uctypes calculates incorrect structure size #17726
Description
Port, board and/or hardware
x86_64 coverage build
MicroPython version
MicroPython v1.26.0-preview.387.ge4e97f5aa7.dirty on 2025-07-20; linux [GCC 12.2.0] version
Reproduction
Use the code below:
>>> import uctypes
>>> si32 = {'v': uctypes.INT32}
>>> si16 = {'v': uctypes.INT16}
>>> si = {'v1': (0, si32), 'v2': (4, si32), 'v3': (8, si16), 'v4': (10, si16)}
>>> uctypes.sizeof(si)
16
and for comparison, this C program:
#include <inttypes.h>
#include <stdio.h>
#include <stddef.h>
typedef struct s16 { int16_t v; } s16;
typedef struct s32 { int32_t v; } s32;
typedef struct si { s32 v1, v2; s16 v3, v4; } si;
int main() {
printf("sizeof(s16) = %zd\n", sizeof(s16));
printf("sizeof(s32) = %zd\n", sizeof(s32));
printf("offsetof(si, v2) = %zd\n", offsetof(si, v2));
printf("offsetof(si, v3) = %zd\n", offsetof(si, v3));
printf("offsetof(si, v4) = %zd\n", offsetof(si, v4));
printf("sizeof(si) = %zd\n", sizeof(si));
}
which prints
sizeof(s16) = 2
sizeof(s32) = 4
offsetof(si, v2) = 4
offsetof(si, v3) = 8
offsetof(si, v4) = 10
sizeof(si) = 12
Expected behaviour
Expected the overall structure size to match C (12 bytes)
Observed behaviour
micropython reports the structure size as 16 bytes
Additional Information
I discovered this while working on m68k-micropython, where there's another problem with struct size calculation (m68k-micropython#15) -- There, 32-bit ints only have a 2-byte alignment requirement.
Code of Conduct
Yes, I agree