[docs] [issue17378] Document that ctypes automatically applies byref() when argtypes declares POINTER

[Eli Bendersky]

New submission from Eli Bendersky:

While playing with ctypes a bit, I noticed a feature that doesn't appear to be documented. Suppose I import the readdir_r function (assuming DIRENT is a correctly declared ctypes.Structure):

DIR_p = c_void_p
DIRENT_p = POINTER(DIRENT)
DIRENT_pp = POINTER(DIRENT_p)

readdir_r = lib.readdir_r
readdir_r.argtypes = [DIR_p, DIRENT_p, DIRENT_pp]
readdir_r.restype = c_int

It seems that I can then call it as follows:

dirent = DIRENT()
result = DIRENT_p()

readdir_r(dir_fd, dirent, result)

Note that while readdir_r takes DIRENT_p and DIRENT_pp as its second and third args, I pass in just DIRENT and DIRENT_p, accordingly. What I should have done is use byref() on both, but ctypes seems to have some magic applied when argtypes declares pointer types. If I use byref, it still works. However, if I keep the same call and comment out the argtypes declaration, I get a segfault.

This behavior of ctypes should be documented.

----------
assignee: docs at python
components: Documentation
keywords: easy
messages: 183661
nosy: docs at python, eli.bendersky
priority: normal
severity: normal
stage: needs patch
status: open
title: Document that ctypes automatically applies byref() when argtypes declares POINTER
type: behavior
versions: Python 2.7, Python 3.2, Python 3.3, Python 3.4

_______________________________________
Python tracker <report at bugs.python.org>
<http://bugs.python.org/issue17378>
_______________________________________