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[doc] ctypes: use_errno=True does not work #52008

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[doc] ctypes: use_errno=True does not work#52008
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theller
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3.11only security fixesdocsDocumentation in the Doc direasytopic-ctypes

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@nikratio
nikratio
mannequin
opened on Jan 23, 2010
BPO 7760
Nosy @amauryfa, @abalkin, @meadori, @eryksun, @iritkatriel
Files
  • test1.py: Test case
    		•

    Note: these values reflect the state of the issue at the time it was migrated and might not reflect the current state.

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    GitHub fields:

    assignee = 'https://github.com/theller'
closed_at = None
created_at = <Date 2010-01-23.00:18:56.292>
labels = ['easy', 'ctypes', 'type-feature', '3.11', 'docs']
title = '[doc] ctypes: use_errno=True does not work'
updated_at = <Date 2021-05-18.20:36:39.807>
user = 'https://bugs.python.org/nikratio'

    bugs.python.org fields:

    activity = <Date 2021-05-18.20:36:39.807>
actor = 'eryksun'
assignee = 'theller'
closed = False
closed_date = None
closer = None
components = ['Documentation', 'ctypes']
creation = <Date 2010-01-23.00:18:56.292>
creator = 'nikratio'
dependencies = []
files = ['15976']
hgrepos = []
issue_num = 7760
keywords = ['easy']
message_count = 8.0
messages = ['98171', '98188', '98195', '189784', '189807', '189886', '393894', '393906']
nosy_count = 8.0
nosy_names = ['amaury.forgeotdarc', 'belopolsky', 'marcin.bachry', 'nikratio', 'meador.inge', 'eryksun', 'Matt.Jones', 'iritkatriel']
pr_nums = []
priority = 'normal'
resolution = None
stage = 'needs patch'
status = 'open'
superseder = None
type = 'enhancement'
url = 'https://bugs.python.org/issue7760'
versions = ['Python 3.11']

    Linked PRs

    Activity

    1. nikratio commented on Jan 23, 2010

      @nikratio
      nikratiomannequin
      on Jan 23, 2010
      MannequinAuthor

      On my system (Ubuntu Karmic, Python 2.6.4 (r264:75706, Dec 7 2009, 18:45:15), Kernel 2.6.31-17-generic, libc6 2.10.1-0ubuntu16) the attached test script produces the following output:

      Traceback (most recent call last):
  File "test1.py", line 27, in <module>
    raise OSError(err, os.strerror(err), path)
OSError: [Errno 0] Success: '/does/not/exist'

      So the function call failed, but the errno provided by ctypes is zero.

      (There are two variants of construction the getxattr() foreign function in the script and both produce the same result).

    2. nikratio
      added
      type-bugAn unexpected behavior, bug, or error
      on Jan 23, 2010
    3. nikratio
      assigned
      theller
      on Jan 23, 2010
    4. nikratio
      added
      topic-ctypes
      on Jan 23, 2010

    5. marcinbachry commented on Jan 23, 2010

      @marcinbachry
      marcinbachrymannequin
      on Jan 23, 2010
      Mannequin

      In the libc case you shouldn't give absolute path in CDLL: CDLL('libc.so.6') is better. You use "/lib/libc.so.6" path, but Python (and ctypes.so) actually uses something like "/lib/tls/i686/cmov/libc.so.6" - these are two separate libraries with separate "errno" variables, mapped to different memory regions. Your call to getxattr modifies errno in the former library, but ctypes get_errno() fetches value from the latter library.

      If I fix CDLL() call, the test works ok for me.

    6. nikratio commented on Jan 23, 2010

      @nikratio
      nikratiomannequin
      on Jan 23, 2010
      MannequinAuthor

      I can confirm that without the path it works for me too.

      But I have to admit that I don't really understand your explanation. Should I generally not use full paths with CDLL? Or just in the case of libc?

      In either case, I think the ctypes dokumentation could be more explicit about this.

    7. BreamoreBoy commented on May 21, 2013

      @BreamoreBoy
      BreamoreBoymannequin
      on May 21, 2013
      Mannequin

      Would someone like to propose a documentation patch that clarifies this situation.

    8. MattJones commented on May 22, 2013

      @MattJones
      MattJonesmannequin
      on May 22, 2013
      Mannequin

      Is this really a documentation issue? Is it not generally understood that using absolute paths to libraries is a bad idea due to the amount of PATH/symlink spaghetti that the average file system contains?

    9. nikratio commented on May 24, 2013

      @nikratio
      nikratiomannequin
      on May 24, 2013
      MannequinAuthor

      Matt, I believe in that case it's still a documentation issue, because then the documentation probably should say that using absolute paths to libraries is a bad idea in general.

    10. vstinner
      changed the title [-]use_errno=True does not work[/-] [+]ctypes: use_errno=True does not work[/+] on May 25, 2013

    11. iritkatriel commented on May 18, 2021

      @iritkatriel
      iritkatriel
      on May 18, 2021
      Member

      The recommendation not to use absolute paths in CDLL is still not mentioned in the docs:
      https://docs.python.org/3/library/ctypes.html#ctypes.CDLL

      Adding the (active) ctypes experts to nosy.

    12. iritkatriel
      added
      easy
      3.11only security fixes
      docsDocumentation in the Doc dir
      on May 18, 2021
    13. iritkatriel
      changed the title [-]ctypes: use_errno=True does not work[/-] [+][doc] ctypes: use_errno=True does not work[/+] on May 18, 2021

    14. 1 remaining item

    15. iritkatriel
      added
      type-featureA feature request or enhancement
      on May 18, 2021

    16. eryksun commented on May 18, 2021

      @eryksun
      eryksun
      on May 18, 2021
      Contributor

      The recommendation not to use absolute paths in CDLL is still
      not mentioned in the docs:

      It's not generally the case. Load just a base filename only if the shared library is in the loader's search path. Otherwise use a qualified path that contains at least one path separator (e.g. "./library.dll").

    17. ezio-melotti
      transferred this issue fromon Apr 10, 2022
    18. serhiy-storchaka
      added this to Locale issues 🗺 and removed this from Locale issues 🗺on May 21, 2022
    19. serhiy-storchaka
      added this to Ctypes issueson Jan 1, 2024
    20. Das-Chinmay
      added 3 commits that reference this issue on Mar 27, 2026

      pythongh-52008: document that absolute paths can break use_errno=True…

      a303722

      pythongh-52008: add news entry

      65fc6a2

      pythongh-52008: fix RST cross-reference and unclosed parenthesis in note

      e45327b
    21. ZeroIntensity
      removed
      type-featureA feature request or enhancement
      on Oct 12, 2026
    22. ZeroIntensity
      added a commit that references this issue on Oct 12, 2026

      gh-52008: Document that `ctypes.CDLL` can only observe `errno` from i…

      41a10ef

    23. ZeroIntensity commented on Oct 12, 2026

      @ZeroIntensity
      ZeroIntensity
      on Oct 12, 2026
      Member

      The issue described here is obsolete since Ubuntu doesn't have multiple libc versions anymore, but I did add a note to the docs saying that the library must use the same errno as ctypes for it to work. Backports are on auto-merge.

    24. ZeroIntensity
      closed this as completedon Oct 12, 2026
    25. ZeroIntensity
      added 2 commits that reference this issue on Oct 12, 2026

      [3.14] gh-52008: Document that `ctypes.CDLL` can only observe `errno`…

      d0888f9

      [3.15] gh-52008: Document that `ctypes.CDLL` can only observe `errno`…

      0073717
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