Incorrect struct definition with bitfields #86098

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1. berthin commented on Oct 4, 2020

   

   berthinmannequin

   on Oct 4, 2020

   MannequinAuthor

   More actions

   Hi all,

   I found one issue related to bitfields and ctypes, and although there is a similar issue already opened (https://bugs.python.org/issue29753), that only covers the case with pragmas.

   Please, consider the following scenario:

   >>> from ctypes import *
   >>> class X(Structure):
   ...     _fields_ = [("a", c_uint8, 8), ("b", c_uint8, 8), ("c", c_uint32, 16)]
   ...
   >>> sizeof(X)
   5

   Which seems to be incorrect, because what natively C does is to generate a 4bytes struct.

   root@0bc6a647abaa:/target/cpython-fix# cat sample.c

   #include <stdio.h>
   #include <stdint.h>
   struct X { 
       uint8_t a; 
       uint8_t b; 
       uint32_t c:16;
   } X;
   
   int main() { 
       printf("%u\n", sizeof(X)); 
   }

   root@0bc6a647abaa:/target/cpython-fix# gcc sample.c -o a && ./a
   4

   Also, if I use different databtypes I get what I expect (a struct of 4 bytes).

   >>> X = type('X', (Structure,), {'_fields_': [('a', c_uint32, 8), ('b', c_uint32, 8), ('c', c_uint32, 16)]})
   >>> sizeof(X)
   4
   >>> X = type('X', (Structure,), {'_fields_': [('a', c_uint16, 8), ('b', c_uint16, 8), ('c', c_uint16, 16)]})
   >>> sizeof(X)
   4
   >>> X = type('X', (Structure,), {'_fields_': [('a', c_uint8, 8), ('b', c_uint8, 8), ('c', c_uint16, 16)]})
   >>> sizeof(X)
   4

   I tried to debug a little bit, adding some prints before and after calling *PyCField_FromDesc* in *Modules/_ctypes/stgdict.c:L603*.=, if someone wants to look at it, the logs are attached / one log file per each test, using the following gdb init:

   # before
   break stgdict.c:601
   commands
   p field_size
   p bitofs
   p size
   p align
   p *dict
   continue
   end

   # after
   break stgdict.c:617
   commands
   p field_size
   p bitofs
   p size
   p align
   p *(CFieldObject*) prop
   continue
   end

   Hope someone could spot the issue, I will also try to investigate it by my own.

   Best,

   P.S. For the tests, I worked on top of bpo-29753 (#19850)
2. berthin

   added

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   3.9 (EOL)end of lifeend of life

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   topic-ctypes

   type-bugAn unexpected behavior, bug, or errorAn unexpected behavior, bug, or error

   on Oct 4, 2020
3. ezio-melotti

   transferred this issue fromon Apr 10, 2022
4. serhiy-storchaka

   added this to Ctypes issueson Jan 1, 2024
5. encukou commented on Sep 10, 2024

   

   encukou

   on Sep 10, 2024

   Member

   More actions

   Fixed in #97702, the reproducer is now part of tests (as the Example_gh_86098* classes)
6. encukou

   closed this as completedon Sep 10, 2024
7. github-project-automation

   moved this to Done in Ctypes issueson Sep 10, 2024