STINNER Victor <vstin...@python.org> added the comment:
Reproducer of test_ctypes undefined behavior:
---
from ctypes import *
class BITS(Structure):
_fields_ = [("A", c_int, 1),
("B", c_int, 2),
("C", c_int, 3),
("D", c_int, 4),
("E", c_int, 5),
("F", c_int, 6),
("G", c_int, 7),
("H", c_int, 8),
("I", c_int, 9),
("M", c_short, 1),
("N", c_short, 2),
("O", c_short, 3),
("P", c_short, 4),
("Q", c_short, 5),
("R", c_short, 6),
("S", c_short, 7)]
b = BITS()
a = getattr(b, "M")
---
> GET_BITFIELD(val, size); // <==== HERE
I expanded the macro:
---
if (NUM_BITS(size)) {
size_t s = sizeof(val)*8;
Py_ssize_t low = LOW_BIT(size);
Py_ssize_t nbits = NUM_BITS(size);
// val=0, size=65553 = (1 << 16) + 17
// s=16, low=17, nbits=1
// s - low - nbits = (size_t)-2
val <<= (s - low - nbits);
val >>= (s - nbits);
}
---
The problem is that (s - low - nbits) is negative (-2), but becomes a very
large number since it's unsigned (s is unsigned: "sizeof(v)*8" in the macro).
C code:
---
#include <stdio.h>
int main()
{
short s = 0x7fff;
size_t z = (size_t)-2;
s <<= z;
printf("s = %hi\n", s);
return 0;
}
---
GCC and clang disagree :-D
---
$ gcc x.c -o x -O3 -Wall -Wextra -Wconversion && ./x
s = 0
$ clang x.c -o x -O3 -Wall -Wextra -Wconversion && ./x
s = -5784
---
Moreover, runtime the binary built by clang produces a different result at each
run...
---
$ ./x
s = -4824
$ ./x
s = 4120
$ ./x
s = -22344
$ ./x
s = -26744
$ ./x
s = -18184
---
----------
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Python tracker <rep...@bugs.python.org>
<https://bugs.python.org/issue46913>
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