The _PyObject_SIZE() and _PyObject_VAR_SIZE() macros should be converted to functions: see PEP 670 for the rationale.

My problem is that I don't know if the return type should be signed (Py_ssize_t) or unsigned (size_t).

CPython usage of _PyObject_SIZE():

• Signed: 18. Implementation of __sizeof__() methods.
• Unsigned: 0
• Implicit cast to unsigned: 2. Calls PyObject_Malloc(_PyObject_SIZE(tp)) and gc_alloc(_PyObject_SIZE(tp), presize) where the first argument type is size_t.

CPython usage of _PyObject_VAR_SIZE():

• Signed: 5
• Unsigned: 1
• Implicit cast to unsigned: 1. Call _PyDebugAllocatorStats(..., _PyObject_VAR_SIZE(&PyTuple_Type, len)) where the argument type is size_t.

To get a container length, the C API uses signed type (Py_ssize_t): PyList_Size(), PyDict_Size(), Py_SIZE(), etc.

To allocate memory, the C API prefers unsigned type (size_t): PyMem_Malloc(), PyObject_Realloc(), etc.

Python allocator functions reject size greater than PY_SSIZE_T_MAX:

void * PyMem_RawMalloc(size_t size) { /* * Limit ourselves to PY_SSIZE_T_MAX bytes to prevent security holes. * Most python internals blindly use a signed Py_ssize_t to track * things without checking for overflows or negatives. * As size_t is unsigned, checking for size < 0 is not required. */ if (size > (size_t)PY_SSIZE_T_MAX) return NULL; return _PyMem_Raw.malloc(_PyMem_Raw.ctx, size); } 

Some "sizeof" functions freely mix signed and unsigned types. Example:

static PyObject * deque_sizeof(dequeobject *deque, void *unused) { Py_ssize_t res; Py_ssize_t blocks; res = _PyObject_SIZE(Py_TYPE(deque)); blocks = (size_t)(deque->leftindex + Py_SIZE(deque) + BLOCKLEN - 1) / BLOCKLEN; assert(deque->leftindex + Py_SIZE(deque) - 1 == (blocks - 1) * BLOCKLEN + deque->rightindex); res += blocks * sizeof(block); return PyLong_FromSsize_t(res); } 

blocks and sizeof(block) are unsigned, but res is signed.

Another problem is that _PyObject_VAR_SIZE() has an undefined behavior on integer overflow. Maybe it should return SIZE_MAX on oveflow, to make sure that Python allocator function fail (return NULL)?

Linked PRs

• gh-99845: Use size_t type in __sizeof__() methods #99846
• gh-99845: Clean up _PyObject_VAR_SIZE() usage #99847
• gh-99845: Change _PyDict_KeysSize() return type to size_t #99848
• gh-99845: PEP 670: Convert PyObject macros to functions #99850
• gh-99845: _PySys_GetSizeOf() uses size_t #99903
• [3.11] gh-99845: _PyObject_DictPointer(): fix dictoffset cast #99922
• [3.10] gh-99845: _PyObject_DictPointer(): fix dictoffset cast (GH-99922) #99924