dynamically loaded libraries

Robert Kern robert.kern at gmail.com
Mon May 29 07:33:39 CEST 2006


mhearne808 wrote:
> I have a question about how dynamically loaded C++ modules work, which
> I will phrase as a hypothetical scenario involving the Numeric module.
>  Please understand that I don't really care about Numeric per se, it's
> just a useful example of a module that defines a generally useful data
> type.
> 
> Let's say I want to create a C++ Python extension module that has
> methods accepting the Numeric array type as input, and also create
> these arrays as output. 
> 
> In order to make this work, do I have to statically link against the
> Numeric source, or do I only have to include the headers, under the
> assumption (??) that the Numeric functionality will be available
> because the Python executable has dynamically loaded it?

Here's how Numeric/numarray/numpy do it (I'll discuss numpy since it is the
currently developed one):

Pointers to each of the API items that we want to export, types and functions
mostly, are packed into a void** array, PyArray_API. A common header file is
used by the numpy extension module that provides the API (multiarraymodule.so)
and each of the modules that use the API. We use #ifdefs to determine whether to
use real prototypes (for multiarray) or #defines that index into PyArray_API and
cast the function pointer there to the appopriate prototype (for everybody else).

In initmultiarray(), we actually pack PyArray_API with the appropriate pointers
and create a PyCObject that points to the beginning of the array. The CObject is
assigned to multiarray._ARRAY_API . We define a function import_array() for the
API-users that will import multiarray, get the object _ARRAY_API and assign
pointer to the beginning of the array to void **PyArray_API. import_array() is
supposed to be called in the initialization function of the other extension
modules. That will make sure that multiarray is imported and the function
pointers are available for dereferencing.

-- 
Robert Kern

"I have come to believe that the whole world is an enigma, a harmless enigma
 that is made terrible by our own mad attempt to interpret it as though it had
 an underlying truth."
  -- Umberto Eco




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