I promised to show how spam_system() is called from Python programs. First, we need to list its name and address in a ``method table'':
static PyMethodDef SpamMethods[] = { ... {"system", spam_system, 1}, ... {NULL, NULL} /* Sentinel */ };
Note the third entry (1). This is a flag telling the interpreter the calling convention to be used for the C function. It should normally always be 1; a value of 0 means that an obsolete variant of PyArg_ParseTuple() is used.
The method table must be passed to the interpreter in the module's initialization function (which should be the only non-static item defined in the module file):
void initspam() { (void) Py_InitModule("spam", SpamMethods); }
When the Python program imports module spam for the first time, initspam() is called. It calls Py_InitModule(), which creates a ``module object'' (which is inserted in the dictionary sys.modules under the key "spam"), and inserts built-in function objects into the newly created module based upon the table (an array of PyMethodDef structures) that was passed as its second argument. Py_InitModule() returns a pointer to the module object that it creates (which is unused here). It aborts with a fatal error if the module could not be initialized satisfactorily, so the caller doesn't need to check for errors.