Segmentation Fault when call iswctype function in libc.so.6 using python ctypes

Question

I'm learning ctypes module in python. I have the following code:

code

import sys
print(sys.version)
import ctypes
libc = ctypes.CDLL("/lib/x86_64-linux-gnu/libc.so.6")
print(libc)
for x in "我学Python!":
    print("'%s'" % x, libc.iswalpha(
       ord(x)
    ))

libc.wctype.argtypes = [ctypes.c_char_p]
alpha_wctype = libc.wctype(b'print')
print(type(alpha_wctype))
libc.iswctype.restype = ctypes.c_int
print(libc.iswctype(ord("我"), alpha_wctype))

output

3.7.7 (default, May  7 2020, 21:25:33) 
[GCC 7.3.0]
<CDLL '/lib/x86_64-linux-gnu/libc.so.6', handle 7f1d5b9e24f0 at 0x7f1d5a498f10>
'我' 1
'学' 1
'P' 1024
'y' 1024
't' 1024
'h' 1024
'o' 1024
'n' 1024
'!' 0
<class 'int'>
Fatal Python error: Segmentation fault

Current thread 0x00007f1d5b9dd740 (most recent call first):
  File "test2.py", line 15 in <module>
[1]    12178 segmentation fault  python -X faulthandler test2.py

So why the last line produced a segmentation fault, how can I use iswctype correctly using ctypes?

Joseph Sible-Reinstate Monica · Accepted Answer · 2021-05-26 22:47:19Z

1

wctype doesn't return an int but rather a wctype_t. Python doesn't have a portable ctypes type for this, but it's probably an unsigned long on your system, so do libc.wctype.restype = ctypes.c_ulong. Similarly, you need to specify the argument types for iswctype, like this: libc.iswctype.argtypes = [ctypes.c_uint, ctypes.c_ulong]. By not doing those things, you were truncating the alpha_wctype value, which was causing the segfault.

answered May 26, 2021 at 22:47

Joseph Sible-Reinstate Monica

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2 Comments

Meng Zhao Over a year ago

Yes, thanks a lot. I thought these types are c_ushort. But how can i makesure these types, without searching the libc code? Is there any pythonic way?

Joseph Sible-Reinstate Monica Over a year ago

@MengZhao I don't think so. I looked through docs.python.org/3/library/ctypes.html but I don't see any better way to do it.

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