|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.
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);
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:
new_callback CB_PMC, CB_SUB, USER_DATA, "signature"
dlfunc C_FUNCTION, "function_name", "signature" C_FUNCTION(CP_PMC, USER_DATA)
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 .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: 1.1 Status: Developing Last Modified: Oct 12, 2004 PDD Format: 1 Language: English