[issue39573] [C API] Make PyObject an opaque structure in the limited C API

[STINNER Victor]

STINNER Victor <vstinner at python.org> added the comment:

Oh and obviously, it's not possible possible to define structures which *include* PyObject or PyVarObject if PyObject and PyVarObject become opaque. Example:

typedef struct {
    PyObject ob_base;
    Py_ssize_t ob_size; /* Number of items in variable part */
} PyVarObject;

This C code requires the PyObject structure to be fully defined (not being opaque).

A new C API and ABI where structures *don't* include PyObject or PyVarObject should be designed to allocate their members "before" the PyObject* pointer value. Something like the current PyGC_Head structure which is excluded from PyObject and stored *before* the "PyObject*" pointer.

Simplified code which allocates memory for an object implementin the GC protocol:

static PyObject *
_PyObject_GC_Malloc(size_t basicsize)
{
    ...
    size_t size = sizeof(PyGC_Head) + basicsize;
    ...
    PyGC_Head *g = (PyGC_Head *)PyObject_Malloc(size);
    ...
    PyObject *op = (PyObject *)(g + 1);
    return op;
}

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Python tracker <report at bugs.python.org>
<https://bugs.python.org/issue39573>
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