🌐 Web query:

CPython ctypes cast function behavior memory address pointer

💡 Result:

Short summary of CPython ctypes.cast behavior:

• ctypes.cast(obj, new_type) returns a new pointer instance of new_type that views the same memory as obj (it does not copy memory). new_type must be a pointer type. [1][2]
• obj may be a ctypes instance, a pointer, an integer address, or some buffer-like Python objects; cast will convert the first arg to c_void_p under the hood for the FFI call. [2][3]
• cast does NOT extend the lifetime of the underlying buffer — you must keep a reference to the original object (or otherwise ensure the memory stays valid) or the pointer becomes dangling. [3]
• pointer.contents returns a freshly-constructed ctypes object each access (ctypes has no OOR — "original object return"), so identity comparisons with is will be False; use ctypes.addressof(...) to compare underlying addresses. [1][7]
• Use ctypes.addressof(obj) to get the integer address of a ctypes instance; use byref(obj, offset) to get a lightweight pointer for calls. [1][2]

Concise example:

• ptr = ctypes.cast(ctypes.pointer(my_struct), ctypes.POINTER(OtherStruct)) — ptr points at the same memory, interpreted as OtherStruct; changing ptr.contents updates underlying memory; but keep my_struct alive. [1][3]

Sources: Python ctypes docs and practical notes. [1] Python ctypes Pointers (docs). [2] Python ctypes Utility functions (addressof, byref, cast). [3] Discussion/examples on using cast with bytes/addresses.

🏁 Script executed:

#!/bin/bash
# Find pointer-related files in ctypes
fd -t f "pointer" crates/vm/src/stdlib/ | head -20

Repository: RustPython/RustPython

Length of output: 186

🏁 Script executed:

#!/bin/bash
# Look for contents attribute handling
rg -n "contents" crates/vm/src/stdlib/ctypes/ -B2 -A2 --type rust

Repository: RustPython/RustPython

Length of output: 186

🏁 Script executed:

#!/bin/bash
# Search for cast function tests
rg -n "test.*cast|cast.*test" crates/vm/src/stdlib/ --type rust -i

Repository: RustPython/RustPython

Length of output: 186

🌐 Web query:

RustPython ctypes cast pointer contents behavior

💡 Result:

Short summary of ctypes cast / pointer / contents behavior you’ll see in CPython (and what RustPython aims to match):

• pointer.contents returns a new ctypes instance that represents the object pointed-to — it is not the original Python object (pi.contents is i is False). Reading .contents constructs a fresh wrapper each access. [1]
• ctypes.cast(obj, POINTER(T)) returns a pointer-wrapper that references the same underlying memory block as obj (no copy of the raw bytes). Using that pointer to read/write (via indexing or .contents) operates on the same memory. [1]
• For “simple” ctypes (immediate subclasses of _SimpleCData such as c_void_p, c_char_p, c_int, etc.) ctypes does automatic conversion/autocast to native Python types in many contexts (returned from functions or read from struct/array fields). That behavior can make .contents or field access return Python ints/bytes rather than ctypes objects and has caused surprises/bugs. A Python tracker discussion explains the autocast and workarounds (subclassing) if you need a ctypes object instead of the native type. [2]
• Practical gotchas seen in real code: repeated cast/create operations can hold references or create wrapper objects that increase memory usage (people have observed apparent leaks when repeatedly casting large buffers). The cast itself references the buffer but constructing many wrapper objects can grow memory use. [3]

If you need guidance for a specific RustPython mismatch or a reproducer (example code showing different behavior vs CPython), share the snippet and I’ll look up/compare exact behavior and point to whether it’s a RustPython bug or a libc/ctypes semantics issue.

Sources:
[1] Python ctypes docs (pointers, cast, contents).
[2] Python tracker discussion on simple-types autocast (issue discussion / workaround).
[3] StackOverflow thread: ctypes.cast memory/leak observations.

🌐 Web query:

RustPython ctypes implementation pointer contents

💡 Result:

Short answer: RustPython's ctypes support is incomplete — pointer.contents semantics from CPython (where .contents returns a new ctypes instance referencing the pointed memory) are not fully implemented/guaranteed in RustPython yet. See the RustPython repo/issues noting "Implement the ctypes module" and ongoing work; for CPython behavior refer to the official ctypes docs (pointer() / POINTER() / .contents). [1][2]

References

• RustPython repo / roadmap mentioning ctypes implementation work. [1]
• CPython ctypes documentation (pointer and .contents behavior). [2]

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