot needs.
The NCI function signature for a callback matching Parrot_callback_C is
vCYZ, while the function signature for a callback matching Parrot_callback_D is
The callback functions take 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, PMC external_data)
The sequence for this is:
dlfunc CALLBACK_REGISTER_FUNC, LIBRARY_PMC, FUNCTION_NAME, 'vCYZ'
newsub $P5, .Sub, _some_parrot_sub # Alternately, if it's in a global # $P5 = global "some::sub::name" new $P6, .PerlInt # The data we get handed .pcc_begin prototyped .arg $P5 .arg $P6 .pcc_call CALLBACK_REGISTER_FUNC .pcc_end
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 # 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.
Maintainer: Dan Sugalski Class: Internals PDD Number: 16 Version: 1.0 Status: Developing Last Modified: December 28, 2003 PDD Format: 1 Language: English