[issue29753] [Linux] ctypes packs bitfields Incorrectly
Sam Price added the comment: I’ve been looking forward to this fix. The old implementation did not match what my LLVM compiler generated. On Sun, Jul 11, 2021 at 2:52 PM Charles Machalow wrote: > > Charles Machalow added the comment: > > Maybe we need to add a __packing__ option to specify how packing should > work and default to legacy behavior. Then allow users to specify if they > want similar behavior cross-os. > > Otherwise changing this does change packing for existing users and can lead > to breakages. > > What exactly was the hit regression here? > > On Sun, Jul 11, 2021, 10:47 AM miss-islington > wrote: > > > > > miss-islington added the > > comment: > > > > > > New changeset 42da46ed522157b057d73e6b623615ef6427999e by Miss Islington > > (bot) in branch '3.10': > > bpo-29753: revert 0d7ad9f (GH-19850) (GH-27085) > > > > > https://github.com/python/cpython/commit/42da46ed522157b057d73e6b623615ef6427999e > > > > > > -- > > > > ___ > > Python tracker > > <https://bugs.python.org/issue29753> > > ___ > > > > ------ > > ___ > Python tracker > <https://bugs.python.org/issue29753> > ___ > -- Thank you, Sam Price (707) 742-3726 -- ___ Python tracker <https://bugs.python.org/issue29753> ___ ___ Python-bugs-list mailing list Unsubscribe: https://mail.python.org/mailman/options/python-bugs-list/archive%40mail-archive.com
[issue29753] Ctypes Packing Bitfields Incorrectly - Linux
Sam Price added the comment:
I ran into this bug on mac and submitted a duplicate issue of 39858
If you add the check *pfield_size != *pbitofs it fixes this bug.
#ifndef MS_WIN32
} else if (bitsize /* this is a bitfield request */
&& *pfield_size /* we have a bitfield open */
&& *pfield_size != *pbitofs /* Current field has been filled, start new
one */
&& dict->size * 8 >= *pfield_size
&& (*pbitofs + bitsize) <= dict->size * 8) {
/* expand bit field */
fieldtype = EXPAND_BITFIELD;
#endif
However this would not fix the results where you expand a bitfield around 3
bytes.
clang produces the 3 bytes if you try and expand around.
#include
#include
typedef struct testA{
uint8_t a0 : 7;
uint8_t a1 : 7;
uint8_t a2 : 7;
uint8_t a3 : 3;
} __attribute__((packed)) testA;
int main(){
printf("%d\n", sizeof(testA)); /* Prints out 3 */
return 0;
}
python prints out a size of 4.
--
nosy: +thesamprice
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[issue39858] bitfield layout wrong in ctypes
Sam Price added the comment:
Does not happen on windows.
Error is in cfield.c
```
#ifndef MS_WIN32
} else if (bitsize /* this is a bitfield request */
&& *pfield_size /* we have a bitfield open */
&& dict->size * 8 >= *pfield_size
&& (*pbitofs + bitsize) <= dict->size * 8) {
/* expand bit field */
fieldtype = EXPAND_BITFIELD;
#endif
```
Specifically dict->size * 8 >= *pfield_size
if *bitofs == *pfield_size then the current field is filled, and expanding the
bitfield should not be done.
Consider adding this *pfield_size != *bitofs
#ifndef MS_WIN32
} else if (bitsize /* this is a bitfield request */
&& *pfield_size /* we have a bitfield open */
&& *pfield_size != *pbitofs /* Current field has been filled, start new
one */
&& dict->size * 8 >= *pfield_size
&& (*pbitofs + bitsize) <= dict->size * 8) {
/* expand bit field */
fieldtype = EXPAND_BITFIELD;
#endif
--
versions: +Python 3.7, Python 3.8, Python 3.9
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[issue39858] bitfield layout wrong in ctypes
New submission from Sam Price : if 8 1 byte fields are included in a ctype field, it allows an extra byte to be included in the packing when there is no room left for the next field. If I put the bitfields in a child structure then I get expected results. In [35]: run ctypeSizeTest.py Size is 4 Expected 3 0 0x1 a0 0 0x10001 a1 0 0x10002 a2 0 0x10003 a3 0 0x10004 a4 0 0x10005 a5 0 0x10006 a6 0 0x10007 a7 0 0x40008 b0 <- Expected to be at offset 1, not 0. 2 0xc b1 <- Expected to be at offset 1, not 2 Size is 3 Expected 3 0 0x1 a 1 0x4 b0 1 0xc0004 b1 -- components: ctypes files: ctypeSizeTest.py messages: 363417 nosy: thesamprice priority: normal severity: normal status: open title: bitfield layout wrong in ctypes type: behavior versions: Python 2.7, Python 3.5, Python 3.6 Added file: https://bugs.python.org/file48954/ctypeSizeTest.py ___ Python tracker <https://bugs.python.org/issue39858> ___ ___ Python-bugs-list mailing list Unsubscribe: https://mail.python.org/mailman/options/python-bugs-list/archive%40mail-archive.com
